Tryptophan as a Central Hub for Host/Microbial Symbiosis

Influenza and the gut microbiota: A hidden therapeutic link

Background

The extensive community of gut microbiota significantly influences various biological functions throughout the body, making its characterization a focal point in biomedicine research. Over the past few decades, studies have revealed a potential link between specific gut bacteria, their associated metabolic pathways, and influenza. Bacterial metabolites can communicate directly or indirectly with organs beyond the gut via the intestinal barrier, thereby impacting the physiological functions of the host. As the microbiota increasingly emerges as a 'gut signature' in influenza, gaining a deeper understanding of its role may offer new insights into its pathophysiological relevance and open avenues for novel therapeutic targets. In this Review, we explore the differences in gut microbiota between healthy individuals and those with influenza, the relationship between gut microbiota metabolites and influenza, and potential strategies for preventing and treating influenza through the regulation of gut microbiota and its metabolites, including fecal microbiota transplantation and microecological preparations.

Methods

We utilized PubMed and Web of Science as our search databases, employing keywords such as "influenza," "gut microbiota," "traditional Chinese medicine," "metabolites," "prebiotics," "probiotics," and "machine learning" to retrieve studies examining the potential therapeutic connections between the modulation of gut microbiota and its metabolites in the treatment of influenza. The search encompassed literature from the inception of the databases up to December 2023.

Results

Fecal microbiota transplantation (FMT), microbial preparations (probiotics and prebiotics), and traditional Chinese medicine have unique advantages in regulating intestinal microbiota and its metabolites to improve influenza outcomes. The primary mechanism involves increasing beneficial intestinal bacteria such as Bacteroidetes and Bifidobacterium while reducing harmful bacteria such as Proteobacteria. These interventions act directly or indirectly on metabolites such as short-chain fatty acids (SCFAs), amino acids (AAs), bile acids, and monoamines to alleviate lung inflammation, reduce viral load, and exert anti-influenza virus effects.

Conclusion

The gut microbiota and its metabolites have direct or indirect therapeutic effects on influenza, presenting broad research potential for providing new directions in influenza research and offering references for clinical prevention and treatment. Future research should focus on identifying key strains, specific metabolites, and immune regulation mechanisms within the gut microbiota to accurately target microbiota interventions and prevent respiratory viral infections such as influenza.

Metabolic regulation of the host-fungus interaction: from biological principles to therapeutic opportunities

Fungal infections present a significant global public health concern, impacting over 1 billion individuals worldwide and resulting in more than 3 million deaths annually. Despite considerable progress in recent years, the management of fungal infections remains challenging. The limited development of novel diagnostic and therapeutic approaches is largely attributed to our incomplete understanding of the pathogenetic mechanisms involved in these diseases. Recent research has highlighted the pivotal role of cellular metabolism in regulating the interaction between fungi and their hosts. In response to fungal infection, immune cells undergo complex metabolic adjustments to meet the energy demands necessary for an effective immune response. A comprehensive understanding of the metabolic circuits governing antifungal immunity, combined with the integration of individual host traits, holds the potential to inform novel medical interventions for fungal infections. This review explores recent insights into the immunometabolic regulation of host-fungal interactions and the infection outcome and discusses how the metabolic repurposing of immune cell function could be exploited in innovative and personalized therapeutic approaches.

Discrete interplay of gut microbiota L-tryptophan metabolites in host biology and disease

The gut microbiota and the host maintain a conjoint relationship and together achieve optimal physiology via a multitude of interactive signalling cues. Dietary-derived L-tryptophan (L-trp) is enzymatically metabolized by the resident symbiotic gut microbiota to indole and various indole derivatives. Indole and indole metabolites secreted by the gut bacteria act locally in the intestinal cells as well as distally and modulate tissue-specific functions which are beneficial to the host. Functions attributed to these microbial indole metabolites in the host include regulation of intestinal permeability, immunity and mucosal roles, inflammation, and insulin sensitivity. On the other hand, dysregulation of gut microbiota L-trp metabolism compromises the optimal availability of indole and indole metabolites and can induce the onset of metabolic disorders, inflammation, liver steatosis, and decrease gut barrier integrity. Gut dysbiosis is regarded as one of the prime reasons for this deregulated microbial-derived indole metabolites. A number of indole metabolites from the gut bacteria have been identified recently displaying variable affinity towards xenobiotic nuclear receptors. Microbial metabolite mimicry concept can be used to design and develop novel indole-moiety-containing compounds with higher affinity towards the receptors and efficacy in preclinical studies. Such compounds may serve as therapeutic drugs in clinical trials in the future. In this article, I review L-trp metabolism in the host and gut microbiota and the various physiological functions, patho-physiologies associated with the microbial-released indole metabolites in the host, including the metabolite mimicry-based concept to develop tailored indole-containing novel experimental drugs.

Host-microbe serotonin metabolism

As a result of a long evolutionary history, serotonin plays a variety of physiological roles, including neurological, cardiovascular, gastrointestinal, and endocrine functions. While many of these activities can be accommodated within the serotoninergic activity, recent findings have revealed an unsuspected role of serotonin in orchestrating host and microbial dialogue at the tryptophan dining table, to the benefit of local and systemic homeostasis. Herein we review the dual role of serotonin at the host-microbe interface and discuss how unraveling the interconnections among the host and microbial pathways of tryptophan degradation may help to accommodate the versatility of serotonin in physiology and pathology.

Variegated overexpression of chromosome 21 genes reveals molecular and immune subtypes of Down syndrome

Individuals with Down syndrome, the genetic condition caused by trisomy 21, exhibit strong inter-individual variability in terms of developmental phenotypes and diagnosis of co-occurring conditions. The mechanisms underlying this variable developmental and clinical presentation await elucidation. We report an investigation of human chromosome 21 gene overexpression in hundreds of research participants with Down syndrome, which led to the identification of two major subsets of co-expressed genes. Using clustering analyses, we identified three main molecular subtypes of trisomy 21, based on differential overexpression patterns of chromosome 21 genes. We subsequently performed multiomics comparative analyses among subtypes using whole blood transcriptomes, plasma proteomes and metabolomes, and immune cell profiles. These efforts revealed strong heterogeneity in dysregulation of key pathophysiological processes across the three subtypes, underscored by differential multiomics signatures related to inflammation, immunity, cell growth and proliferation, and metabolism. We also observed distinct patterns of immune cell changes across subtypes. These findings provide insights into the molecular heterogeneity of trisomy 21 and lay the foundation for the development of personalized medicine approaches for the clinical management of Down syndrome.

Grafted Sertoli Cells Exert Immunomodulatory Non-Immunosuppressive Effects in Preclinical Models of Infection and Cancer

The Sertoli cells (SeCs) of the seminiferous tubules secrete a multitude of immunoregulatory and trophic factors to provide immune protection and assist in the orderly development of germ cells. Grafts of naked or encapsulated SeCs have been proved to represent an interesting therapeutic option in a plethora of experimental models of diseases. However, whether SeCs have immunosuppressive or immunomodulatory effects, which is imperative for their clinical translatability, has not been demonstrated. We directly assessed the immunopotential of intraperitoneally grafted microencapsulated porcine SeCs (MC-SeCs) in murine models of fungal infection (Aspergillus fumigatus or Candida albicans) or cancer (Lewis lung carcinoma/LLC or B16 melanoma cells). We found that MC-SeCs (i) provide antifungal resistance with minimum inflammatory pathology through the activation of the tolerogenic aryl hydrocarbon receptor/indoleamine 2,3-dioxygenase pathway; (ii) do not affect tumor growth in vivo; and (iii) reduce the LLC cell metastatic cancer spread associated with restricted Vegfr2 expression in primary tumors. Our results point to the fine immunoregulation of SeCs in the relative absence of overt immunosuppression in both infection and cancer conditions, providing additional support for the potential therapeutic use of SeC grafts in human patients.

Relationship between Compound α-Ketoacid and Microinflammation in Patients with Chronic Kidney Disease

Chronic kidney disease (CKD) refers to the presence of structural or functional abnormalities in the kidneys that affect health, lasting for more than 3 months. CKD is not only the direct cause of global incidence rate and mortality, but also an important risk factor for cardiovascular disease. Persistent microinflammatory state has been recognized as an important component of CKD, which can lead to renal fibrosis and loss of renal function, and plays a crucial role in the pathophysiology and progression of the disease. Simultaneously, compound α-Ketoacid can bind nitrogen-containing metabolites in the blood and accelerate their excretion from the body, thereby reducing the level of metabolic waste, alleviating gastrointestinal reactions in patients, and reducing the inflammatory response and oxidative stress state of the body. Compound α-Ketoacid contains amino acids required by CKD patients. In this review, we explore the relationship between compound α-Ketoacid and microinflammation in patients with CKD. The review indicated that compound α-Ketoacid can improve the microinflammatory state in CKD patients by improving the nutritional status of CKD patients, improving patient's acid-base balance disorder, regulating oxidative stress, improving gut microbiota, and regulating abnormal lipid metabolism.

Elevated indoleamine 2,3-dioxygenase activity is associated with endothelial dysfunction in people living with HIV and ROS production in human aortic endothelial cells in vitro

The precise role of indoleamine 2,3-dioxygenase (IDO) in cardiovascular diseases (CVD) among people living with HIV (PLWH) is still under debate, despite recognized links. This study aimed to investigate the impact of elevated IDO activity on endothelial dysfunction in PLWH. A total of 38 PLWH, who had not previously received anti-retroviral therapy (ART), were enrolled in the study. These participants were monitored for 36 months following the initiation of ART. Measurements including plasma levels of IDO activity, markers of endothelial dysfunction, inflammatory factors, and lipids. In vitro, human aortic endothelial cells (HAEC) were exposed to interferon-γ, an IDO inhibitor, a kynurenine 3-hydroxylase (KMO) inhibitor, as well as different concentrations of kynurenine. Pre-ART, PLWH demonstrated notably elevated plasma concentrations of soluble intercellular adhesion molecule 1 (sICAM-1), soluble vascular cell adhesion molecule 1(sVCAM-1), and IDO activity in comparison to healthy controls. Post-ART, both IDO activity and sICAM-1 levels experienced a significant decrease, with IDO activity reaching levels comparable to those observed in healthy controls. Furthermore, a positive correlation was observed between IDO activity and sICAM-1 (p = 0.0002), as well as sVCAM-1 (p < 0.0001) before ART. In vitro, the augmentation of kynurenine concentration in the medium and the induction of IDO expression in HAEC resulted in increased production of reactive oxygen species (ROS), with minimal impact on endothelial dysfunction. From these findings, it can be concluded that long-term ART has the potential to restore the heightened IDO activity observed in PLWH. The overexpression of IDO primarily influences the expression of ROS in HAEC.

Gut microbiota metabolites, redox status, and the related regulatory effects of probiotics

Oxidative stress is a state of imbalance between oxidation and antioxidation. It is caused by excess levels of free radicals and leads to the damage of DNA, proteins, and lipids. The crucial role of gut microbiota in regulating oxidative stress has been widely demonstrated. Studies have suggested that the redox regulatory effects of gut microbiota are related to gut microbiota metabolites, including fatty acids, lipopolysaccharides, tryptophan metabolites, trimethylamine-N-oxide and polyphenolic metabolites. In recent years, the potential benefits of probiotics have been gaining increasing scientific interest owing to their ability to modulate gut microbiota and oxidative stress. In this review, we summarise the adverse health effects of oxidative stress and discuss the role of the gut microbiota and its metabolites in redox regulation. Based on the influence of gut microbiota metabolites, the roles of probiotics in preventing oxidative stress are highlighted.

Indoleamines Impart Abiotic Stress Tolerance and Improve Reproductive Traits in Hazelnuts

Hazelnuts have recently gathered tremendous attention due to the expansion of the confectionary industry. However, the sourced cultivars fail to perform in initial phase of cultivation as they enter bare survival mode due to changes in climatic zones, for example, Southern Ontario, where the climate is continental, as opposed to the milder climate in Europe and Turkey. Indoleamines have been shown to counter abiotic stress and modulate vegetative and reproductive development of plants. Here, we examined the effect of indoleamines on the flowering response of the dormant stem cuttings of sourced hazelnut cultivars in controlled environment chambers. The stem cuttings were exposed to sudden summer-like conditions (abiotic stress) and the female flower development was assessed in relation to endogenous indoleamine titers. The sourced cultivars responded well to serotonin treatment by producing more flowers compared to the controls or other treatments. The probability of buds resulting in female flowers was highest in the middle region of the stem cuttings. It is interesting to note that the tryptamine titers of the locally adapted, and N-acetyl serotonin titers of native hazelnut cultivars, provided the best explanation for adaptation to the stress environment. Titers of both compounds were compromised in the sourced cultivars which resorted mostly to serotonin concentrations to counter the stress. The indoleamines tool kit identified in this study could be deployed in assessing cultivars for stress adaptation attributes.

Local and systemic effects of microbiome‐derived metabolites

Commensal microbes form distinct ecosystems within their mammalian hosts, collectively termed microbiomes. These indigenous microbial communities broadly expand the genomic and functional repertoire of their host and contribute to the formation of a “meta‐organism.” Importantly, microbiomes exert numerous biochemical reactions synthesizing or modifying multiple bioactive small molecules termed metabolites, which impact their host's physiology in a variety of contexts. Identifying and understanding molecular mechanisms of metabolite–host interactions, and how their disrupted signaling can contribute to diseases, may enable their therapeutic application, a modality termed “postbiotic” therapy. In this review, we highlight key examples of effects of bioactive microbe‐associated metabolites on local, systemic, and immune environments, and discuss how these may impact mammalian physiology and associated disorders. We outline the challenges and perspectives in understanding the potential activity and function of this plethora of microbially associated small molecules as well as possibilities to harness them toward the promotion of personalized precision therapeutic interventions.

From the nose to the lungs: the intricate journey of airborne pathogens amidst commensal bacteria

The recent COVID-19 pandemic has dramatically brought the pitfalls of airborne pathogens to the attention of the scientific community. Not only viruses but also bacteria and fungi may exploit air transmission to colonize and infect potential hosts and be the cause of significant morbidity and mortality in susceptible populations. The efforts to decipher the mechanisms of pathogenicity of airborne microbes have brought to light the delicate equilibrium that governs the homeostasis of mucosal membranes. The microorganisms already thriving in the permissive environment of the respiratory tract represent a critical component of this equilibrium and a potent barrier to infection by means of direct competition with airborne pathogens or indirectly via modulation of the immune response. Moving down the respiratory tract, physicochemical and biological constraints promote site-specific expansion of microbes that engage in cross talk with the local immune system to maintain homeostasis and promote protection. In this review, we critically assess the site-specific microbial communities that an airborne pathogen encounters in its hypothetical travel along the respiratory tract and discuss the changes in the composition and function of the microbiome in airborne diseases by taking fungal and SARS-CoV-2 infections as examples. Finally, we discuss how technological and bioinformatics advancements may turn microbiome analysis into a valuable tool in the hands of clinicians to predict the risk of disease onset, the clinical course, and the response to treatment of individual patients in the direction of personalized medicine implementation.

Tuberculosis and Autoimmunity

Tuberculosis remains a common and dangerous chronic bacterial infection worldwide. It is long-established that pathogenesis of many autoimmune diseases is mainly promoted by inadequate immune responses to bacterial agents, among them Mycobacterium tuberculosis. Tuberculosis is a multifaceted process having many different outcomes and complications. Autoimmunity is one of the processes characteristic of tuberculosis; the presence of autoantibodies was documented by a large amount of evidence. The role of autoantibodies in pathogenesis of tuberculosis is not quite clear and widely disputed. They are regarded as: (1) a result of imbalanced immune response being reactive in nature, (2) a critical part of TB pathogenicity, (3) a beginning of autoimmune disease, (4) a protective mechanism helping to eliminate microbes and infected cells, and (5) playing dual role, pathogenic and protective. There is no single autoimmunity-mechanism development in tuberculosis; different pathways may be suggested. It may be excessive cell death and insufficient clearance of dead cells, impaired autophagy, enhanced activation of macrophages and dendritic cells, environmental influences such as vitamin D insufficiency, and genetic polymorphism, both of Mycobacterium tuberculosis and host.

A High-Risk Profile for Invasive Fungal Infections Is Associated with Altered Nasal Microbiota and Niche Determinants

It is becoming increasingly clear that the communities of microorganisms that populate the surfaces exposed to the external environment, termed microbiota, are key players in the regulation of pathogen-host cross talk affecting the onset as well as the outcome of infectious diseases. We have performed a multicenter, prospective, observational study in which nasal and oropharyngeal swabs were collected for microbiota predicting the risk of invasive fungal infections (IFIs) in patients with hematological malignancies. Here, we demonstrate that the nasal and oropharyngeal microbiota are different, although similar characteristics differentiate high-risk from low-risk samples at both sites. Indeed, similar to previously published results on the oropharyngeal microbiota, high-risk samples in the nose were characterized by low diversity, a loss of beneficial bacteria, and an expansion of potentially pathogenic taxa, in the presence of reduced levels of tryptophan (Trp). At variance with oropharyngeal samples, however, low Trp levels were associated with defective host-derived kynurenine production, suggesting reduced tolerance mechanisms at the nasal mucosal surface. This was accompanied by reduced levels of the chemokine interleukin-8 (IL-8), likely associated with a reduced recruitment of neutrophils and impaired fungal clearance. Thus, the nasal and pharyngeal microbiomes of hematological patients provide complementary information that could improve predictive tools for the risk of IFI in hematological patients.

Xenobiotic receptors and the regulation of intestinal homeostasis: harnessing the chemical output of the intestinal microbiota

The commensal bacteria that reside in the gastrointestinal tract exist in a symbiotic relationship with the host, driving the development of the immune system and maintaining metabolic and tissue homeostasis in the local environment. The intestinal microbiota has the capacity to generate a wide array of chemical metabolites to which the cells of the intestinal mucosa are exposed. Host cells express xenobiotic receptors, such as the aryl hydrocarbon receptor (AhR) and the pregnane X receptor (PXR), that can sense and respond to chemicals that are generated by nonhost pathways. In this review, we outline the physiological and immunological processes within the intestinal environment that are regulated by microbial metabolites through the activation of the AhR and the PXR, with a focus on ligands generated by the stepwise catabolism of tryptophan.

Gut microbiota as a potential target for developing anti-fatigue foods

Fatigue has many negative effects on human health. As such, it is desirable to develop anti-fatigue foods and understand the mechanisms of their action. Based on a comprehensive review of the literature, this article discusses the important roles of gut microbiota in fatigue and anti-fatigue. Studies have shown that an increase in pathogenic bacteria and a decrease in beneficial bacteria co-exist when fatigue is present in both rodents and humans, whereas changes in gut microbiota were reported after intervention with anti-fatigue foods. The roles of gut microbiota in the activities of anti-fatigue foods can also be explained in the causes and the effects of fatigue. Among the causes of fatigue, the accumulation of lactic acid, decrease of energy, and reduction of central nervous system function were related to gut microbiota metabolism. Among the harmful effects of fatigue, oxidative stress, inflammation, and intestinal barrier dysfunction were related to gut microbiota dysbiosis. Furthermore, gut microbiota, together with anti-fatigue foods, can inhibit pathogen growth, convert foods into highly anti-oxidative or anti-inflammatory products, produce short-chain fatty acids, maintain intestinal barrier integrity, inhibit intestinal inflammation, and stimulate the production of neurotransmitters that regulate the central nervous system. Therefore, it is believed that gut microbiota play important roles in the activities of anti-fatigue foods and may provide new insights on the development of anti-fatigue foods.

An Investigation into the Temporal Reproducibility of Tryptophan Metabolite Networks Among Healthy Adolescents

Tryptophan and its bioactive metabolites are associated with health conditions such as systemic inflammation, cardiometabolic diseases, and neurodegenerative disorders. There are dynamic interactions among metabolites of tryptophan. The interactions between metabolites, particularly those that are strong and temporally reproducible could be of pathophysiological relevance. Using a targeted metabolomics approach, the concentration levels of tryptophan and 18 of its metabolites across multiple pathways was quantified in 24-hours urine samples at 2 time-points, age 17 years (baseline) and 18 years (follow-up) from 132 (52% female) apparently healthy adolescent participants of the DOrtmund Nutritional and Anthropometric Longitudinally Designed (DONALD) Study. In sex-specific analyses, we applied 2 network approaches, the Gaussian graphical model and Bayesian network to (1) explore the network structure for both time-points, (2) retrieve strongly related metabolites, and (3) determine whether the strongly related metabolites were temporally reproducible. Independent of selected covariates, the 2 network approaches revealed 5 associations that were strong and temporally reproducible. These were novel relationships, between kynurenic acid and indole-3-acetic acid in females and between kynurenic acid and xanthurenic acid in males, as well as known relationships between kynurenine and 3-hydroxykynurenine, and between 3-hydroxykynurenine and 3-hydroxyanthranilic acid in females and between tryptophan and kynurenine in males. Overall, this epidemiological study using network-based approaches shed new light into tryptophan metabolism, particularly the interaction of host and microbial metabolites. The 5 observed relationships suggested the existence of a temporally stable pattern of tryptophan and 6 metabolites in healthy adolescent, which could be further investigated in search of fingerprints of specific physiological states. The metabolites in these relationships may represent a multi-biomarker panel that could be informative for health outcomes.

Tryptophan: From Diet to Cardiovascular Diseases

Cardiovascular disease (CVD) is one of the major causes of mortality worldwide. Inflammation is the underlying common mechanism involved in CVD. It has been recently related to amino acid metabolism, which acts as a critical regulator of innate and adaptive immune responses. Among different metabolites that have emerged as important regulators of immune and inflammatory responses, tryptophan (Trp) metabolites have been shown to play a pivotal role in CVD. Here, we provide an overview of the fundamental aspects of Trp metabolism and the interplay between the dysregulation of the main actors involved in Trp metabolism such as indoleamine 2, 3-dioxygenase 1 (IDO) and CVD, including atherosclerosis and myocardial infarction. IDO has a prominent and complex role. Its activity, impacting on several biological pathways, complicates our understanding of its function, particularly in CVD, where it is still under debate. The discrepancy of the observed IDO effects could be potentially explained by its specific cell and tissue contribution, encouraging further investigations regarding the role of this enzyme. Thus, improving our understanding of the function of Trp as well as its derived metabolites will help to move one step closer towards tailored therapies aiming to treat CVD.

A Tryptophan-Deficient Diet Induces Gut Microbiota Dysbiosis and Increases Systemic Inflammation in Aged Mice

The gut microflora is a vital component of the gastrointestinal (GI) system that regulates local and systemic immunity, inflammatory response, the digestive system, and overall health. Older people commonly suffer from inadequate nutrition or poor diets, which could potentially alter the gut microbiota. The essential amino acid (AA) tryptophan (TRP) is a vital diet component that plays a critical role in physiological stress responses, neuropsychiatric health, oxidative systems, inflammatory responses, and GI health. The present study investigates the relationship between varied TRP diets, the gut microbiome, and inflammatory responses in an aged mouse model. We fed aged mice either a TRP-deficient (0.1%), TRP-recommended (0.2%), or high-TRP (1.25%) diet for eight weeks and observed changes in the gut bacterial environment and the inflammatory responses via cytokine analysis (IL-1a, IL-6, IL-17A, and IL-27). The mice on the TRP-deficient diets showed changes in their bacterial abundance of Coriobacteriia class, Acetatifactor genus, Lachnospiraceae family, Enterococcus faecalis species, Clostridium sp genus, and Oscillibacter genus. Further, these mice showed significant increases in IL-6, IL-17A, and IL-1a and decreased IL-27 levels. These data suggest a direct association between dietary TRP content, the gut microbiota microenvironment, and inflammatory responses in aged mice models.

Aryl hydrocarbon receptor: Its roles in physiology

Aryl hydrocarbon receptor (AHR) was initially discovered as a cellular protein involved in mediating the detoxification of xenobiotic compounds. Extensive research in the past two decades has identified several families of physiological ligands and uncovered important functions of AHR in normal development and homeostasis. Deficiency in AHR expression disrupts major signaling systems and transcriptional programs, which appear to be responsible for the development of numerous developmental abnormalities including cardiac hypertrophy and epidermal hyperplasia. This mini review primarily summarizes recent advances in our understanding of AHR functions in normal physiology with an emphasis on the cardiovascular, gastrointestinal, integumentary, nervous, and immunomodulatory systems.

The Microbiota/Host Immune System Interaction in the Nose to Protect from COVID-19

Coronavirus disease 2019 (COVID-19) is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and is characterized by variable clinical presentation that ranges from asymptomatic to fatal multi-organ damage. The site of entry and the response of the host to the infection affect the outcomes. The role of the upper airways and the nasal barrier in the prevention of infection is increasingly being recognized. Besides the epithelial lining and the local immune system, the upper airways harbor a community of microorganisms, or microbiota, that takes an active part in mucosal homeostasis and in resistance to infection. However, the role of the upper airway microbiota in COVID-19 is not yet completely understood and likely goes beyond protection from viral entry to include the regulation of the immune response to the infection. Herein, we discuss the hypothesis that restoring endogenous barriers and anti-inflammatory pathways that are defective in COVID-19 patients might represent a valid strategy to reduce infectivity and ameliorate clinical symptomatology.

Drug Mimicry: Promiscuous Receptors PXR and AhR, and Microbial Metabolite Interactions in the Intestine

Significant attrition limits drug discovery. The available chemical entities present with drug-like features contribute to this limitation. Using specific examples of promiscuous receptor-ligand interactions, a case is made for expanding the chemical space for drug-like molecules. These ligand-receptor interactions are poor candidates for the drug discovery process. However, provided herein are specific examples of ligand-receptor or transcription-factor interactions, namely, the pregnane X receptor (PXR) and the aryl hydrocarbon receptor (AhR), and itsinteractions with microbial metabolites. Discrete examples of microbial metabolite mimicry are shown to yield more potent and non-toxic therapeutic leads for pathophysiological conditions regulated by PXR and AhR. These examples underscore the opinion that microbial metabolite mimicry of promiscuous ligand-receptor interactions is warranted, and will likely expand the existing chemical space of drugs.