key: cord-0906418-6d6dwmq1
authors: Barengolts, Elena; Smith, Emily Daviau
title: Considerations for Gut Microbiota and Probiotics in Patients with Diabetes Amidst the Covid-19 Pandemic: A Narrative Review
date: 2020-12-28
journal: Endocr Pract
DOI: 10.4158/ep-2020-0336
sha: 230b28c81bc62b0f4e3977ca842cc59caef8e284
doc_id: 906418
cord_uid: 6d6dwmq1

Objective: To review data implicating microbiota influences on Coronavirus Disease 2019 (COVID-19) in patients with diabetes. Methods: Primary literature review included topics: “COVID-19,” “SARS,” “MERS,” “gut micro-biota,” “probiotics,” “immune system,” “ACE2,” and “metformin.” Results: Diabetes was prevalent (~11%) among COVID-19 patients and associated with increased mortality (about 3-fold) compared to patients without diabetes. COVID-19 could be associated with worsening diabetes control and new diabetes diagnosis that could be linked to high expression of angiotensin-converting enzyme 2 (ACE2) receptors (coronavirus point of entry into the host) in the endocrine pancreas. A pre-existing gut microbiota imbalance (dysbiosis) could contribute to COVID-19–related complications in patients with diabetes. The COVID-19 virus was found in fecal samples (~55%), persisted for about 5 weeks, and could be associated with diarrhea, suggesting a role for gut dysbiosis. ACE2 expressed on enterocytes and colonocytes could serve as an alternative route for acquiring COVID-19. Experimental models proposed some probiotics, including Lactobacillus casei, L. plantarum, and L. salivarius, as vectors for delivering or enhancing efficacy of anti-coronavirus vaccines. These Lactobacillus probiotics were also beneficial for diabetes. The potential mechanisms for interconnections between coronavirus, diabetes, and gut microbiota could be related to the immune system, ACE2 pathway, and metformin treatment. There were suggestions but no proof supporting probiotics benefits for COVID-19 infection. Conclusion: The data suggested that the host environment including the gut microbiota could play a role for COVID-19 in patients with diabetes. It is a challenge to the scientific community to investigate the beneficial potential of the gut microbiota for strengthening host defense against coronavirus in patients with diabetes.

Coronavirus Disease 2019 infection emerged at the end of 2019 and progressed to a pandemic (1) (2) (3) . The cause of COVID-19, a novel coronavirus (CoV)-2 zoonotic virus with predominant reservoir in bats, was found to have the largest known viral RNA genome among about 30 coronaviruses capable of infecting humans and animals (1) (2) (3) . Four human coronaviruses (OC43, 229E, NL63, and HKU1) instigated predominantly mild respiratory infections and, after rhinoviruses, were a leading cause of common colds (10 to 30% of cases) (1) (2) (3) . However, coronaviruses could cause severe and highly contagious disease, including severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and Coronavirus Disease 2019 (COVID-19) (1) (2) (3) (4) (5) . The genetic sequence of the COVID-19 coronavirus showed approximately 80% and 50% shared identity with SARS and MERS coronaviruses, respectively (1) (2) (3) (4) (5) .

The natural course of COVID-19 varies from the asymptomatic carrier to acute upper respiratory infection (ARI), pneumonia, acute respiratory distress syndrome, and multi-organ failure (1) (2) (3) (4) (5) . The reported mortality rate was greater in MERS and SARS than COVID-19; however, total death toll could be higher in COVID-19 due to the larger number of populations being affected by the COVID-19 pandemic. Severity of disease was positively associated with older age and the presence of comorbid conditions, including diabetes mellitus (DM) (1) (2) (3) (4) (5) (6) .

Gut bacteria (gut microbiota) implicated in diabetes and immune responses could be important for modulating the response to coronavirus via interactions with receptors used by coronaviruses as points of entry into the host, angiotensin-converting enzyme 2 (ACE2) for COVID- 19 and SARS and dipeptidyl peptidase 4 (DPP4) for MERS (7, 8) . The same receptors played a significant role in pathophysiology and treatment of diabetes (7, 8) . There were no randomized controlled trials (RCTs) investigating use of beneficial bacteria (probiotics) for COVID-19; however, valuable insights could be gained by evaluating data from other coronavirus infections. This review summarizes existing data on the interplay of COVID-19 with diabetes, microbiota, and probiotics and provides some insights into the possible role of the gut microbiota for coronavirus infection in patients with diabetes.

We searched manuscripts from PubMed, Science Direct, and Google Scholar. The terms and keywords included combinations of the following specific terminologies: "COVID-19," "SARS-CoV-2," "SARS-nCoV," "SARS-CoV," "MERS-CoV," "coronavirus," "diabetes mellitus," "type 2 diabetes (T2D)," "type 1 diabetes (T1D)," "gastrointestinal (GI) symptoms," "comorbidity," "mortality," "gut microbiota," "microbiome," "innate and adaptive immunity," "mechanisms," "vaccination," "probiotics," "prebiotics," and "synbiotics." Articles published in English between January 2002 and August 15, 2020, were retrieved and reviewed, as were relevant articles from the reference lists. The majority of studies for coronavirus did not differentiate between T2D and T1D; therefore, we used terms "diabetes," "T1D," and "T2D" as appropriate for available information. The method was consistent with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines (9) .

The published manuscripts were predominantly in the following topic areas: coronavirus epidemiology, clinical course, comorbidity and mortality; spontaneous coronavirus infection in animals; microbiota; use of probiotics; and the role of immune system, ACE2, and metformin in coronavirus infection. Below, we summarize the key findings from reviewed manuscripts, highlighting the findings that in our opinion were of the greatest importance for the field.

Diabetes is a highly prevalent disease throughout the world, with a particular predominance of over 90% T2D in the middle aged and elderly. In the world, approximately 463 million (9.3%) individuals have diabetes, the majority being older than 65 years (10). In the U.S., about 34 million people (10.5%) have T2D, with 27% in those 65 years or older (10) . Diabetes and older age are both risk factors for severe disease and increased mortality in patients with COVID-19 (Table 1) (1-6). The Centers for Disease Control of China reported that among 44,672 confirmed cases, the crude case fatality rate was 2.3% (1,023 deaths), while it was 14.8% in those age 80 years or older (2) . The crude case fatality rate for patients with no comorbidities was approximately 0.9%, whereas it was 10.5% for those with cardiovascular disease and 7.3% for those with diabetes (2). In the U.S., a Centers for Disease Control report estimated an 11% prevalence of diabetes among COVID-19 patients, while it reported that mortality was 6.2%, 28.8%, and 41.7% for patients without diabetes, with diabetes, and with uncontrolled hyperglycemia (blood glucose >180 mg/dL), respectively (3). In previous epidemics of SARS and MERS, diabetes was positively associated with risk for infection, development of complications, and higher mortality (Table 1) (4, 5) . There was evidence suggesting that infection could directly impact pancreatic endocrine function. Newly diagnosed diabetes was reported in COVID-19 and SARS (6, 11) . In addition, receptors for coronaviruses were identified in endocrine pancreas with expression of ACE2 in βand δ-cells (insulin-and somatostatin-producing cells, respectively) (6) and DPP4 in βand α-cells (glucagon producing) (12) . Overall, these data suggested that coronavirus-induced pathophysiologic changes and/or direct impairment of pancreatic islets could be contributing to adverse outcomes of coronavirus infection in patients with diabetes.

There could be multiple potential interactions of coronaviruses with gut microbiota, as shown by data from human and spontaneous animal infection and experimental studies (Fig. 1) . The coronaviruses could cause microbiota imbalance (dysbiosis), disrupt the gut barrier, and promote secondary bacterial invasion, whereas the microbiota could modify coronaviruses entry into the host (including lung) and modify the immune response to coronaviruses (13, 14) . In addition, alteration of the gut microbiota with antibiotics and/or other therapeutics could modify resistance against viral and bacterial pathogens as well as drug metabolism and pharmacokinetics (15) . Finally, beneficial microbiota (probiotics) from food and/or supplements could modify the gut microbiota and influence clinical course (16) .

Coronavirus was a common pathogen in stool specimens of 331 patients seen in general practice with an ARI (17) . In these patients, laboratory-confirmed compared to nonconfirmed coronavirus infection was associated with almost 3-fold increased GI symptoms, suggesting that coronavirus could be associated with dysbiosis in patients with common ARI (17) . Comparably, all three epidemic-related coronaviruses were identified in stool samples and were associated with similar GI symptoms (i.e., nausea, vomiting, and diarrhea), suggesting that the GI tract could serve as an alternative route for acquiring coronaviruses, including those causing COVID-19 (18) , SARS (19) , and MERS (14) ( Table 1 ). In a prospective study comparing COVID-19 patients with healthy controls and pneumonia controls, COVID-19 disease severity and fecal viral load correlated with gut microbiota changes, and dysbiosis persisted after patients' recovery (20) . The baseline (antibiotic-naïve) abundance of microbiota was increased for multiple opportunistic pathogens known to cause bacteremia. The top two bacteria correlating strongly positively with disease severity were Clostridium ramosum and Clostridium hathewayi. Conversely, the top two bacteria correlating strongly negatively with disease severity were Alistipes onderdonkii and Faecalibacterium prausnitzii, known as beneficial for immune homeostasis and anti-inflammatory properties (20) . The data suggested a potential role for gut microbiota in determining host response to COVID-19. A study comparing bronchoalveolar lavage fluid from 8 patients with COVID-19, 25 patients with community-acquired pneumonia (CAP), and 20 healthy controls showed that the microbiota in COVID-19 and CAP patients were similar and either dominated by pathogens or had elevated levels of oral and upper respiratory commensal bacteria (21) . In the upper respiratory tract (collected from oropharynx by using swabs), a comparison of microbiota from 57 healthy asymptomatic people with that from 59 patients acutely sick with corona and other viruses found that the healthy harbored primarily Streptococcus, whereas the acutely sick had an enrichment of Haemophilus or Moraxella (22) . The differences in microbiota were not associated with the virus type but were rather linked to patient age, with Moraxella nonliquefaciens exhibiting unprecedentedly high abundance in children younger than 6 years (22). 

The studies investigating spontaneous coronaviral diseases and gut microbiota in animals frequently interacting with people (cats, dogs, pigs, calves, and horses) showed similarities with human infection (Table 2 ) (13, (23) (24) (25) (26) (27) (28) . Coronavirus infection in animals varied from asymptomatic to severe and was predominantly asymptomatic in young horses (24) , similar to rare COVID-19 in children (29) . The alterations in intestinal histology as well as local intestinal and systemic immunology also resembled those reported in human infection (Table 2 ) (13,24). Severe gut dysbiosis was shown in multiple animal studies, with changes in microbiota occurring at each taxonomic level ( Table 2 ). In cats and pigs, coronavirus disease was associated with a low Bacteroidetes to Firmicutes ratio (23, 28) , similar to patients with diabetes (30) , suggesting that preexisting dysbiosis of diabetes could possibly contribute to COVID-19-related complications. Overall, the data of microbiota differences between healthy adults and patients with diabetes (30) , between adults and youngsters (22) , and those reported for coronaviruses (23, 24, 28, 29) suggested that coronavirus and microbiota interactions could potentially be contributing to variable coronavirus infectivity and clinical presentation in patients with diabetes.

There were no clinical trials investigating probiotic use in coronavirus infection in people (31, 32) . A cohort of 70 Italian patients hospitalized for COVID-19 was treated with similar standard therapy, and in addition, a group of randomly chosen patients was treated with a probiotic (33) . The probiotic and control groups were comparable for anthropometric and COVID-19-related factors. The probiotic compared to control group had fast improvement of symptoms, 8-fold lower estimated risk of developing respiratory failure, and all patients in this group survived (vs. ~9% mortality in controls) (33 (33) . Probiotics were suggested yet not proven as potentially helpful for ventilator-associated pneumonia (34) .

Probiotics were proposed as an empiric supplement for spontaneous coronavirus infection in domestic animals; however, no studies provided proof of their efficacy. There were several reviews of in vivo (15, 35) and in vitro (15) models of coronavirus infection. Particularly promising in vivo models appeared to be genetically engineered (using the CRISPR-Cas9 gene-editing tool) mouse models that could expedite preclinical development of candidate COVID-19 vaccines and drugs (15) . Ex vivo three-dimensional organoid systems, mini-tracheobronchial tree (15) and mini-gut (with morphological and functional properties of in vivo GI tract) (14) , and various cells lines (15) were used for facilitating mechanistic insights and therapeutics investigations. Probiotics were used in experimental studies predominantly for developing anti-coronavirus vaccines rather than direct treatment of coronavirus infection. Animal models and cell line experiments showed that some probiotics including strains of L. casei, L. plantarum, and L. salivarius could be helpful for developing anti-coronavirus vaccines and diabetes treatment (Table 3 ) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48) (49) (50) (51) . The mechanisms of these beneficial effects were dependent on specific strains and involved improved circulating glucose and glucose tolerance as well as potential beneficial effects on gut microbiota ecology, barrier function, and inflammation (Table 3) . Of interest, a majority of probiotics were found among Lactobacillus and Bifidobacterium. Both species had been major components of the human gut microbiota and recognized for their antibacterial and antiviral activities (52) . Multiple RCTs and meta-analyses showed that probiotics could decrease the incidence of upper respiratory infections (53) and enhance the immune response to influenza vaccination (54) . In everyday food, Lactobacillus and Bifidobacterium are mostly present in fermented dairy products (e.g., yogurt, kefir, cheese) as well as sourdough bread and sauerkraut, which can be included in a balanced diet (55) .

Mechanistic insights into the pathophysiology of COVID-19 are evolving and can be changing with further exploration into the unique biology of SARS-CoV-2 ( Fig.  1) . Multiple potential mechanisms, connections, and interactions between the host and the virus influence COVID-19 infection risk, severity of the course, and clinical outcomes in patients with diabetes. Among these mechanisms, an important role is played by metabolic endotoxemia characteristic of T2D (56), as well as by interactions in the gut-lung and gut-brain axes that have been implicated in neuroinflammation and the lung inflammatory response in COVID-19 (33, (57) (58) (59) . This review, however, will focus on three possible mechanisms of interconnections for coronavirus, diabetes, gut microbiota, and probiotics, including the immune system, ACE2 receptors, and the antidiabetes drug metformin.

The interaction between coronaviruses and the immune system portends the severity of infection (Fig. 1) . Coronavirus antigen is recognized by the cells of the innate immune system, and downstream of this pathway, cells of the adaptive immune response produce antibodies against the virus. The proper response of innate and adaptive immunity stops viral replication, prompts viral clearance, promotes tissues repair, and stimulates antiviral immunity (1) (2) (3) . In severe infection, coronaviruses can trigger secretion of high amounts of inflammatory cytokines ("cytokine storm"), contributing to respiratory and multisystem failure (1-6). The emerging data suggest that immune system dysregulation could be a link connecting COVID-19 infection severity, diabetes, and the microbiota. Both innate and adaptive immunity are dysregulated in T2D (60, 61) and T1D (62, 63) , although the mechanisms of immune dysregulation are different. Viral clearance assessed by viral RNA measurement is delayed in T2D patients with COVID-19, suggesting attenuated immune function in these patients (60) . Natural killer (NK) cells, well-known

CoV caused devastating disease with vomiting, diarrhea, dehydration and death in PED and TGEV (13, 23) .

CoV infection: Low B/F ratio (23), decreased butyrateproducing bacteria (23) in PED, as well as decreased Lactobacillus and increased Enterobacteriaceae in TGEV (13) .

CoV was frequently asymptomatic (4-80%), whereas symptoms included fever, anorexia, and lethargy (80-90%), diarrhea (20%), and encephalitis (3%) (24) . Horses <1 year old rarely had symptomatic CoV infection (24) .

CoV infection: Increased abundance of ammonia-producing microbiota (24) .

CoV was frequently asymptomatic (46%) yet was found in 64% fecal samples of calves presenting with diarrhea (25).

CoV diarrhea was associated with changes in commensal Escherichia coli that acquired pathogenic features (genes for curli, cellulose, and fimbriae and antimicrobial resistance) (26) .

CoV found in stool of 40% cats and 5% dogs presenting with diarrhea (27, 28) . CoV diarrhea in cats was associated with low B/F ratio (28) .

Intestinal histology varied from mild to severe inflammation (13, 24) .

Local intestinal immunity and barrier function were impaired and characterized by decreased number of IgA-positive cells, CD3 + T cells, and dendritic cells (13, 24) . Systemic immunity was characterized by enhanced mRNA expression levels of cytokine IL-1β, IL-6, TNF-α, IL-10, and TGF-β. Impairment of intestinal integrity and immune function resulted in increased pathogenic bacterial loading and facilitated secondary infections (13, 24) .

Abbreviations: B/F = Bacteroidetes to Firmicutes ratio; CoV = coronavirus; IL = interleukin; PED = porcine epidemic diarrhea; TGEV = transmissible gastroenteritis virus; TGF = transforming growth factor; TNF = tumor necrosis factor. CoV: L. casei Shirota anti-TGEV activity involved increased production of ROS (38) . T2D patients: increased serum SIRT1 and decreased fetuin-A (41), decreased total count of blood bacteria (45) . DM mice and rats: reversed dysbiosis (46, 47) , increased B/F ratio (46, 47) , enriched Bifidobacterium, Lactobacillus, and SCFAproducing GMB: Allobaculum and Bacteroides (46) , Butyricimonas (47) .

Recombinant a oral vaccine induced efficient immune anti-TGEV responses in piglets (39) . L. plantarum Probio-38 had anti-TGEV d activity in cell lines (40) . L. plantarum PCA236 had anti-TGEV c activity in cell lines (38) .

T2D/PreDM patients: decreased FBG and insulin resistance (43) .

T2D/PreDM patients: decreased proinflammatory cytokines (IL-6, MCP-1) (43). DM mice: increased GMB alpha diversity, reversed gut dysbiosis and decreased proinflammatory markers expression: pJNK and iNOS in the intestine, Fmo3 in the liver, IL-1β in Kupffer cells, and ICAM in the aorta and liver (48) . DM rats: decreased serum pro-inflammatory IL-1β, IL-6, CRP, increased anti-inflammatory IL-10 and downregulated the inflammasome signaling pathway (49) .

L. salivarius L. salivarius Probio-37 had anti-TGEV d activity in cell lines (40) .

T2D patients e : decreased insulin resistance (44) .

DM mice: reversed dysbiosis, increased mucosal antibacterial proteins (e.g., Reg3β), decreased endotoxin levels and Klebsiella pneumoniae translocation (50).

Multistrain lowered the risk of respiratory failure and mortality in a cohort of COVID-19 patients (33) .

T2D/PreDM patients: decreased FBG and insulin resistance with similar multistrain (51).

DM mice: reduction of oxidative stress and inflammation via activation of transcription factor Nrf2 system and its target gene HO-1 leading to NF-kB inhibition (33) . as defense against viral pathogens and immunoregulators, are also associated with the development of diabetes (T1D and T2D) and with changes in the gut microbiota (62, 63) .

Clostridiales microbiota abundance in the gut of nonobese diabetic mice that spontaneously develop T1D (63) . In a T1D nonobese diabetic mouse model, a single early-life antibiotic exposure produced gut dysbiosis and accelerated T1D development (62) . In this model, gut dysbio-sis was persistent, induced changes in genes regulating innate and adaptive immunity, and was not due to antibiotic effects (62) . Moreover, evaluation of the gut microbiota showed that four taxonomic groups (Enterococcus, Blautia, Enterobacteriaceae, and Akkermansia) were significantly overrepresented, and four other groups (S24-7, Clostridiales, Oscillospira, and Ruminococcus) were significantly underrepresented in intervention mice versus controls, implicating an important role for early life gut microbiome perturbations in T1D development (62) . Correspondingly, in a mouse model of T2D, hyperglycemia and insulin resistance were controlled by a mechanism comprising the adaptive immune system and gut microbiota (61) . In this model, the mouse immune system was triggered by a process of 'immunization,' subcutaneous injection of a homogenized diluted gut microbiota extract from the diabetic mice. A month later, two groups of mice (immunized and nonimmunized) were fed a high-fat diet (HFD) for 2 months in order to challenge the development of metabolic features. Immunization prevented hyperglycemia and insulin resistance in a dosedependent manner in response to the HFD. The transfer of immune cells harvested from the spleen of the microbiotaimmunized mice to naïve mice produced immune system changes, CD4+ and CD8+ T cell proliferation, cytokine production (including IL-17) and antibody secretion, and protected the recipients from HFD-induced impairments. Also, HFD-induced dysbiosis was partially corrected with favorable reversal toward normal abundance for specific genera, Anaerotruncus, Lachnospiraceae incertae, and Mucispirillum (61) . Similarly, studies in mice showed that the gut microbiota per se could affect and counteract a genetically determined condition that predisposes mice to the T2D phenotype (64) . The gut microbiota changes in these murine models indicated lower richness, alpha diversity, and a depletion of Allobaculum, Lactobacillus, and enrichment with Bacteroides genera in T2D mice compared to wild-type mice. The intestinal IL-17-producing Th17 cells were involved in limiting gut dysbiosis, lipopolysaccharide translocation to visceral adipose tissues, and reducing obesity and T2D in murine models (64) .

Mechanistic studies in models of T1D and T2D showed that the gut microbiota and/or its components activate receptors on gut endothelium such as toll-like receptors and nucleotide-oligomerization domain (NOD)-like receptors, which results in activation of IL-17-producing Th17 cells suggested to be critically involved in the pathogenesis of T1D and T2D (63) (64) (65) . The downstream pathway, including multiple intermediaries and receptors (e.g., interferon-g, Foxp3, NOD1, and NOD2), is involved in empowering microbiota Th17-related mechanisms (64, 65) . Th17 cells also play a role in the gut-lung axis. A constituent of the gut microbiota, segmented filamentous bacteria, stimulate the migration of Th17 cells to the lung, enhancing the autoimmune response and aggravating lung inflammation in a mouse model (57) . Overall, the data demonstrate the causality of microbiota-stimulated adaptive immune system modifications for pathogenesis and protection against diabetes.

ACE2 is a point of cellular entry for COVID-19 and SARS viruses (6) , is important in diabetes and its complications (7) , and is linked to the gut amino acid transport and microbiota ecology (66) . ACE and ACE2, two different enzymes with multiple roles in health and disease, are best known for their role in the renin-angiotensin system (RAS). ACE converts angiotensin-I to angiotensin-II (Ang-II), the main bioactive molecule of the RAS, which stimulates vasoconstriction. By contrast, ACE2 hydrolyzes Ang-II into an inactive metabolite, thereby promoting vasodilation. ACE2 counteracts many functions of ACE with antifibrotic, antiproliferative, and anti-inflammatory effects (7, (66) (67) (68) . Studies of mouse models lacking ACE2 receptors (ACE2 knockout, ACE2-KO) confirmed the receptor's role in the pathophysiology of diabetes complications (67) . The gut of ACE2-KO mice showed reduced epithelial integrity and leakage of bacterial products into the circulation and a marked increase in potentially deleterious peptidoglycan-producing bacteria (66, 67) . Of interest, in a cohort of 1,128 adults with COVID-19 and hypertension, all-cause mortality was lower in users compared to nonusers of ACE inhibitors and ACE receptor blockers (adjusted hazard ration, 0.42; P = .03) (68) . Presently, ACE2 is under active investigation for potential development of an anti-COVID-19 therapy (15) .

Metformin, a known regulator of bowel function and gut microbiota (69) , was associated with reduced mortality from lower respiratory tract disease in patients with T2D (70) , with decreased all-cause mortality in patients with T2D, and with reduced all-cause mortality and diseases of aging, independent of its effect on diabetes control (71) . Metformin could be connected to coronaviruses and ACE2 via the silent information regulator T1 (SIRT1) pathway. SIRT1, an enzyme of the sirtuin family of deacetylating proteins acting on >80 substrates, was proposed as an important mechanistic link in diabetes, with positive impacts on glucose homeostasis, insulin sensitivity, energy metabolism, chronic stresses (e.g., oxidative stress), as well as acting as a promoter of longevity (41, (72) (73) (74) . Metformin is a direct stimulator of SIRT1 (72), while SIRT1 is involved in controlling ACE2 expression (74) and functionally linked to MERS-CoV expression (75) . SIRT1 had antiviral effects against influenza virus infection in a study of senescent human bronchial epithelial cells and dermal fibroblasts (76) . SIRT1-microbiota interactions appear to play a role in diabetes. The use of L. casei probiotic resulted in improved fasting glucose and insulin resistance and was associ-ated with increased circulating SIRT1 in L. casei-treated patients compared to the placebo group in a double-blind RCT of T2D patients (41) . Correspondingly, lack of SIRT1 catalytic activity (SIRT1Y/Y mice) resulted in modulation of the gut microbiota (73) . In SIRT1Y/Y compared to SIRT1+/+ mice fed normal chow, gut Fusobacteria were decreased and Prevotella were increased, while Firmicutes and Bacteroidetes were the dominant phyla in both mouse strains. After HFD feeding, the relative proportion of phylum Firmicutes increased, whereas that of Bacteroidetes decreased in both strains, but the shift occurred faster in SIRT1Y/Y mice. Ruminococcaceae and Alistipes showed the same trends as the above phyla. The HFD was associated with an increase in the genus Enterococcus only in the SIRT1Y/Y mice, whereas the Verrucomicrobia phylum, including the Akkermansia genus, disappeared completely in the SIRT1Y/Y mice after only 1 week on the HFD, a result consistent with data showing decreased Akkermansia in obesity and T2D (73) . The mechanism of the anti-DM effect of metformin also involves, among many other actions, attenuation of endotoxemia, activation of the anti-oxidative nuclear factor erythroid 2p45related factor 2 (Nrf2) system, and increased abundance of the beneficial bacteria Lactobacillus and Akkermansia muciniphila (77) . The activation of the Nrf2 system and its target heme oxygenase-1 was similarly suggested as a mechanism for the beneficial effect of multistrain probiotics used in a cohort of Italian COVID-19 patients (33, 59) . Taken together, these data suggest that a metformin-related mechanistic pathway (including microbiota) could modulate the coronavirus course in patients with diabetes.

The available data suggest that the environment within the human gut, including the microbiota, could play an important role in the development and severity of COVID-19 in patients with diabetes. There are no data and no scientific rationale to suggest that probiotics could provide protection and/or treatment of coronavirus infection. Based on data of probiotic benefits for viral and other infections and diabetes, a balanced diet inclusive of probiotics could be proposed as relevant for controlling glucose and supporting the immune system during the COVID-19 pandemic. It is a challenge to the scientific community to uncover the beneficial potential of the gut microbiota for strengthening host defenses not only against COVID-19 but also against other novel infections. Future studies of the multifaceted network of interplay between the microbiota, probiotics, and the host could lead to new discoveries in the pathogenesis and management of coronavirus and other infections in patients with diabetes.

The authors have no multiplicity of interest to disclose.

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NOD2 deficiency promotes intestinal CD4+ T lymphocyte imbalance, metainflammation, and aggravates type 2 diabetes in murine model

Evaluating the causal role of gut microbiota in type 1 diabetes and its possible pathogenic mechanisms

ACE2--from the renin-angiotensin system to gut microbiota and malnutrition

Bone marrow-derived cells restore functional integrity of the gut epithelial and vascular barriers in a model of diabetes and ACE2 deficiency

Association of inpatient use of angiotensin converting enzyme inhibitors and angiotensin II receptor blockers with mortality among patients with hypertension hospitalized with COVID-19

Disentangling type 2 diabetes and metformin treatment signatures in the human gut microbiota

Reduced mortality from lower respiratory tract disease in adult diabetic patients treated with metformin

Metformin reduces all-cause mortality and diseases of ageing independent of its effect on diabetes control: a systematic review and meta-analysis

Metformin is a direct SIRT1-activating compound: computational modeling and experimental validation

The SIRT1 deacetylase protects mice against the symptoms of metabolic syndrome

Epigenetic regulation of angiotensin-converting enzyme 2 (ACE2) by SIRT1 under conditions of cell energy stress

A yeast suppressor screen used to identify mammalian sirt1 as a proviral factor for Middle East respiratory syndrome coronavirus replication

Enhanced viral replication by cellular replicative senescence

Metformin exerts glucose-lowering action in high-fat fed mice via attenuating endotoxemia and enhancing insulin signaling