key: cord-286136-gm6w590s
authors: Aleksic Sabo, Verica; Knezevic, Petar
title: Antimicrobial activity of Eucalyptus camaldulensis Dehn. plant extracts and essential oils: A review
date: 2019-03-05
journal: Ind Crops Prod
DOI: 10.1016/j.indcrop.2019.02.051
sha: 
doc_id: 286136
cord_uid: gm6w590s

Eucalyptus has become one of the world’s most widely planted genera and E. camaldulensis (The River Red Gum) is a plantation species in many parts of the world. The plant traditional medical application indicates great antimicrobial properties, so E. camaldulensis essential oils and plant extracts have been widely examined. Essential oil of E. camaldulensis is active against many Gram positive (0.07–1.1%) and Gram negative bacteria (0.01–3.2%). The antibacterial effect is confirmed for bark and leaf extracts (conc. from 0.08 μg/mL to 200 mg/mL), with significant variations depending on extraction procedure. Eucalyptus camaldulensis essential oil and extracts are among the most active against bacteria when compared with those from other species of genus Eucalyptus. The most fungal model organisms are sensitive to 0.125–1.0% of E. camaldulensis essential oil. The extracts are active against C. albicans (0.2–200 mg/mL leaf extracts and 0.5 mg/mL bark extracts), and against various dermatophytes. Of particular importance is considerable the extracts’ antiviral activity against animal and human viruses (0.1–50 μg/mL). Although the antiprotozoal activity of E. camaldulensis essential oil and extracts is in the order of magnitude of concentration several hundred mg/mL, it is considerable when taking into account current therapy cost, toxicity, and protozoal growing resistance. Some studies show that essential oils’ and extracts’ antimicrobial activity can be further potentiated in combinations with antibiotics (beta-lactams, fluorochinolones, aminoglycosides, polymyxins), antivirals (acyclovir), and extracts of other plants (e.g. Annona senegalensis; Psidium guajava). The present data confirm the river red gum considerable antimicrobial properties, which should be further examined with particular attention to the mechanisms of antimicrobial activity.

The continuing increase in the degree of bacterial resistance to conventional antibiotics is a problem of global significance. The crisis of antimicrobial resistance has been ascribed to the misuse of these agents and today resistant strains are common, such as methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, drug resistant Streptococcus pneumoniae and Mycobacterium tuberculosis, carbapenemresistant and extended-spectrum beta-lactamase producing enterobacteria, multidrug-resistant Pseudomonas aeruginosa and Acientobacter baumannii etc. It was estimated that the medical cost per patient with an antibiotic-resistant infection is up to $29,069 and infections are usually life treating (Ventola, 2018; Aslam et al., 2018) . Similarly, fungi have become resistant to poliens, azoles and echinocandins, and the emergence of drug-resistant strains has been reported in all fungal species (Robbins et al., 2017) . Beside the noticeable emergence of resistant protozoa and viruses, there is a problem with camaldulensis is highly variable (Table 1 ). In the past, this character has been used to break up the group into different varieties or subspecies. The entire complex is currently under revision and new varieties or subspecies may be described or extant ones rationalised. Until this work is completed, EUCLID (2006) decided to adopt a conservative view of E. camaldulensis.

Used for centuries as a traditional Aboriginal herbal remedy, eucalyptus leaves and their essential oils have found various applications in everyday life due to their antiseptic, anti-inflammatory and antipyretic properties (Jeane et al., 2003; Kumar et al., 1988) . Ancient Aboriginal society in Australia used E. camaldulensis plant in medicines to treat gastrointestinal symptoms (including colic, diarrhea, and dysentery), respiratory disease (colds, coughs, asthma, laryngalgia, laryngitis, pharyngitis, sore throat, trachalgia), arrest bleeding, open wounds, and cuts, as well as its decoctions for the relief of spasms, aches, and pains in muscles, but also pains in joints and even tooth (Duke and Wain, 1981) .

As previously stated, E. camaldulensis plant is also known as red gum eucalyptus, murray red gum, and river red gum, because it produces red kino i.e. red gum, in significant amount. Thus besides different plant parts Aborigines used its secondary products as folk remedies. They made incisions in the tree trunks for obtaining the red kino and applied it directly to abrasions and cuts. Except fresh kino, the dried, dehydrated kino was prepared and used in the same way as fresh, but after softening in water (Williams, 2011; Clarke, 2014) . Another significant folk remedy is young leaves which were used for smoke bath, where burning leaves smoke surrounds patient. The smoking medicine was used for fevers, colds, flu and general sickness (Williams, 2011; Pennacchio et al., 2010; Duke and Wain, 1981) . Being useful for treating various health conditions, E. camaldulensis and its folk remedies then were transferred and introduced to other parts of the world, such as Africa. In Sudan the red kino was used for sore throat and diarrhea, while the smoke of burnt leaves was inhaled in case of respiratory problems. In Senegal for stomach-ache decoctions from leaves were prepared with sugar, while in Zimbabwe for cough, flu, and fever a decoction of E. camaldulensis leaves were combined with Citrus limon (L.) Burm. f. fruits and Psidium guajava L. leaves (Doran and Wongkaev, 2008; Maroyi, 2013) . Further more, to prevent tooth decay and periodontitis in Nigeria teeth cleaning sticks were made from tree (Bukar et al., 2004) , and in traditional medicine for healing wound infections, poultice of leaves containing eucalyptus oil have been used (Adeniyi et al., 2006) .

Nowdays, E. camaldulensis has been the subject of numerous studies, to confirm plant's usefulness in traditional medicine for the treatment V. Aleksic Sabo and P. Knezevic Industrial Crops & Products 132 (2019) [413] [414] [415] [416] [417] [418] [419] [420] [421] [422] [423] [424] [425] [426] [427] [428] [429] of various ailments (Coelho-de-Souza et al., 2005; Ghani, 2003; Ito et al., 2000) . Its essential oils are reported to be anesthetic, antiseptic and astringent (Jeane et al., 2003; Kumar et al., 1988) . In addition, a decoction of the leaves is reported to be a remedy for sore throat and other bacterial infections of the respiratory and urinary tracts (Bruneton, 1999) . Due to numerous contemporary data regarding the antimicrobial activity of E. camaldulensis, this subject will be discussed in special section/chapter. Except antimicrobial effects, E. camaldulensis plant extracts (PEx) and essential oils (EOs) and its constituents possess numerous other beneficial effects. One such effect is certainly gastrointestinal effect. In animal models, extracts of the leaves of E. camaldulensis and E. torelliana R. Muell are reported to decrease gastric acid production and thus appear useful for the treatment of gastric ulcers (Adeniyi et al., 2006) . It has been proven that E. camaldulensis leaves methanol extracts possessed ulcer-healing promoting effect when investigated in acetic acid induced-ulcer in rat (Rattus norvegicus domesticus) (Table 2) . Similarly, the poultice of the leaves is applied over wounds and ulcers (Gill, 1992) .

The antioxidant effect represents one more useful E. camaldulensis characteristic. The free radical scavenging activities of the essential oils is assessed by measuring their scavenging abilities for 2,2-diphenyl-1picrylhydrazyl (DPPH) radicals. The scavenging activity for the Eucalyptus camaldulensis was characterized as high (81.9%) (Ghaffar et al., 2015) . The results of E. camaldulensis EO antioxidant effect evaluation indicated that the EO had high potent ferrous ions chelating and total antioxidant activities comparing to ascorbic acid and BHT (El-Baz et al., 2015) . Also, the E. camaldulensis var. brevirostris leaves ethanol extract possessed antioxidant activity, where the prevailing antioxidants in the extract were gallic and ellagic acid (El-Ghorab et al., 2003) . There are several more studies dealing with antioxidant activity of E. camaldulensis (Barra et al., 2010; Salem et al., 2015; Siramon and Ohtani, 2007; Olawore and Ololade, 2017) , but it is interesting to mention here that E. camaldulensis flower EO inhibited melanogenesis through its antioxidant properties and by down-regulating both mitogen-activated protein kinases (MAPK) and protein kinase A (PKA) signaling pathways. This study indicated that the essential oil has the potential to be developed into a skin care product (Huang et al., 2015) . Thus, except the medical application, E. camaldulensis extracts are also currently used in cosmetic formulations, and leaf extracts have been approved as food additives (Takahashi et al., 2004) . Also, essential oils and their constituents have been used as flavoring agents in the formulation of different pharmaceutical products, cosmetics, and food industry (Cowan, 1999) . Except this, E. camaldulensis PEx and EOs have the potential to be used as antibacterial and antifungal agents in cosmetic and pharmaceutical products. It is interesting to mention here the activity of E. camaldulensis EOs (2-8 mg/mL) on dental biofilm formation in vivo where detected inhibition was 14.5-39.2% after four weeks, comparing to chlorhexidine (2 mg/mL), which inhibited maximum 13.9% of dental biofilm formation . Extracts and essential oils of this aromatic plant can be used as food preservatives in order to reduce the dependency on synthetic chemicals in food preservation. In Australia, it is also used as sources of wild honey, providing bees with good quality pollens and heavy yields of nectar (Boland et al., 1984) . Moreover, in industry, the wood of E. camaldulensis has been used for heavy construction, railway sleepers, flooring, framing, fencing, plywood, and veneer manufacture, wood turning, firewood, and charcoal production (Boland et al., 1984) . Eucalyptus camaldulensis wood burns well and make a good fuel, also its dense wood and coppicing ability make it an excellent species for fuelwood production used in several countries such as Brazil (Jacobs, 1981; Eldridge et al., 1993) . Furthermore, Eucalyptus biomass residues from agro-forest and pulping industries represent a valuable source of highvalue compounds such as triterpenic compounds (Domingues et al., 2011; Ferreira et al., 2018) .

Due to the diversity of E. camaldulensis beneficial effects, all effects reported in the literature are summarized in Table 2 . Beside the beneficial effects, essential oils and plant extracts can exert potentially unfavorable effects as complex mixtures of different compounds. A risk assessment of their hazard is always necessary before commercialization, so the estimation of EOs toxicity has been already conducted resulting in human oral and dermal dose limit recommendations. For most EOs the recommended dose is in range 1-4%, but for cineole-rich Eucalyptus sp. EOs, including E. camaldulensis, the limit dose is 10%, indicating a generally low application risk (Lis-Balchin, 2006) . The safe daily oral dose in human adults is 300-600 mg, while semisolid preparations for topical use may contain 5-20% of Eucalyptus oil (Blumenthal and Busse, 1998; Tisserand and Young, 2014) . Similarly, E. camaldulensis bark methanolic extract LD50 value for Swiss albino mice (Mus musculus) is very high -1120 mg/kg, indicating its low host toxic effects (Islam et al., 2014) . The low toxicity and natural origin of E. camaldulensis essential oil and extracts, in contrast to synthetic antimicrobials, favor their application as antimicrobial agents.

Eucalyptus camaldulensis leaves contain 0.1-0.4% essential oil, of which 77% is 1,8-cineole. There is considerable amount of cuminal, phellandrene, aromadendren (or aromadendral), valerylaldehyde, geraniol, cymene, and phellandral (Council of Scientific and Industrial Research, 1948 -1976Council of Scientific and Industrial Research (S.I.R, 1948Council of Scientific and Industrial Research, 1948 -1976 Slee et al., 2006) . Leaves contain 5-11% tannin. The kino (a class of wood exudates), contains 45% kinotannic acid as well as kino red, a Ebrahimi et al. (2013) Repellency against the adult females of Culex pipiens Dried fruits essential oil Two different treatment levels (5 and 10 μl) in six exposure times (15, 75, 135, 195 , 255 and 315 s) Erler et al. (2006) Anti-diabetic effect Albino rats

Oral 500 mg/kg of body weight Dawoud (2015) Alloxan-induced diabetic rats Leaves ethanol extract 500 mg/kg of body weight in distilled water orally, E. camaldulensis leaves supplement incorporated into the diet 5 g/kg/day Dawoud and Shayoub (2017) V. Aleksic Sabo and P. Knezevic Industrial Crops & Products 132 (2019) 413-429 glucoside, catechol, and pyrocatechol. Leaves and fruits test positive for flavonoids and sterols. The bark contains 2.5-16% tannin, the wood 2-14%, and the kino 46.2-76.7% (Watt and Breyer-Brandwijk, 1962) . Some of the reported phytoconstituents of the tree included essential oils, sterols, alkaloids, glycosides, flavonoids, tannins, and phenols.

A considerable variation in the yield of leaf essential oil from E. camaldulensis has been reported (Boland et al., 1984; Shieh, 1996; Moudachirou et al., 1999; Farah et al., 2002) , depending on multiple biotope factors, and also genetic and/or epigenetic characteristics of the plant. The yields of E. camaldulensis leaves EO (0.90-0.98%) originating from Pakistan and Morocco were similar (Ashraf et al., 2010; Farah et al., 2002) , while Moudachirou et al. (1999) reported a variable oil content of 0.6-1.4% from different locations of Benin. The oil yield of E. camaldulensis from Jerusalem was 0.5% (Chalchat et al., 2000) and significantly higher oil yield was reported for E. camaldulensis from Taiwan: 2.3-3.0% with respect to different seasons (Shieh, 1996) . Similar EOs yield (0.77-2.53%) has been reported for E. globulus, as one of the economically important plants for essential oil production (Joshi, 2012; Selvakumar et al., 2012; Harkat-Madouri et al., 2015) . The reported essential oil yield for other Eucalyptus species is slightly higher, ranging from 1.2% to 3% (w/w): the highest yield was obtained from E. cinerea F. (Sebei et al., 2015) .

All the E. camaldulensis essential oil single compounds belong to chemical class of hydrocarbons terpenes, further devided according to the number of isoprene units (C 5 H 8 ) to monoterepenes (C 10 H 16 ), sesquterepenes (C 15 H 24 ), and longer chains of isoprene units. Oxygenated terpenes (oxygenated monoterepenes and oxygenated sesquterepenes) are called trepenoids. According to the literature, in E. camaldulensis EOs dominates 1,8-cineole (eucalyptol), trans-pinocarveol and terpinen-4-ol from chemical class of oxygenated monoterepenes. The oils also contain considerable amount of monoterpene hydrocarbons (β-pinene, α-thujene, γ-terpinene, p-cymene), while sesquterepene hydrocarbons and oxygenated sesquterepenes are detected in significantly lower amounts (Table 3) .

When the composition of five Eucalyptus essential oils (Eucalyptus camaldulensis, Eucalyptus astringens Maiden, Eucalyptus leucoxylon, Eucalyptus lehmannii and Eucalyptus rudis Endl) are compared, a high percentage of monoterpenes, mainly oxygenated compounds, with lower quantities of sesquiterpenes were recorded throughout the four seasons (Ben Jemaa et al., 2012) . In E. camaldulensis oil, monoterpenes were prevalent (34.6-56.3% for all seasons) while sesquiterpene hydrocarbons (6.6-16.5%) and oxygenated sesquiterpenes (2.1-11.1%) were present in less extent. Eucalyptus astringens oil had resembling chemical characteristic to E. camaldulensis oil, the abundant quantity of monoterpenes, which represented more than 50% of the total oil amounts for the all seasons (53.4-63.6 %). Similarlly, E. leucoxylon essential oil, contained mainly oxygenated monoterpenes with a prevalence of 1,8 cineole (13.1-17.6%), such as in E. camaldulensis oil (15.5-20.6%). The essential oil of E. lehmannii also was made up largely of monoterpenes (hydrocarbons 27.6-44.9% and oxygenated 30.9-62.8%), with smaller amounts of sesquiterpene hydrocarbons and absence of oxygenated sesquiterpenes, unlike the oils of other Eucalyptus species (Ben Jemaa et al., 2012) .

The difference in the chemical composition of the E. camaldulensis EOs may be due to many reasons, which can generally be classified in five main groups: (1) the change in plant genes through generations and hybridizations (naturally and induced) may result in production a variety of volatile oils compared with those of different habitat; (2) nutrients of different soils and their accumulation in the leaves may result in different plant metabolism and consequently production of different bio-products and also EOs made of diverse compounds in variable amounts; (3) acclimation of species to the Australian environment in which it is growing in the past, compared with the introduced and/or worldwide planted trees on plantations; (4) different ecotypes of the E. camaldulensis and (5) differences may be due to plant part used for essential oils extraction and its stage of development (maturity). Knowing that these factors express significant effect on the percent compositions of some EO components for this species, E. camaldulensis can be grown in the corresponding areas and in specific conditions to enhance EO production.

The variations in the chemical composition of eucalyptus EOs with respect to seasons have also been reported (Tsiri et al., 2003) , but the most commonly detected as major components in the eucalyptus essential oil are 1,8-cineole, β-pinene, γ-terpinene, and p-cymene. It is important to emphasize here that according to chemical composition the Eucalyptus camaldulensis EO generally can be divided in two different types. Type I is a cineole-rich essential oil containing 80-90% 1,8-cineole plus pinene, and Type II is a cineole-poor essential oil, containing significantly less cineol (Williams, 2011) . Plant genotype is important factor influencing the final chemical composition of EO (Djilani and Dicko, 2012) . Due to genetic and epigenetic factors same plant species can produce a similar EO, but with different chemical composition and therapeutic activities. Brophy and Southwell (2002) examined essential oils of two variations, Eucalyptus camaledulensis var. camaldulensis and Eucalyptus camaledulensis var. obtuse, and the main compounds of Eucalyptus camaledulensis var. camaldulensis were pcymene (22%), cryptone (14%), and spathulenol (17%), while Eucalyptus camaledulensis var. obtusa had different main compounds: 1,8cinole (52%), α-pinene (15%), and aromadendrene (3%), suggesting that these subspecies belong to different types of eucalyptus essential oils. The content of 1,8-cineole was also variable but in the same range for essential oils reported from Greece (25.3-44.2%) (Tsiri et al., 2003) , Pakistan (34.4-40.0%) (Ashraf et al., 2010) , Mozambique (37.1-40.0%) (Pegula et al., 2000) , Nigeria (32.8-70.4%) (Oyedeji et al., 1999) , and Taiwan (34.0-68.2%) (Shieh, 1996) . Similarly, the major component of Eucalyptus camaldulensis oils originating form Burundi, Morocco, and Benin was 1,8-cineole ranging from 31.0 to 72.5% with no presence of cryptone (Dethier et al., 1994; Zrira and Benjilali, 1991; Zrira et al., 1992; Moudachirou et al., 1999) . Cryptone has been shown to be present in low-cineole varieties of EOs from Australia (Bignell et al., 1996) , Uruguay (Dellacassa et al., 1990) and South Florida (Pappas and Sheppard-Hanger, 2000) . For example, the major constituents identified in the essential oil from South Florida included p-cymene (35.0%), cryptone (13.7%), terpinen-4-ol (5.7%), spathulenol (4.3%), and cuminaldehyde (3.7%), with a very low amount of 1,8-cineole (2.7%) (Pappas and Sheppard-Hanger, 2000) . It was proven that essential oils of different plant parts have different chemical composition (Table 3) . Previous studies on the essential oil of E. camaldulensis flowers revealed the presence of 1,8-cineole, βpinene, and spathulenol as the most abundant constituents (Giamakis et al., 2001) . The essential oil of the leaves was found to contain pcymene, γ-terpinene, α-pinene, 1,8-cineole, terpinen-4-ol, α-terpineol, carvacrol, and thymol as the major components (Siramon and Ohtani, 2007) . The major components of the fruits essential oil were aromadendrene, α-pinene, drimenol, and cubenol (El-Ghorab et al., 2002) .

Plant developmental stage and maturity of plant parts used for essential oil extraction also affect essential oil chemical composition. Giamakis et al. (2001) analyzed the immature flowers and calli grown in Athens (Greece), and they found that the main monoterpenes produced in E. camaldulensis calli, cultured in darkness and under light conditions, were 1,8-cineole, 62.70 and 69.26% as well as β-pinene, 27.09 and 25.31%, respectively. In lower amounts α-pinene (1.2 and 1.1%, respectively) and the terpenoids, camphene, myrcene, isocineole, myrtenol, bicyclogermacrene, spathulenol, and trans-pinocarveol were present (less than 1%). Apart from isocineole, these constituents were also determined in the essential oil from immature flowers. This is remarkable because Giamakis et al. (2001) showed that undifferentiated calli are capable of producing high amounts of monoterpenoid compounds (approx. one third of that produced by the explant). This makes immature flowers from E. camaldulensis an interesting candidate for the development of calli able to produce high percentages of these two important monoterpenes, 1,8-cineole and β-pinene (Giamakis et al., 2001) . Also, the influence of light conditions on essential oil chemical composition showed that light conditions did not considerably affect the production of monoterpenes and sesquiterpenes compounds by calli developed from immature flowers (Giamakis et al., 2001) .

Furthermore, plant culture conditions can influence the essential oil chemical composition. Soil salinity is a key ecological stress that severely influences plant productivity (Williams, 2011) . Ashraf et al. (2010) showed that salinity had a considerable effect on the percent compositions of some components of E. camaldulensis leaves essential oil. The mean values of 1,8-cineole content of EO from saline and nonsaline provenances of Pakistan were 34.42 and 40.05%, respectively (Ashraf et al., 2010) . Therefore, they recommend stressing of this species by growing in the saline areas to enhance essential oil production for its various medicinal and pharmacological uses.

Extraction represents the primary step in obtaining the crude mixture of compounds from plants. Quality and quantity of the extracts dependent of the target compound structures, natural sources, and type of processes (Karacabey et al., 2013) , explaining the different phenolic composition in the extracts obtained with different procedures. The most commonly plant extract have been obtained by conventional solvent extraction methods (infusion, decoction, digestion, maceration, and percolation) (Azwanida, 2015) using solvents such as water, ethanol, methanol, chloroform, dimethyl-sulfoxide etc. However, these techniques are demanding regarding the extraction process duration, organic solvent consumption, and lack of extraction automation. The potential and powerful alternative to conventional liquid solvent extraction methods are Ultrasonic-Assisted Extraction (UAE) and Microwave-Assisted Extraction (MAE), especially in the case of plant material (Hao et al., 2002; Eskilsson and Bjrklund, 2000) . Interest in MAE has increased significantly over the past 5-10 years in particular medicinal plant research, as a result of its inherent advantages as special heating mechanism, moderate capital cost, and its good performance under atmospheric conditions (Ballard et al., 2010; Chan et al., 2011) . In addition, MAE technique possesses many advantages compared with other methods for the extraction of compounds such as bioactive compounds (Sanchez-Aldana et al., 2013) saving in processing time and solvent, higher extraction rate, better products with lower cost, reduced energy consumption (up to 85-fold savings), and waste generation (Yan et al., 2010; Yemis and Mazza, 2012) . This is confirmed for E. camaldulensis extraction of phenolic and flavonoid compounds, where the compounds were extracted with MAE for 5 min which was equivalent with UAE (60 min) and traditional extraction (24 h) methods (Gharekhani et al., 2012) .

Many authors reported the chemical composition of E. camaldulensis extracts. The leaves of E. camaldulensis from the zoo-botanical garden in Giza (Egypt) yielded 4 major fractions (Singab et al., 2011) . The major components of the first fraction (eluted with water) were identified as HHDP-glucopyranose, chlorogenic acid, and phloroglucinol derivatives. The second 30% methanol fraction was found to contain different galloyl-HHDP-glucopyranose positional isomers and pedunculagin as major components. The third 60% methanol fraction was predominantly composed of digalloyl-HHDP-glucopyranose (tellimagrandin I) α and β anomers, while the last 100% methanol fraction was composed of a mixture of ellagitannin dimers. The profiling of the obtained fractions by HPLC-PDA-ESI/MS/MS indicated that ellagitannins were the most predominant components of all three methanol fractions (Singab et al., 2011) .

The secondary metabolites screening of E. camaldulensis leaf extracts from Nigeria confirmed presence of tannin, saponins, and cardiac glycosides (Ayepola and Adeniyi, 2008) . Analyzed n-hexane, chloroform, and methanol extracts of E. camaldulensis stem bark and leaf also grown in Nigeria showed the presence of tannins and saponins in the stem bark and in the leaf of E. camaldulensis with absence of alkaloids in all extracts (Adeniyi et al., 2009) . Furthermore, the crude methanol leaf extracts also from Nigeria contained in addition volatile oils and balsam (gum) (Babayi et al., 2004) . Similarly, the phytochemical screening of ethanol, methanol, and petroleum ether leaf extracts from Nigeria contained in moderate to high amount secondary metabolites: alkaloids, saponins, tannins, flavonoids, steroid, carbohydrates, and cardiac glycosides, and not anthraquinones (Chuku et al., 2016) . Crude methanol leaf extracts of E. camaldulensis from Iran had saponins, tannins, volatile oils, and balsam (gum), while the components such as anthraquinones, hydrolysable tannin, flavonoid, alkaloid, and glycosides were not detected (Jouki and Khazaei, 2010) ; crude methanolic leaves extract of E. camaldulensis from India contained anthraquinones, flavonoids, saponins, and terpenoids, while alkaloids, cardiac glycosides, and tannins were not detected (Singh and Thakur, 2016) . Phytochemical screening of the crude stem barks methanol extract of E. camaldulensis from Bangladesh indicated presence of saponins, flavonoids, tannins, and also volatile oils, while anthraquinones, hydrolysable tannins, alkaloids, and glycosides were not present (Islam et al., 2014) . The polyphenolic composition (flavonoids and phenolic acids and aldehydes) also was studied in the soluble fractions of the methanolic extracts of Eucalyptus camaldulensis originating from two Spain provinces, Huelva and Pontevedra: gallic, protocatechuic, vanillic and ellagic acids, and protocatechic aldehyde were identified, along with eriodictyol, quercetin, naringenin, vanillin, naringin, quercitrin, luteolin, and kaempferol (Cadahia et al., 1997) . Eucalyptus camaldulensis extracts are generally rich in tannins which vary qualitatively and quantitatively according to the origin of the samples, and consequently protoanthocyanidin levels were influenced by the geographical origin (Cadahia et al., 1997) .

Antimicrobial activity of E. camaldulensis EO and extracts are well documented against many microorganisms listed in the Table 4 . For easier comparison, all data commented here for EOs were re-calculated from microliter per milliliter or microgram/milligram per milliliter to percentage (v/v or w/v), using the equitations presented in Fig. 2 . We considered here only minimal inhibitory and minimal bactericidal concentrations, while results obtained using disc or agar diffusion methods were not discussed. However, MIC/MBC results vary between several micrograms to several milligrams. Such high variation does not seem as real, even taking into account variation in oil composition, and are rather a consequence of erroneous equalization of one microliter and one microgram, or typographical errors. For instance, there are some nonsense MICs, such as 2000 μL/mL, that is practically impossible to obtain (Salem et al., 2015) . In some manuscripts even a species was not precisely indicated (Harkenthal et al., 1999; Karpanen et al., 2008; Warnke et al., 2009; Tadtong et al., 2016) and these results were not considered in the present review.

Eucalyptus camaldulensis plant extracts and EOs were tested against wide range of bacteria. The most frequently included Gram positive bacterium in screening is S. aureus. Minimal inhibitory concentrations in most studies were in range 0.07-0.5% indicating moderately high activity against this bacterium. Beside S. aureus, antibacterial effect was confirmed against B. subtilis (0.17-0.34%), M. luteus (0.2-0.4%), and S. pyogenes (0.4-1.1%) Akin et al., 2010; Knezevic et al., 2016; Khubeiz et al., 2016; Reda et al., 2017; Ostad Asiaei et al., 2018) . Activity of EOs was examined aginst L. monocytogenes in only one study and MIC was not obtained with the highest used concentration of 1.0%, and the same was reported for Enterococcus durans (Akin et al., 2010) .

Activity against Gram negative bacteria has been documented in a greater extent and MICs for the most frequently used model organism E. coli was in range 0.15-3.2%. Sensitivity of other enterobacteria is similar, with MICs in range from 0.05 to 0.32% for K. pneumoniae (Khubeiz et al., 2016; Ostad Asiaei et al., 2018) , 0.16-0.32% for Salmonella enterica subsp. enterica serovar Typhimurium (Khubeiz et al., 2016) ; 0.35-0.4% for serovars Typhi, Paratyphi, and 0.6% for S. Enteritis (Ostad Asiaei et al., 2018). Other enterobacteria are similarily sensitive: Shigella soneii was inhibited with 0.3% of EO (Ostad Asiaei et al., 2018) , while Proteus vilgaris was sensitive in a broad range from 0.25% (Ostad Asiaei et al., 2018) to more than 1.28% (Khubeiz et al., 2016) . The published data for P. aeruginosa are in the same range as for P. vulgaris, which is not surprising, as both species are generally highly resistant to antimicrobial agents. On the contrary, A. baumannii which is also known to be multi-drug resistant, showed sensitivity to EO in a range 0.05-0.1% . The lowest MIC among Gram negative bacteria was recorded for a gastrointestinal pathogen V. parachaemoliticus (0.01%) (Khubeiz et al., 2016) .

Although the Gram positive bacteria are consider more sensitive to EOs in comparison to Gram negative, it cannot be applied for essential oils of E. camaldulensis, since the most sensitive bacteria are Gram negative -A. baumannii and V. parahaemolyticus. The minimal bactericidal concentrations for most bacteria are equal to, or rarely up to 4 times greater than MICs. This low range in MIC/MBC < 4 indicates that E. camaldulensis EOs act as bactericidal agents (Pankey and Sabath, 2004) .

Antibacterial effects of E. camaldulensis extracts have been examined in a wider extent than essential oils. Inhibitory concentrations varied depending on extraction method, plant properties, and model organism, being in broad range from 0.08 μg/mL to 200 mg/mL. Crude aqueous leaf extracts show lower activity against various bacteria (range 25-50 mg/mL) (Abubakar, 2010) , while bark aqueous extracts were more effective, with MIC from 0.1 mg/mL for Propionobacterium acnes to 4.0 mg/mL for P. aeruginosa (Mabona et al., 2013) . The antimicrobial activity of methanol, ethanol or petroleum leaf extracts of E. camaldulensis showed significant variation; for instance, MICs against B. subtilis varied from 0.04 up to 200 mg/mL or against S. aureus 1.25-25 mg/mL (Chuku et al., 2016; Ayepola and Adeniyi, 2008) . For most examined Gram negative bacteria MICs were in range 10-200 mg/mL, while P. aeruginosa was even more susceptible (MICs 10-100 mg/mL). Similar or better antibacterial effect showed acetone leaf extract, being active against examined bacteria in range 15-50 mg/mL. The highest activity was obtained with dichloromethen extracts, with MICs for Staphylococcus spp. in range 0.25-1.0 mg/mL, and even lower for B. subtilis, P. acnes and B. agri (MICs 0.10-0.79 mg/mL). Interestingly, generally highly resistant M. tuberculosis was sensitive to methanol, n-hexane or chloroform extract with MICs in range 0.004-0.064 mg/mL, while M. bovis was slightly less sensitive with MICs 0.01-0.05 mg/mL (Lawal et al., 2012; Gemechu et al., 2013) .

The progresses made on the investigation of essential oils mode of action, especially against bacterial cell targets, gave new perspectives in this combat. The knowledge about the essential oils and their target(s) on bacterial cell is crucial to understand which parts of bacterial cell are affected. The essential oils antibacterial action is linked to oil hydrophobicity that increases cell permeability and consequent leakage of cell constituents (Dorman and Deans, 2000; Lambert et al., 2001; Helander et al., 1998; Turgis et al., 2009; Ultee et al., 2002; Faleiro, 2011) . It is important to perceive that a disturbed cell structures may affect stability of other cellular structures in a cascade type of action (Carson et al., 2002) . Essential oils have several target sites on bacterial cells; however it seems that all are directly or indirectly connected to the primary effect of essential oils on the bacterial envelopes. First of all, EOs may cause cell wall and membrane disturbance (Lambert et al., 2001; Oussalah et al., 2006) , which further can lead to the significant loss of intracellular ATP (Oussalah et al., 2006; Turgis et al., 2009) , induction the synthesis of heat shock proteins (Burt et al., 2007) , pH disturbance (Turgis et al., 2009; Oussalah et al., 2006) , and intracytoplasmic changes (e.g. coagulation, periplasmic space enlargement) (Becerril et al., 2007) . Although the number of studies dealing with the EO mechanisms of action is increasing, there are still many questions to be answered before the precise mechanism is revealed. This is at the same time one of the main limitations for EOs and extracts wide usage as antimicrobials. Thus, the so far knowledge must be further improved enabling the combat bacterial pathogens and its resistance. Contemporary research of EOs and extracts are also focused on mechanism that allows bacteria to regulate some physiological activities, such as virulence, competition amongst populations, motility, sporulation, conjugation, antibiotic production, and biofilm formation (Rodriguez-Garcia et al., 2014) . This system of intercellular communication, quorum sensing (QS), is based on production of the signal molecules, called autoinducers (Abraham et al., 2011) . This communication system is relative to cell density and some compounds interfere with the QS communication system and attenuating the bacterial pathogenicity, in the phenomenon known as anti-QS compounds (Abraham et al., 2011) . There are reports of anti-QS activity of various plants species EOs from genus Eucalyptus, such as Eucalyptus globulus L. (Luís et al., 2016; Cervantes-Ceballos et al., 2015) , Eucalyptus radiata D. (Luís et al., 2016) , Eucalyptus citriodora Hook., Eucalyptus smithii R. T. Baker and Eucalyptus staigeriana F. Muell. ex Bailey (Luís et al., 2016) . Unfortunately, there is still no data regarding E. camaldulensis anti-QS activity.

In one study, antibacterial and in vivo biofilm preventive efficacies of E. camaldulensis oil were significantly higher than that of M. spicata oil and chlorhexidine, suggesting that E. camaldulensis EO is capable of Fig. 2 . Schematic diagram for re-calculation from microliter or milligram per milliliter to percentage (v/v or w/v). affecting biofilm formation . Considering the anti-QS activity of above mentioned Eucalyptus species and also detected antimicrobial and anti-biofilm effect of E. camaldulensis it can be assumed that it possess similar or even higher anti-QS activity. However, future detailed studies of anti-QS and anti-biofilm E. camaldulensis activity are needed in order to confirm this assumption.

The E. camaldulensis leaves extracts and EOs have a potential as antifungal agents. They are able to act as a moderate antifungal agent against household molds, wood rot fungi (Siramon et al., 2013) , and phytopathogenic fungi (Gakuubi et al., 2017; Mehani et al., 2014) .

Eucalyptus camaldulensis EO has been studied for antifungal activity and was active in concentration 0.125-1.0% against most model organisms. The most sensitive seems to be F. sporotrichioides with MIC 0.125% (Mehani et al., 2014) , while R. oryzae is the most resistant, as 1% of EO was ineffective against this fungus (Siramon et al., 2013) . The best known human and animal pathogenic yeast Candida sp. showed considerable sensitivity to E. camaldulensis EO, with MIC approx. 0.5% (Siramon et al., 2013; Nasir et al., 2015) . Most EOs from Sardinia inhibited growth of A. niger and B. cinerea in concentration of 20 μL/plate, while lower concentrations of the oils, such as 5 μL/plate were ineffective (Barra et al., 2010) .

It is interesting to notice that liposomes containing E. camaldulensis EO were prepared (Moghimipour et al., 2012) for antifungal oil activity. The particle size varied from 40.5 to 298 nm for the different formulations, with approx. 95% of the essential oil entrapped. Inhibition of Microsporum canis, M. gypseum, Trichophyton rubrum, and T. verrucosum was achieved with 125 μL, and the liposomal gel formulation of the EO was proposed to improve antifungal activity.

Aqueous and organic extracts of E. camaldulensis have been reported to have antifungal activity (Table 4 ). Methanol leaves extracts showed inconsistent activity against C. albicans: in one study it was in range 50-200 mg/mL (Chuku et al., 2016) , while in another it was 0.2 mg/mL (Babayi et al., 2004) , indicating difference in active concentration of one thousand times. The bark methanole extract was active in concentration 0.5 mg/mL. Methanol leaf and bark extracts showed considerable activity against dermatophytes: 0.8-1.6 mg/mL against Microsporum spp, 0.125-1.6 mg/mL against Trichophyton spp., and 0.2 mg/mL against Epidemophyton flocossum (Takahashi et al., 2004; Falahati et al., 2005; Mabona et al., 2013) .

Beside the antifungal activity of EOs and extracts of E. camaldulensis, it is worth noticing that this species, along with E. blakelyi M., E. gomphocephala A. DC, E. rudis Endl., and E. tereticornis Sm., is a reservoir of an emerging pathogenic fungus -Cryptococcus gattii. This fungus is usually related to tropic and subtropic regions, affecting the respiratory and nervous systems of the immunocompetent humans and domestic animals (Sorrell et al., 1996; Chakrabarti et al., 1997; Bielska and May, 2016; Roe et al., 2018) . Similarily, C. neoformans var. grubii can be isolated from flowers and bark of E. camaldulensis (Gugnani et al., 2005) . According to the literature, E. camaldulensis is natural reservoir of Cryptococcus and is considered responsible for occurring cryptococcosis worldwide. In this context, it is interesting to observe that C. neoformans is moderately sensitive to E. citriodora Hook EOs, with MIC90 0.5% (wt/v) (Pattnaik et al., 1996; Luqman et al., 2008) or E. globulus Labill. with MIC 0.13% (wt/v) (Suliman et al., 2010) . Unfortunately, data on Cryptococcus sp. sensitivity to EOs and plant extracts of reservoir species of Eucalyptus, including E. camaldulensis, remains unknown.

Although there are no reports regarding E. camaldulensis mode of antifungal action, according to previous studies with other essential oils and fungi, the plasma membrane and the mitochondria are the probable antifungal targets of EOs. According to recent studies, the antifungal activity of EOs results from its ability to disrupt the permeability barrier of the plasma membrane and from the mitochondrial dysfunction-induced ROS accumulation in A. flavus and C. albicans (Tian et al., 2012; Chen et al., 2013) . The exact mechanism of E. camaldulensis antifungal activity is unknown and should be elucidated in the future.

Infections caused by viruses are very common and sometimes life threatening, especially in immunocompromised patients and neonates (Snoeck, 2000; Khan et al., 2005) . Despite the recent significant progress in antiviral drug development, viral infections are considered as one of the major causes of death worldwide (Müller et al., 2007; Meyers et al., 1982) . Thus, novel natural antiviral agents need to be found urgently. Many natural products possess antiviral activity and some of them are already in use for treatment of human viral infections with both RNA and DNA viruses (e.g. myricetin against coronavirus, linalool, urosolic acid, and apigenin against coxsackievirus, quercetin and narasin against denge virus, curcumin against hepatitis B and C virus) (Lin et al., 2014; Kitazato et al., 2007) . Numerous secondary plant metabolites such as essential oils, flavonoids, saponins, tannins, alkaloids, lignans, terpenes, and phenolic acids express significant antiviral activity against different viruses (Jassim and Naji, 2003; Chiang et al., 2003; Sanchez Palomino et al., 2002) . Recently few studies confirmed the E. camaldulensis EOs and plant extracts antiviral activity (Table 4) .

Eucalyptus camaldulensis EOs reduce coxackie B4 and rotavirus Wa multiplication for 50%, herpes simplex virus 1 for 90%, but have no effect on adenovirus 7 multiplication (El-Baz et al., 2015) . Similarly, methanolic extracts showed 50% inhibition of HSV 1 and 2 in concentration 0.1-0.3 μg/mL, and against Varicella zoster virus at concentration 1.0 μL/ml (Abu-Jafar and Huleihel, 2017). Dimethyl sulfoxide (DMSO) extracts inhbit multiplication of animal New Castle virus in concentration range 50-500 μg/mL (Al-Hadid, 2016) . Antiviral activity has been observed for E. camaldulensis methanolic extracts against polio virus, coxsackie B, and echovirus 6 (Adeniyi et al., 2015) . The data on antiviral activity of E. camaldulensis EO and extracts, although scarce, indicate their great potential, and necessity for further studies in this context.

Despite the fact that many plant extracts and essential oils were previously reported for their antiviral activities, the mechanism of action still remains poorly understood. There are many factors that influencing the EOs mode of action, which should be taken in consideration when antiviral activity of plant antimicrobials is examined. One of such factors is the difference between enveloped and non-enveloped viruses, because the observed antiviral effect has usually been greater for enveloped viruses (Yamada et al., 2009; Siddiqui et al., 1996) . In majority of the studies dealing with antiviral mode of action, the focus has been on either the inhibition of viral adsorption to host cells or examination of the plant antimicrobials effectiveness against intracellular virus multiplication (Gilling et al., 2014) . So, most commonly described modes of antiviral actions are virus inactivation and the impaired virus adsorption to host cells, which is often difficult to distinguish. Like in other antimicrobial modes of action, antiviral mechanism is also dependent on EOs or extracts compounds activity. This is one more factor that should be considered, because it affects the final antiviral mechanism of action. For some compounds the potential mechanism is reported, i.e. carvacrol acts directly upon the virus capsid and subsequently the nucleic acid (Gilling et al., 2014) . When EOs antiviral mechanism of action is considered, EOs may cause the loss of the viral capsid integrity, ultimately leading to exposure of the viral genome. In addition, some EOs subsequently act directly upon the viral nucleic acid (DNA or RNA). According to one study, E. camaldulensis EOs may be promising antiviral agent against RNA viruses with no effect against DNA virus (El-Baz et al., 2015) . Also, with shorter periods of exposure to the antimicrobial, the virus is able to adsorb specifically to host cells; however, it may or may not be able to cause successful infection depending upon the integrity of the viral genome. On the other hand, after exposure to some EOs virus capsid and genome remains intact. These antimicrobials appear to exert their antiviral effect by coating the capsid and thereby preventing specific adsorption of the virus to host cells (Gilling et al., 2014) . Although, there are still no reports regarding E. camaldulensis EOs antiviral mode of action, there are some promising results about its antiviral effect (Table 4) . These promising results represent foundation for further research and potential application of E. camalulensis EOs as potent antiviral agent.

Among all deverse benefitial activities, E. camaldulensis expresess also an antiprotozoal effect as well. The reports regarding this effect are listed in Table 4 . Eucalyptus camaldulensis methanolic and aqueous extracts were active against Leishmania major with IC50 values (50% inhibitory concentration) 586.2 ± 47.6 and 1108.6 ± 51.9 μg/mL, respectively (Nosratabadi et al., 2015) . This was characterized as moderate leishmanicidal activity, but considering fact that present therapy consist antimony compounds which are expensive, toxic, and drug resistance is prevalent, E. camaldulensis plant extract derivatives represent safe, inexpensive, and promising alterantive solution.

There are also several reports regarding antiprotozoal activity against Trichomonas vaginalis, a causative agent of trichomoniasis which is the most prevalent nonviral sexual infection. The first report on E. camaldulensis aqueous extract anti-trichomonas activity showed that extract was active at concentration 500 mg/mL after 24 h (Mahdi et al., 2006) . However, recent studies reported better effect of the E. camaladulensis extracts (Hassani et al., 2013; Youse et al., 2012) . Five different E. camaldulensis leaves extracts including total extract, diethyl ether, chloroform, ethyl acetate, and water fractions were active in concentration range 12.5-50 mg/mL, with growth inhibiton achived after 24-72 h (Hassani et al., 2013) . Ethyl acetate fraction showed the highest percentage of growth inhibition with the lowest concentration (12.5 mg/mL) after 24 and 48 h (Hassani et al., 2013) . Even better antitrichomonas activity was detedted for wather and ethanolic extracts, where concentration 60-90 μg/mL killed Trichomonas vaginalis for 72 h (Youse et al., 2012) . The mainstay medication for trichomoniasis is metronidazole; however some resistant strains to this treatment have been detected making these results of E. camaldulensis anti-trichomonas activity very promising as antiprotozoal agent.

Except extracts, significant antiprotozoal activity was reported for E. camaldulensis EOs. The essential oils were found to possess antitrypanosomal activity in vitro in a dose-dependent manner in a short time. The decrease of Trypanosoma evansi number over time was achieved in doses of 400 mg/mL for 3 min, 200 mg/mL for 4 min, and 100 mg/mL for 15 min. Against Trypanosoma brucei brucei EOs was more potent in the concentration of 400 mg/mL, decreasing the number of parasite for 3 min, 200 mg/mL for 4 min, and 100 mg/mL for 11 min (Habila et al., 2010) . Such prompt decrease in parasite number in the in vitro tests for both Trypanosoma brucei brucei and T. evansisuggests that the EOs kill the parasites efficiently, but by an unknown mechanism. There are suggestions of some potential mechanisms of antiprotozoal EOs action. The activity of EOs could be due to the hydrophobic nature of the cyclic hydrocarbons, which allow EOs to interact with the protozoans causing conformational changes in the parasite membrane structure, resulting in the loss of membrane stability (Calsamiglia et al., 2007) . The essential oils also can act by inhibiting some key enzymes in the parasite glycolytic pathway (Smith-Palmer et al., 2004) . Furthermore, some EOs components inhibit acetylcholinesterase activity and act on other vulnerable sites, such as cytochrome p450 (Maciel et al., 2010) . This multicomponent nature of plant EOs is an advantage for several target sites on protozoans, which is of great importance.

It was shown that EOs of E. camaldulensis are more potent against B. subtilis, S. aureus, E. coli, P. aeruginosa, S. lutea, and P. carotovorum in comparison to EOs from E. gomphocephala DC., but not against A. tumefaciens. Furthermore, the E. camaldulensis var. obtuse showed even better effect than E. camaldulensis (Salem et al., 2015) . Among seven examine essential oils of various Eucalyptus species, EO of E. camaldulensis was among the best against B. subtilis, A. niger, and R. solani, but not against E. coli and S. aureus (Ghaffar et al., 2015) . Similar activity of E. camaldulensis and E. torelliana F. Muell. was recorded for extracts against six strains of H. pylori (Adeniyi et al., 2009) .

It is worth to notice that among 132 extracts from 42 plants growing in southern Africa, E. camaldulensis bark extract showed considerable activity against bacteria and fungi, with an exception against P. aeruginosa and M. canis (Mabona et al., 2013) . Finaly, in many studies regarding antimicrobial activity of various Eucalyptus species, E. camaldulensis was not always included (Ashour, 2008; Safaei-Ghomi and Ahd, 2010; Mulyaningsih et al., 2010; Elaissi et al., 2012; Sebei et al., 2015) .

Due to rapid emerging of microbial resistance to conventional drugs, the necessity for efficient solution(s) is rising. The main strategy represents finding new antimicrobial agents; however another strategy goes in the direction of reducing degree of bacterial resistance and/or bacterial re-sensitization to conventional antibiotics. This can be achieved using combined therapies. The most of the tested combinations are dual combinations, but the combinations of three, four or more agents also can be efficient (Lesjak et al., 2016) . This is in line with antimicrobial efficiency and potential of EOs and plant extracts which are complex mixtures of numerous compounds. Combination strategy could be very promising regarding the diversity of agents that could be tested: (1) conventional non-antimicrobial agent (e.g. anti-inflammatory or anti-psychotic drug) + conventional antimicrobial agent (antibiotic); (2) conventional antimicrobial agent (antibiotic) + natural antimicrobial agent (essential oil, plant extract, bacteriophage, antimicrobial peptide ect.); (3) conventional antimicrobial agent (antibiotic) + single compound isolated from natural antimicrobial agent (with previously confirmed antimicrobial activity); (4) combination of two or more single compounds isolated from natural antimicrobial agents (with previously confirmed antimicrobial activity).

Being a plant with already detected and evaluated antimicrobial activity, it can be assumed that E. camaldulensis also have a potential in combined therapy. This assumption is confirmed in some studies in vitro and in vivo (Table 5) . Eucalyptus camaldulensis EOs and extracts in vitro reduced resistance of MDR A. baumannii in combination with conventional antibiotics: β-lactams, ciprofloxacin, gentamicin, polymyxin B . Similarily, the increase of β-lactamase produing MRSA and E. coli sensitivity to cephalexin, cefuroxime, amoxicillin, and ampicillin has been obtained through combination with E. camaldulensis EOs (Chaves et al., 2018) . Synergistic activity has been recorded between E. camaldulensis plant extracts and gentamicin or ceftriaxone against MDR S. aureus and P. aeruginosa (Reda et al., 2017; Ibrahim et al., 2014) , as well as in combination with ampicillin against S. aureus (Ibrahim et al., 2014) .

Furthermore, the combination of E. camaldulensis extract with another plant extract Psidium guajava L. was also efficient against MDR bacteria (Bala et al., 2014) . Except antibacterial activity, other activities of E. camaldulensis extracts in combination were detected and characterized as efficient. Antiviral activity was confirmed for the combination of E. camaldulensis 80% methanol leaves extract and acyclovir against herpes simplex virus -1 and -2 and varicella-zoster virus (Abu-Jafar and Huleihel, 2017). Similaily, the combination of Annona senegalensis L. leaf methanol extract and E. camaldulensis extract efficiently cured in vivo albino mice infection with parasite Trypanosoma brucei brucei (Lafia strain) (Kabiru et al., 2012) . All these data are very promising, but the methods used in different studies and results interpretation vary (Table 5) . As a consequence, the data should be taken with precautions, as can not be compared and properly discussed. To avoid this problem, the testing combinations of different agents should be conducted using standardized methods, such as time-kill method (CLSI, 1999; Verma, 2007) , checkerboard method (Verma, 2007; EUCAST, 2000) , Chou-Talalay method (Chou, 2010) or Boik method (Boik, 2010) . All these methods possess some shortfalls, such as timeconsuming, labor-intensive, limitations regarding the number of the agents in combination, etc. Unfortunately, there is no one gold standard for synergy testing and prior the further application of different phytochemicals, this issue should be overcome.

Summarizing the available data on antimicrobial properties of Eucalyptus camaldulensis essential oil and extracts, it is obvious that this plant is a valuable source of phytotherapeutics. The essential oil, as well as leaf and bark extracts are particularly valuable as antibacterial, antiviral, and antifungal agents, and their antiprotozoal activity should not be neglected taking into account current therapy cost, toxicity, and protozoal growing resistance. Some E. camaldulensis plant characteristics such as easy cultivation, wide distribution by plantation, and rapid growth additionally support further examination of antimicrobial activity, in order to enhance the commercial production of E. camaldulensis based pharmaceuticals. The future studies should be focused on determination of the mechanisms of antimicrobial activity, paticularly potential anti-biofilm and anti-QS effects, as well as the activity enhancement in combination with other available agents.

Effects of Eucalyptus camaldulensis and other antimicrobial agents in combination. Two-dimensional checkerboard method (Verma, 2007) Synergism (FICI ≤ 0.5) Knezevic et al. (2016) Leaf essential oil from Brazil

The MIC of the antibiotics were determined in the presence and absence of sub-inhibitory concentrations (125 μg mL −1 ) of the essential oil (Coutinho et al., 2010) Combining the essential oil with β-lactams reduced the resistance of tested strains 

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Medicinal Plants of Bangladesh

Microwave-assisted extraction of phenolic and flavonoid compounds from Eucalyptus camaldulensis Dehn. leaves as compared with ultrasound-assisted extraction

Eucalyptus camaldulensis: volatiles from immature flowers and high production of 1,8-cineole and β-pinene by in vitro cultures

Ethnomedical Uses of Plants in Nigeria

Antiviral efficacy and mechanisms of action of oregano essential oil and its primary component carvacrol against murine norovirus

Isolation of Cryptococcus gattii and Cryptococcus neoformans var. grubii from the flowers and bark of Eucalyptus trees in India

Evaluation of in vitro activity of essential oils against trypanosoma brucei brucei and Trypanosoma evansi

Microwave assisted extraction of artemisinin from Artemisia annua L

Chemical composition, antibacterial and antioxidant activities of essential oil of Eucalyptus globulus from Algeria

Comparative study on the in vitro antibacterial activity of Australian tea tree oil, cajuput oil, niaouli oil, manuka oil, kanuka oil, and eucalyptus oil

Effects of different extracts of Eucalyptus camaldulensis on Trichomonas vaginalis parasite in culture medium

Characterization of the action of selected essential oil components on gram-negative bacteria

Myrtus communis and Eucalyptus camaldulensis cytotoxicity on breast cancer cells

Investigation of the anti-melanogenic and antioxidant characteristics of Eucalyptus camaldulensis flower essential oil and determination of its chemical composition

Study of the synergistic effect of antibiotics and plant extracts against clinical Staphylococcus aureus strains

Antiviral drug resistance as an adaptive process

Growth inhibition and apoptosis of Ehrlich ascites carcinoma cells by the methanol extract of Eucalyptus camaldulensis

Cypellocarpins A-C, phenol glycosides esterified with oleuropeic acid, from Eucalyptus cypellocarpa

Growth Habits of the Eucalypts

Eucalypts for Planting. Eucalypts for Planting

Novel antiviral agents: a medicinal plant perspective

Analgesic and anti-inflammatory effects of essential oils of Eucalyptus

Aroma profile of Eucalyptus globulus: collected from north west Karnataka

The antimicrobial activities of methanolic extracts of Eucalyptus camaldulensis against Bacillus subtilis, Staphylococcus aureus and Escherichia coli

Evaluation of the efficacy of combination therapy in T. b. brucei -infected mice using extracts of Annona senegalensis and Eucalyptus camaldulensis

Anti-trypanosomal potential of Eucalyptus camaldulensis

Modeling solid-liquid extraction kinetics of trans-resveratrol and trans-e-viniferin from grape cane

Antimicrobial efficacy of chlorhexidine digluconate alone and in combination with eucalyptus oil, tea tree oil and thymol against planktonic and biofilm cultures of Staphylococcus epidermidis

Evaluation of eucalyptus essential oil against some plant pathogenic fungi

Extracts and molecules from medicinal plants against herpes simplex viruses

Chemical compositions and antimicrobial activity of leaves Eucalyptus camaldulensis essential oils from four syrian samples

Antibiotc susceptibility testing; a standard single disc method

Viral infectious disease and natural products with antiviral activity

Antimicrobial activity of Eucalyptus camaldulensis essential oils and their interactions with conventional antimicrobial agents against multi-drug resistant Acinetobacter baumannii

Antimicrobial properties of different Eucalyptus oils

Antimicrobial activity of diosquinone and plumbagin from Diospyros mespiliformis (Hostch) (Ebenaceae)

A study of the minimum inhibitory concentration and mode of action of oregano essential oil, thymol and carvacrol

In vitro susceptibility of Mycobacterium tuberculosis to extracts of Eucalyptus camaldulensis and Eucalyptus torelliana and isolated compounds

Ulcer-healing promoting activities of methanol extracts of Eucalyptus camaldulensis Dehnh. and Eucalyptus torelliana F

Binary and tertiary mixtures of Satureja hortensis and Origanum vulgare essential oils as potent antimicrobial agents against Helicobacter pylori

Essential oils composition and antibacterial activities of Eucalyptus camaldulensis Dehn

Antiviral natural products and herbal medicines

Aromatherapy Science: A Guide for Healthcare Professionals

Chemical composition, antioxidant, antibacterial and anti-quorum sensing activities of Eucalyptus globulus and Eucalyptus radiata essential oils

Antimicrobial activity of Eucalyptus citriodora essential oil

Antimicrobial activity of South African medicinal plants with dermatological relevance: from a screening approach, to combination studies and bioactive isolation

Chemical composition of Eucalyptus spp. essential oils and their insecticidal effects on Lutzomyia longipalpi

Alternative drugs against Trichomonas vaginalis

Traditional use of medicinal plants in south-central Zimbabwe: review and perspectives

Phytochemistry and larvicidal activity of Eucalyptus camaldulensis against malaria vector, Anopheles stephensi. Asian Pac

Antifungal effect of essential oil Eucalyptus camaldulensis plant on Fusarium graminearum and Fusarium sporotrichioide

Multicenter collaborative trial of intravenous acyclovir for treatment of mucocutaneous herpes simplex virus infection in the immunocompromised host

Preparation and characterization of liposomes containing essential oil of Eucalyptus camaldulensis leaf

Chemical composition of essential oils of Eucalyptus from Benin: Eucalyptus citriodora and E. camaldulensis. Influence of location, harvest time, storage of plants and time of steam distillation

Antibiograms of faecal Escherichia coli and Enterococci species isolated from pastoralist cattle in the interface areas of the Kafue basin in Zambia

Evaluation of antiviral activity of South American plant extracts against herpes simplex virus type 1 and rabies virus

Synergistic properties of the terpenoids aromadendrene and 1,8-cineole from the essential oil of Eucalyptus globulus against antibiotic-susceptible and antibiotic-resistant pathogens

Firewood Crops. Shrub and Tree Species for Energy Production

Antimicrobial potential of the Ethiopian Thymus schimperi essential oil in comparison with others against certain fungal and bacterial species

In vitro antileishmanial activity of methanolic and aqueous extracts of Eucalyptus camaldulensis against Leishmania major

Eucalyptus camaldulensis var. nancy and Eucalyptus camaldulensis var. petford seed essential oils: phytochemicals and therapeutic potentials

Evaluation of antimicrobial activity of Eucalyptus camaldulensis essential oil against the growth of drug-resistant bacteria

Antimicrobial effects of selected plant essential oils on the growth of a Pseudomonas putida strain isolated from meat

Antimicrobial characteristics of some herbal oils on Pseudomonas aeruginosa with special reference to their chemical compositions

Antimicrobial activity of essential oils of five Eucalyptus species growing in Nigeria

Clinical relevance of bacteriostatic versus bactericidal mechanisms of action in the treatment of gram-positive bacterial infections

The essential oil of Eucalyptus camaldulensis Dehn. from South Florida: a high cryptone/low cineole Eucalyptus

Antibacterial and antifungal activity of tan essential oils in vitro

Essential oil composition of Eucalyptus camalddunensis

Uses and Abuses of Plant-Derived Smoke: Its Ethnobotany as Hallucinogen, Perfume, Incense and Medicine

Antimicrobial activity of Eucalyptus camaldulensis and Calotropis gigantea against various pathogens

The effect of Mentha spicata and Eucalyptus camaldulensis essential oils on dental biofilm

Synergistic effect of combined antibiotic and methanol extract of Eucalyptus camaldulensis leaf against Staphylococcus aureus and Pseudomonas aeruginosa

Molecular evolution of antifungal drug resistance

GAP, an aequorin-based fluorescent indicator for imaging Ca2+ in organelles

Dating the Cryptococcus gattii dispersal to the North American Pacific Northwest

Antimicrobial and antifungal properties of the essential oil and methanol extracts of Eucalyptus largiflorens and Eucalyptus intertexta

Chemotyping of diverse Eucalyptus species grown in Egypt and antioxidant and antibacterial activities of its respective essential oils

Screening of South American plants against Human Immunodeficiency Virus: preliminary fractionation of aqueous extract from Baccharis trinervis

Comparative extraction of pectic and polyphenols from Mexican lime pomace and bagasse

Chemical composition and antibacterial activities of seven Eucalyptus species essential oils leaves

Studies on the antidandruff activity of the essential oil of Coleus amboinicus and Eucalyptus globulus

Yields and chemical components of essential oils in Eucalyptus camaldulensis leaves

Effect of essential oils on the enveloped viruses: antiviral activity of oregano and clove oils on herpes simplex virus type 1 and Newcastle disease virus

Phenolic constituents of Eucalyptus camaldulensis Dehnh, with potential antioxidant and cytotoxic activities

Antibacterial activity of Eucalyptus camaldulensis against bacteria causing food-borne illness

Antioxidative and antiradical activities of Eucalyptus camaldulensis leaf oils from Thailand

Chemical composition and antifungal property of Eucalyptus camaldulensis leaf oils from Thailand

River Red Gum. Eucalyptus camaldulensis var. obtusa. Centre for Plant Biodiversity Research

Influence of subinhibitory concentrations of plant essential oils on the production of enterotoxins A and B and α-toxin by Staphylococcus aureus

Antiviral therapy of herpes simplex

Natural environmental sources of Cryptococcus neoformans var. gattii

Validating the in vitro antimicrobial activity of Artemisia afra in polyherbal combinations to treat respiratory infections

Antimicrobial constituents and effects of blnded Eucalyptus, rosemary, patchouli, pine, and cajuput essential oils

Antimicrobial activities of eucalyptus leaf extracts and flavonoids from Eucalyptus maculata

Phenylpropanoid glycosides from Lantana camara and Lippia multiflora

Effects of supplementation of Eucalyptus (E. camaldulensis) leaf meal on feed intake and rumen fermentation efficiency in swamp buffaloes

The Mechanism of antifungal action of essential oil from dill (Anethum graveolens L.) on Aspergillus flavus

Essential Oil Safety: A Guide for Health Care Professionals

Composition of fruit volatiles and annual changes in the volatiles of leaves of Eucalyptus camaldulensis Dehn. growing in Greece

Antimicrobial activity of mustard essential oil against Escherichia coli O157:H7 and Salmonella typhi

The phenolic hydroxyl group of carvacrol is essential for action against the food-borne pathogen Bacillus cereus

The antibiotic resistance crisis: part 1: causes and threats

Methods for determining bactericidal activity and antimicrobial interactions: synergy testing, time-kill curves, and population analysis

The battle against multi-resistant strains: renaissance of antimicrobial essential oils as a promising force to fight hospital-acquired infections

The Medicinal and Poisonous Plants of Southern and Eastern Africa

Medicinal Plants in Australia Volume 2: Gums, Resins, Tannin and Essential Oils. 2

Mechanism of the antiviral effect of hydroxytyrosol on influenza virus appears to involve morphological change of the virus

Optimisation of the microwave-assisted extraction process for four main astragalosides in Radix astragali

Optimization of furfural and 5-hydroxymethylfurfural production from wheat straw by a microwave-assisted process

Effect of Echinophora platyloba, Stachys lavandulifolia, and Eucalyptus camaldulensis plants on Trichomonas vaginalis growth in vitro

In-vitro sensitivity pattern of some urinary tract isolates to Carica papaya extracts

The essential oil of the leaves and the fruits of E. camaldulensis

Essential oils of twentyseven Eucalyptus species grown in Morocco