Kashyap, Purna

Abstract The human microbiome consists of the microorganisms associated with the body, such as bacteria, fungi, archaea, protozoa, and viruses, along with their gene content and products. These microbes are abundant in the digestive, respiratory, renal/urinary, and reproductive systems. While microbes found in other organs/tissues are often associated with diseases, some reports suggest their presence even in healthy individuals. Lack of microbial colonization does not indicate a lack of microbial influence, as their metabolites can affect distant locations through circulation. In a healthy state, these microbes maintain a mutualistic relationship and help shape the host's physiological functions. Unlike the host's genetic content, microbial gene content and expression are dynamic and influenced by factors such as ethnicity, genetic background, sex, age, lifestyle/diet, and psychological/physical conditions. Therefore, defining a healthy microbiome becomes challenging as it is context dependent and can vary over time for an individual. Although differences in microbial composition have been observed in various diseases, these changes may reflect host alterations rather than causing the disease itself. As the field is evolving, there is increased emphasis on understanding when changes in the microbiome are an important component of pathogenesis rather than the consequence of a disease state. This article focuses on the microbial component in the digestive and respiratory tracts—the primary sites colonized by microorganisms—and the physiological functions of microbial metabolites in these systems. It also discusses their physiological functions in the central nervous and cardiovascular systems, which have no microorganism colonization under healthy conditions based on human studies. © 2024 American Physiological Society. Compr Physiol 14:5491‐5519, 2024.

Background: Fecal microbiota transplantation (FMT) is a successful therapy for Clostridioides difficile infection (CDI). FMT from overweight donors is speculated to influence the recipient’s body mass index (BMI) after administration for CDI. Objectives: We investigated changes in the recipient’s BMI after FMT in relation to the donor’s BMI. Design: We conducted a retrospective cohort study involving patients who underwent FMT for recurrent CDI at Mayo Clinic between 2012 and 2019. Methods: We analyzed demographic and donor data for patients undergoing FMT at Mayo Clinic (2012–2019). Recipient BMI (pre- and post-FMT) and donor BMI were extracted from medical records. Mixed-effects linear regression was used to evaluate the impact of donor BMI, donor BMI category, recipient baseline BMI, time before and after FMT, and interactions between these variables on overall BMI change and BMI change per month. Kaplan–Meier curves were used to assess BMI changes (⩾5 units) based on the last recorded post-FMT BMI. Results: We analyzed data from 401 patients with recorded BMI measurements before and after FMT. The median age of the recipients at the time of FMT was 59.1 years (interquartile range (IQR): 40.5–70.1 years), with 61.6% being female. The median BMI for recipients prior to FMT was 26.7 kg/m² (IQR: 22.7–31.6 kg/m²), while the median BMI of the donors was 24.5 kg/m2 (IQR: 23.9–27.5 kg/m2). Stool from donors with a normal BMI was used for 58.2% of recipients, while 41.8% received stool from pre-obese donors. Donor BMI data were missing for 3.2% of recipients. Donor BMI was not significantly associated with changes in recipient BMI; for each 1-unit increase in donor BMI, a 0.01-unit monthly increase was observed (95% confidence interval: −0.0003, 0.02; p = 0.11). The log-rank test for BMI increases (⩾+5) and decreases (⩽−5) revealed no significant differences among the donor BMI groups (Chi-squared = 4.4, p = 0.1 for increases, Chi-squared = 2, p = 0.4 for decreases). Conclusion: The lack of impact of donor BMI on BMI changes post-FMT suggests that these changes are more dependent on the recipient’s metabolic profile. Prospective, controlled trials are required to analyze these results more comprehensively.

AbstractBackgroundThis SRMA reviewed and assessed the changes in the severity of disorders of gut‐brain interaction (DGBI) symptoms during the COVID‐19 pandemic, and evaluated factors associated with symptom severity changes.MethodsElectronic databases were searched until February 2024, for articles reporting on changes in symptom severity in DGBI patients during the COVID‐19 pandemic. The proportion of DGBI patients who reported a change in their symptom severity were pooled using a random‐effects model, and subgroup analyses were conducted to assess the effect of socio‐cultural modifiers on symptom severity in DGBI.Key ResultsTwelve studies including 3610 DGBI patients found that 31.4% (95% CI, 15.9–52.5) of DGBI patients experienced symptom deterioration, while 24.3% (95% CI, 10.2–47.5) experienced improvement. Countries with high gross domestic product (GDP) had a 43.5% (95% CI, 16.3–75.2) likelihood of symptom deterioration, compared to 9.2% (95% CI, 1.4–42.2) in lower GDP countries. Similarly, countries with low COVID fatality rates had a 60.1% (95% CI, 19.7–90.3) likelihood of symptom deterioration, compared to 18.3% (95% CI, 7.8–36.9) in higher fatality rate countries. Countries with lenient COVID policies had a 58.4% (95% CI, 14.1–92.3) likelihood of symptom deterioration, compared to 19% (95% CI, 8.2–38.1) in countries with stricter policies. Patients in high vaccine hesitancy countries had a 51.4% (95% CI, 19.5–82.2) likelihood of symptom deterioration, compared to 10.6% (95% CI, 2.7–33.4) in low vaccine hesitancy countries.Conclusions & InferencesThis meta‐analysis reveals that a significantly higher proportion of DGBI patients experienced deterioration of symptoms during the COVID‐19 pandemic. Various sociocultural, economic and environmental factors potentially modify the effects of the COVID‐19 pandemic on DGBI.

AbstractBackgroundDisorders of gut–brain interaction (DGBI) are characterized by debilitating symptoms not explained by structural or biochemical abnormalities. While functional conditions present with complex, likely heterogeneous pathophysiology, we aimed to investigate if proxy measures of sociocultural and environmental factors are associated with the prevalence of various DGBI in populations across the world.MethodsWe performed an ecological study utilizing peer‐reviewed published datasets reporting for 26 countries prevalence rates of DGBI (Rome Foundation Global Epidemiology Study, RFGES), with six independent variables: Helicobacter pylori prevalence and household size as proxy measures for orofecal infections, gross domestic product per capita (GDP), and median age as a proxy measures for socioeconomic development, density of fast food outlets (FFO) per 100,000 population as proxy measure for processed food exposure, and suicide mortality rate per 100,000 people, and world happiness scores were used as a proxy for psychological stress. The data were retrieved from publicly accessible datasets (United Nations, CIA World Factbook, World Bank, World Happiness Report, commercial/financial reports of a global FFO chain). We used linear regression to assess variables in univariate and multivariate analysis and report standardized β coefficients with 95% confidence intervals (CI).Key ResultsThe regression model revealed that the overall prevalence of DGBI was inversely associated with both GDP per capita (β = −0.57, 95% CI: −0.92, −0.22, p = 0.002) and happiness scores (β = −0.433 95% CI: 0.821, −0.065, p = 0.023), while being positively associated with H. pylori prevalence (β = 0.40, 95% CI: 0.008, 0.81, p = 0.046). The prevalence of functional constipation (FC) was also inversely associated with GDP per capita (β = −0.50, 95% CI: −0.86, −0.13, p = 0.01) and happiness scores (β = −0.497, 95% CI: −0.863, −0.132, p = 0.01), while being positively associated with H. pylori prevalence (β = 0.53, 95% CI: 0.16, 0.91, p = 0.007). The Multivariate model analysis revealed that combining the factors of H. pylori prevalence, suicide rate, household size and happiness scores showed statistically significant association with FC (p = 0.039). Household size (β = −0.43, 95% CI: −0.82, 0.038, p = 0.033) and suicide rates (β = 0.55, 95% CI: 0.19, 0.90, p = 0.004) were statistically significantly associated with functional diarrhea. Irritable bowel syndrome (IBS) was associated with GDP per capita (β = −0.40, 95% CI: −0.79, −0.014, p = 0.043) and happiness scores (β = −0.390, 95% CI: −0.778, −0.003, p = 0.049).Conclusions & InferencesUtilizing publicly available data, the prevalence of DGBI across diverse countries is linked to various socio‐cultural and environmental factors. Collectively, the data suggests that the prevalence of DGBI is increased in less prosperous regions of the world.

AbstractBackground and AimPatients with intestinal failure (IF) have abnormal intestinal anatomy, secretion, and dysmotility, which impairs intestinal homeostatic mechanisms and may lead to small intestinal bacterial overgrowth (SIBO). We conducted a systematic review and meta‐analysis to determine the prevalence of SIBO in patients with IF and to identify risk factors for SIBO.MethodsMEDLINE (PubMed) and Embase electronic databases were searched from inception to December 2023 for studies that reported the prevalence of SIBO in IF. The prevalence rates, odds ratio (OR), and 95% confidence intervals of SIBO in IF and the risk factors for SIBO in IF were calculated using random effects model.ResultsFinal dataset included nine studies reporting on 407 patients with IF. The prevalence of SIBO in IF was 57.5% (95% CI 44.6–69.4), with substantial heterogeneity in this analysis (I2 = 80.9, P = 0.0001). SIBO prevalence was sixfold higher in patients with IF who received parenteral nutrition (PN) compared with IF patients not on PN (OR = 6.0, 95% CI 3.0–11.9, P = 0.0001). Overall, the prevalence of SIBO in patients with IF using PPI/acid‐suppressing agents (72.0%, 95% CI 57.5–83.8) was numerically higher compared with IF patients not using these agents (47.6%, 95% CI 25.7–70.2).ConclusionsThis systematic review and meta‐analysis suggests that there is an increased risk of SIBO in patients with IF and that PN, and potentially, the use of PPI/acid‐suppressing agents is risk factors for SIBO development in patients with IF. However, the quality of evidence is low and can be attributed to lack of case–control studies and clinical heterogeneity seen in the studies.

Disorders of Gut‐Brain Interaction (DGBI) are widely prevalent and commonly encountered in gastroenterology practice. While several peripheral and central mechanisms have been implicated in the pathogenesis of DGBI, a recent body of work suggests an important role for the gut microbiome. In this review, we highlight how gut microbiota and their metabolites affect physiologic changes underlying symptoms in DGBI, with a particular focus on their mechanistic influence on GI transit, visceral sensitivity, intestinal barrier function and secretion, and CNS processing. This review emphasizes the complexity of local and distant effects of microbial metabolites on physiological function, influenced by factors such as metabolite concentration, duration of metabolite exposure, receptor location, host genetics, and underlying disease state. Large‐scale in vitro work has elucidated interactions between host receptors and the microbial metabolome but there is a need for future research to integrate such preclinical findings with clinical studies. The development of novel, targeted therapeutic strategies for DGBI hinges on a deeper understanding of these metabolite‐host interactions, offering exciting possibilities for the future of treatment of DGBI.

AbstractBackgroundThere is compelling evidence that microbe‐host interactions in the intestinal tract underlie many human disorders, including disorders of gut‐brain interactions (previously termed functional bowel disorders), such as irritable bowel syndrome (IBS). Small intestinal bacterial overgrowth (SIBO) has been recognized for over a century in patients with predisposing conditions causing intestinal stasis, such as surgical alteration of the small bowel or chronic diseases, including scleroderma and is associated with diarrhea and signs of malabsorption. Over 20 years ago, it was hypothesized that increased numbers of small intestine bacteria might also account for symptoms in the absence of malabsorption in IBS and related disorders. This SIBO‐IBS hypothesis stimulated significant research and helped focus the profession's attention on the importance of microbe‐host interactions as a potential pathophysiological mechanism in IBS.PurposeHowever, after two decades, this hypothesis remains unproven. Moreover, it has led to serious unintended consequences, namely the widespread use of unreliable and unvalidated breath tests as a diagnostic test for SIBO and a resultant injudicious use of antibiotics. In this review, we examine why the SIBO hypothesis remains unproven and, given the unintended consequences, discuss why it is time to reject this hypothesis and its reliance on breath testing. We also examine recent IBS studies of bacterial communities in the GI tract, their composition and functions, and their interactions with the host. While these studies provide important insights to guide future research, they highlight the need for further mechanistic studies of microbe‐host interactions in IBS patients before we can understand their possible role in diagnosis and treatment of patient with IBS and related disorders.

Abstract The conventional bottom-up approach to probing the human gut microbiome’s link with hosts in germ-free models is hampered by considerable costs and time. To address this, our study introduces the nematode Caenorhabditis elegans as an innovative high-throughput model for exploring the gut microbiome’s impact on functional phenotypes. Traditionally, C. elegans studies have used continuous feeding for bacterial administration, a method that is unsuitable for anaerobes. For the first time, we have standardized a protocol for colonizing C. elegans with human gut anaerobes. By screening a microbial culturomics library representing 70% of the gut microbiome’s functional capacity, we showed successful colonization for 46% of the library. Functional phenotyping revealed that 5 of 10 strains, previously identified in vitro as inhibiting C. difficile , also inhibited in vivo . Validation of a selected strain in a germ-free mouse model confirmed colonization resistance and an immune response consistent with findings in C. elegans , underscoring the model’s translational potential.