Small intestinal bacterial overgrowth is associated with Diarrhea-predominant irritable bowel syndrome by increasing mainly Prevotella abundance

Differences in clinical manifestations and the fecal microbiome between irritable bowel syndrome and small intestinal bacterial overgrowth

BACKGROUND

Irritable bowel syndrome (IBS) and small intestinal bacterial overgrowth (SIBO) share similar abdominal symptoms; however, their differentiation remains controversial.

AIMS

To illustrate the differences between the two conditions.

METHODS

Patients and healthy controls completed questionnaires and provided stool samples for analysis.

RESULTS

IBS presented with the most severe symptoms and was specifically characterized by intense abdominal pain and frequent episodes of diarrhea. Patients with IBS displayed more dysregulated taxonomy within the fecal microbiota than SIBO. Opportunistic pathogens, including Lachnoclostridium, Escherichia-Shigella, and Enterobacter were enriched in the IBS group which contributed to increased bacterial pathogenicity and positively correlated with abdominal pain and bloating, meanwhile, Lachnoclostridium and Escherichia-Shigella were found to be associated with metabolites affiliated to bile acids, alcohols and derivatives. Bacteria enriched in SIBO group correlated with constipation. The bacterial co-occurrence network within the SIBO group was the most intricate. Ruminococcaceae Group were defined as core bacteria in SIBO. Differential metabolites affiliated to androstane steroids and phenylacetic acids were associated with core bacteria.

CONCLUSIONS

Our study elucidates the differences between IBS and SIBO in terms of symptoms, microbiota and functions, which provides insights into a better understanding of both diseases and evidence for different treatment strategies.

Small Intestinal Bacterial Overgrowth (SIBO) and Twelve Groups of Related Diseases-Current State of Knowledge

The human gut microbiota creates a complex microbial ecosystem, characterized by its high population density, wide diversity, and complex interactions. Any imbalance of the intestinal microbiome, whether qualitative or quantitative, may have serious consequences for human health, including small intestinal bacterial overgrowth (SIBO). SIBO is defined as an increase in the number of bacteria (103-105 CFU/mL), an alteration in the bacterial composition, or both in the small intestine. The PubMed, Science Direct, Web of Science, EMBASE, and Medline databases were searched for studies on SIBO and related diseases. These diseases were divided into 12 groups: (1) gastrointestinal disorders; (2) autoimmune disease; (3) cardiovascular system disease; (4) metabolic disease; (5) endocrine disorders; (6) nephrological disorders; (7) dermatological diseases; (8) neurological diseases (9); developmental disorders; (10) mental disorders; (11) genetic diseases; and (12) gastrointestinal cancer. The purpose of this comprehensive review is to present the current state of knowledge on the relationships between SIBO and these 12 disease groups, taking into account risk factors and the causal context. This review fills the evidence gap on SIBO and presents a biological-medical approach to the problem, clearly showing the groups and diseases having a proven relationship with SIBO, as well as indicating groups within which research should continue to be expanded.

Long-Term Impact of Childhood Adversity on the Gut Microbiome of Nursing Students

Adverse childhood experiences (ACEs) encompass negative, stressful, and potentially traumatic events during childhood, impacting physical and mental health outcomes in adulthood. Limited studies suggest ACEs can have short-term effects on children's gut microbiomes and adult cognitive performance under stress. Nevertheless, the long-term effects of ACEs experienced during adulthood remain unexplored. Thus, this study aimed to assess the long-term effects of ACEs on the gut microbiota of adult nursing students. We employed a multidimensional approach, combining 16S rRNA sequencing, bioinformatics tools, and machine learning to predict functional capabilities. High-ACE individuals had an increased abundance of Butyricimonas spp. and Prevotella spp. and decreased levels of Clostridiales, and Lachnospira spp. Prevotella abundance correlated negatively with L-glutamate and L-glutamine biosynthesis, potentially impacting intestinal tissue integrity. While nursing students with high ACE reported increased depression, evidence for a direct gut microbiota-depression relationship was inconclusive. High-ACE individuals also experienced a higher prevalence of diarrhea. These findings highlight the long-lasting impact of ACEs on the gut microbiota and its functions in adulthood, particularly among nursing students. Further research is warranted to develop targeted interventions and strategies for healthcare professionals, optimizing overall health outcomes.

The opportunistic nature of gut commensal microbiota

The abundance of gut commensals has historically been associated with health-promoting effects despite the fact that the definition of good or bad microbiota remains condition-specific. The beneficial or pathogenic nature of microbiota is generally dictated by the dimensions of host-microbiota and microbe-microbe interactions. With the increasing popularity of gut microbiota in human health and disease, emerging evidence suggests opportunistic infections promoted by those gut bacteria that are generally considered beneficial. Therefore, the current review deals with the opportunistic nature of the gut commensals and aims to summarise the concepts behind the occasional commensal-to-pathogenic transformation of the gut microbes. Specifically, relevant clinical and experimental studies have been discussed on the overgrowth and bacteraemia caused by commensals. Three key processes and their underlying mechanisms have been summarised to be responsible for the opportunistic nature of commensals, viz. improved colonisation fitness that is dictated by commensal-pathogen interactions and availability of preferred nutrients; pathoadaptive mutations that can trigger the commensal-to-pathogen transformation; and evasion of host immune response as a survival and proliferation strategy of the microbes. Collectively, this review provides an updated concept summary on the underlying mechanisms of disease causative events driven by gut commensal bacteria.

Gut Dysbiosis in Irritable Bowel Syndrome: A Narrative Review on Correlation with Disease Subtypes and Novel Therapeutic Implications

Irritable bowel syndrome (IBS) is a prevalent functional gastrointestinal disorder characterized by chronic abdominal pain and altered bowel habits. It can be subclassified in different subtypes according to the main clinical manifestation: constipation, diarrhea, mixed, and unclassified. Over the past decade, the role of gut microbiota in IBS has garnered significant attention in the scientific community. Emerging research spotlights the intricate involvement of microbiota dysbiosis in IBS pathogenesis. Studies have demonstrated reduced microbial diversity and stability and specific microbial alterations for each disease subgroup. Microbiota-targeted treatments, such as antibiotics, probiotics, prebiotics, synbiotics, fecal microbiota transplantation, and even diet, offer exciting prospects for managing IBS. However, definitive conclusions are hindered by the heterogeneity of these studies. Further research should focus on elucidating the mechanisms, developing microbiome-based diagnostics, and enabling personalized therapies tailored to an individual's microbiome profile. This review takes a deep dive into the microscopic world inhabiting our guts, and its implications for IBS. Our aim is to elucidate the complex interplay between gut microbiota and each IBS subtype, exploring novel microbiota-targeted treatments and providing a comprehensive overview of the current state of knowledge.

Evaluation of the effectiveness and mechanism of action of the Chang-Kang-Fang formula combined with bifid triple viable capsules on diarrhea-predominant irritable bowel syndrome

Introduction

The Chang-Kang-Fang (CKF) formula, a traditional Chinese herbal formula, can decrease serotonin (5-HT) levels and treat irritable bowel syndrome (IBS). Probiotics have a better synergistic effect on diarrhea-predominant IBS (IBS-D) when combined with 5-HT₃ receptor antagonists. The present study aimed to elucidate the efficacy and the mechanisms of action of the CKF formula combined with bifid triple viable capsules (PFK) against IBS-D.

Methods

The rat models of IBS-D were induced by gavage with senna decoction plus restraint stress. The CKF formula, PFK and their combination were administered to the rats. Their effects were evaluated based on general condition of the rats and the AWR score. The levels of 5-HT and fos protein in the colon and hippocampus were measured by immunohistochemistry. The levels of SP and VIP, as well as ZO-1 and occludin in the colon, were determined by enzyme-linked immunosorbent assay and immunohistochemistry. The intestinal microbiota in faeces was analyzed by 16S rRNA high-throughput sequencing.

Results

The results showed that the oral CKF formula combined with PFK (CKF + PFK) could significantly relieve the symptoms of IBS-D, including elevating the weight rate and decreasing the AWR score. Compared with the MC group, administration of CKF + PFK significantly reduced the expression of fos in the colon and hippocampus and that of 5-HT, SP and VIP in the colon and increased the levels of 5-HT in the hippocampus and ZO-1 and occludin in the colon. The above indexes exhibited statistical significance in the CKF + PFK group relative to those in the other groups. Moreover, treatment with CKF + PFK improved the diversity of intestinal microbiota and the abundance of Firmicutes, Lachnospiraceae and Ruminococcaceae but decreased those of Bacteroidetes and Prevotellaceae.

Conclusions

The CKF formula combined with PFK may have a synergistic effect on IBS-D by slowing gastrointestinal motility, lowering visceral hypersensitivity, enhancing the intestinal barrier function and modulating the composition of intestinal microbiota.

A multi-omic brain gut microbiome signature differs between IBS subjects with different bowel habits

Alterations of the brain-gut-microbiome system (BGM) have been implicated in the pathophysiology of irritable bowel syndrome (IBS), yet bowel habit-specific alterations have not been elucidated. In this cross-sectional study, we apply a systems biology approach to characterize BGM patterns related to predominant bowel habit. Fecal samples and resting state fMRI were obtained from 102 premenopausal women (36 constipation-predominant IBS (IBS-C), 27 diarrhea-predominant IBS (IBS-D), 39 healthy controls (HCs)). Data integration analysis using latent components (DIABLO) was used to integrate data from the phenome, microbiome, metabolome, and resting-state connectome to predict HCs vs IBS-C vs IBS-D. Bloating and visceral sensitivity, distinguishing IBS from HC, were negatively associated with beneficial microbes and connectivity involving the orbitofrontal cortex. This suggests that gut interactions may generate aberrant central autonomic and descending pain pathways in IBS. The connection between IBS symptom duration, key microbes, and caudate connectivity may provide mechanistic insight to the chronicity of pain in IBS. Compared to IBS-C and HCs, IBS-D had higher levels of many key metabolites including tryptophan and phenylalanine, and increased connectivity between the sensorimotor and default mode networks; thus, suggestingan influence on diarrhea, self-related thoughts, and pain perception in IBS-D ('bottom-up' mechanism). IBS-C's microbiome and metabolome resembled HCs, but IBS-C had increased connectivity in the default mode and salience networks compared to IBS-D, which may indicate importance of visceral signals, suggesting a more 'top-down' BGM pathophysiology. These BGM characteristics highlight possible mechanistic differences for variations in the IBS bowel habit phenome. This article is part of the Special Issue on 'Microbiome & the Brain: Mechanisms & Maladies'.

Significant Differences in Gut Microbiota Between Irritable Bowel Syndrome with Diarrhea and Healthy Controls in Southwest China

BACKGROUND

Irritable bowel syndrome (IBS) is a heterogeneous disease, which is closely related to environmental factors and gut microbiota.

OBJECTIVE

To study gut microbiota in IBS-D of Han nationality in Southwest China and explore its relationship with environmental factors.

METHODS

One hundred and twenty cases of IBS-D and 63 cases of HCs were recruited; baseline data such as age, height, and weight were collected. HAMA, HAMD, IBS-SSS, IBS-QOL, and laboratory tests were performed. Feces were collected for 16S rDNA sequencing. Then, the differences of gut microbiota were analyzed and looked for biomarkers of each. FAPROTAX was used to predict the functional differences of gut microbiota. Spearman analysis was conducted between the phylum level and environmental factor.

RESULTS

There were significant differences in daily life between IBS-D and HCs, especially in the spicy taste. The scores of HAMA and HAMD, urea, and transaminase in IBS-D were significantly higher than those of HCs. The richness of gut microbiota in IBS-D was significantly lower than that of HCs, as well as the beta diversity, but not diversity. The biomarkers of IBS-D were Prevotella, Clostridiales, and Roseburia, and the biomarkers of HCs were Veillonellaceae, Bacteroides coprocola, and Bifidobacteriales. The functions of gut microbiota in IBS-D were significantly different from HCs. Correlation analysis showed that multiple gut microbiota were closely related to HAMA, IBS-SSS, IBS-QOL, inflammatory indexes, and liver enzymes.

CONCLUSION

There are significant differences in richness of gut microbiota, flora structure, and flora function between IBS-D and HCs in Southwest China. These differences may be closely related to environmental factors such as eating habits, living habits, and mental and psychological factors.

CLINICAL TRIAL REGISTRATION

The trial was registered and approved in China Clinical Trial Registry (Registration No. ChiCTR2100045751).

Patients with breath test positive are necessary to be identified from irritable bowel syndrome: a clinical trial based on microbiomics and rifaximin sensitivity

BACKGROUND

As a non-invasive and effective diagnostic method for small intestinal bacterial overgrowth (SIBO), wild-use of breath test (BT) has demonstrated a high comorbidity rate in patients with diarrhea-predominant irritable bowel syndrome (IBS-D) and SIBO. Patients overlapping with SIBO respond better to rifaximin therapy than those with IBS-D only. Gut microbiota plays a critical role in both of these two diseases. We aimed to determine the microbial difference between IBS-D overlapping with/without SIBO, and to study the underlying mechanism of its sensitivity to rifaximin.

METHODS

Patients with IBS-D were categorized as BT-negative (IBSN) and BT-positive (IBSP). Healthy volunteers (BT-negative) were enrolled as healthy control. The patients were clinically evaluated before and after rifaximin treatment (0.4 g bid, 4 weeks). Blood, intestine, and stool samples were collected for cytokine assessment and gut microbial analyses.

RESULTS

Clinical complaints and microbial abundance were significantly higher in IBSP than in IBSN. In contrast, severe systemic inflammation and more active bacterial invasion function that were associated with enrichment of opportunistic pathogens were seen in IBSN. The symptoms of IBSP patients were relieved in different degrees after therapy, but the symptoms of IBSN rarely changed. We also found that the presence of IBSN-enriched genera ( Enterobacter and Enterococcus ) are unaffected by rifaximin therapy.

CONCLUSIONS

IBS-D patients overlapping with SIBO showed noticeably different fecal microbial composition and function compared with IBS-D only. The better response to rifaximin in those comorbid patients might associate with their different gut microbiota, which suggests that BT is necessary before IBS-D diagnosis and use of rifaximin.

REGISTRATION

Chinese Clinical Trial Registry, ChiCTR1800017911.

Gut dysbiosis and small intestinal bacterial overgrowth as independent forms of gut microbiota disorders in cirrhosis

BACKGROUND

Gut dysbiosis and small intestinal bacterial overgrowth (SIBO) are commonly observed in patients with cirrhosis. Despite the substantial number of articles describing the relations between disorders of gut microbiota and various manifestations of cirrhosis, dysbiosis and SIBO were always studied separately.

AIM

To study the relationship of gut dysbiosis and SIBO in cirrhosis.

METHODS

This observational study included 47 in-patients with cirrhosis. Stool microbiome was assessed using 16S rRNA gene sequencing. SIBO was assessed using the lactulose hydrogen breath test.

RESULTS

SIBO was found in 24/47 (51.1%) patients. Patients with SIBO had a higher abundance of Firmicutes (P = 0.017) and Fusobacteria (P = 0.011), and a lower abundance of Bacteroidetes (P = 0.013) than patients without SIBO. This increase in the abundance of Firmicutes occurred mainly due to an increase in the abundance of bacteria from the genus Blautia (P = 0.020) of the Lachnospiraceae family (P = 0.047), while the abundance of other major families of this phylum [Ruminococcaceae (P = 0.856), Peptostreptococcaceae (P = 0.066), Clostridiaceae (P = 0.463), Eubacteriaceae (P = 0.463), Lactobacillaceae (P = 0.413), and Veillonellaceae (P = 0.632)] did not differ significantly between the patients with and without SIBO. Reduced level of Bacteroidetes in samples from patients with SIBO was a result of the decrease in bacterial numbers from all the major families of this phylum [Bacteroidaceae (P = 0.014), Porphyromonadaceae (P = 0.002), and Rikenellaceae (P = 0.047)], with the exception of Prevotellaceae (P = 0.941). There were no significant differences in the abundance of taxa that were the main biomarkers of cirrhosis-associated gut dysbiosis [Proteobacteria (P = 0.790), Bacilli (P = 0.573), Enterobacteriaceae (P = 0.632), Streptococcaceae (P = 0.170), Staphylococcaceae (P = 0.450), and Enterococcaceae (P = 0.873)] between patients with and without SIBO.

CONCLUSION

Despite the differences observed in the gut microbiome between patients with and without SIBO, gut dysbiosis and SIBO are most likely independent disorders of gut microbiota in cirrhosis.

Interplay between gut microbiota and bile acids in diarrhoea-predominant irritable bowel syndrome: a review

Irritable bowel syndrome (IBS) is a common functional gastrointestinal disease that disturbs the physiology and psychology of patients and increases the burden on families, the healthcare system, society, and economic development, affecting more and more people around the world. Despite the multiple factors that account for IBS remaining incompletely studied, emerging evidence demonstrated the abnormal changes in gut microbiota and bile acids (BAs) metabolism closely associated with IBS. Moreover, microbiota drives significant modifications for BAs, consisting of deconjugation, 7α-dehydroxylation, oxidation, epimerization, desulfation, esterification, and so on, while BAs, in turn, affect the microbiota directly or indirectly. In light of the complex connection among gut microbiota, BAs, and IBS, it is urgent to review the latest research progress in this field. In this review, we described the disorders of intestinal microecology and BAs profiles in IBS-D and also highlighted the cross-talk between gut microbiota and BAs in the context of IBS-D. Integrating these, we suggest that new therapeutic strategies targeting the microbiota-BAs axis for IBS-D, even for other related diseases caused by bacteria-bile acid dysbiosis should be expected.

Long-Term Follow-Up Results of Fecal Microbiota Transplantation for Irritable Bowel Syndrome: A Single-Center, Retrospective Study

Objective: This study aimed to investigate the long-term efficacy of fecal microbiota transplantation (FMT) in patients with irritable bowel syndrome (IBS).

Study Methods: In this single-center long-term follow-up study, FMT treatment was administered to patients with moderate to severe IBS (IBS severity scoring system (IBS-SSS) > 175). After 1 year of treatment, it was decided whether to repeat FMT based on IBS-SSS score (IBS-SSS > 175). Baseline characteristics before and after FMT and questionnaires were completed at 1, 3, 6, 12, 24, 36, 48, and 60 months after FMT. The study outcomes included treatment efficacy rates, change of IBS-SSS, IBS-specific quality of life and fatigue, effect on stool frequency, Bristol Stool Scale for IBS-C and IBS-D, and side effects.

Results: A total of 227 patients (47.58% IBS-C, 39.21% IBS-D, and 13.22% IBS-M) were recruited (142 females and 85 males with a mean age of 41.89 ± 13.57 years). The efficacy rates were 108 (51.92%), 147 (74.62%), 125 (74.41 %), 88 (71.54%), 78 (75.00%), 65 (73.03%), 45 (61.64%), and 37 (62.71%) at different follow-up time points. The total IBS-SSS score was 321.37 ± 73.89 before FMT, which significantly decreased after 1 month. The IBS-specific quality of life (IBS-QoL) score was 40.24 ± 11.34 before FMT, increased gradually, and was significantly higher at 3 months compared to before FMT. The total Fatigue Assessment Scale (FAS) score was 47 ± 8.64 before FMT and was significantly lower at 3 months. During follow-up, 89 (39.21%) side effects occurred that were alleviated by symptomatic treatment, and no serious adverse events were detected.

Conclusion: Based on 60 months of long-term follow-up, the safety and efficacy of FMT for IBS was established. However, as the treatment effect declines over time, periodic and repetitive FMT is required for a sustained effect.

The effectiveness of rotating versus single course antibiotics for small intestinal bacterial overgrowth

BACKGROUND

Small intestinal bacterial overgrowth treatment is usually based on antibiotics with no guidelines available.

OBJECTIVE

This study aimed to investigate the efficacy of different antibiotics to treat small intestinal bacterial overgrowth.

METHODS

Consecutive patients referred to our tertiary center and diagnosed with intestinal bacterial overgrowth were retrospectively included. Patients were diagnosed using a 75 g glucose breath test. Patients were treated either with a single antibiotic (quinolone or azole) or rotating antibiotics (quinolone and azole, one after the other) for 10 consecutive days per month for 3 months. A negative glucose breath test after antibiotic treatment was considered as remission. Quality of life (GIQLI) and gastrointestinal severity (IBS-SSS) were assessed before and after antibiotic treatment. Symptomatic evaluation was realized in simple blind of glucose breath test result: patients were unaware of their results.

RESULTS

Between August 2005 and February 2020, 223 patients were included in the analysis (female 79.8%, mean age 50.2 ± 15.7 years). Remission was observed in 119 patients (53.4%) after one course of antibiotics and was more frequent in patients receiving rotating antibiotics than in patients receiving a single antibiotic (70.0% vs. 50.8%, p = 0.050). Remission was associated with a significant improvement in quality of life (p = 0.035) and in bloating (p = 0.004).

CONCLUSION

In this study, the treatment of small intestinal bacterial overgrowth using rotating antibiotics was more effective than treatment using a single course of antibiotic. Remission was associated with improvement in both quality of life and bloating.

The relationship between gut microbiota and proteolytic activity in irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common functional gastrointestinal disease that affects 3.8-9.2% of the world population. It affects the physiology and psychology of patients and increases the burden on families, the healthcare system, society, and economic development. Presently, a large number of studies have shown that compared to healthy individuals, the composition and diversity of gut microbiota in IBS patients have changed, and the proteolytic activity (PA) in fecal supernatant and colonic mucosa of IBS patients has also increased. These findings indicate that the imbalance of intestinal microecology and intestinal protein hydrolysis is closely related to IBS. Furthermore, the intestinal flora is a key substance that regulates the PA and is associated with IBS. The current review described the intestinal microecology and intestinal proteolytic activity of patients with IBS and also discussed the effect of intestinal flora on PA. In summary, this study proposed a pivotal role of gut microbiota and PA in IBS, respectively, and provided an in-depth insight into the diagnosis and treatment targets of IBS as well as the formulation of new treatment strategies for other digestive diseases and protease-related diseases.

Clinical efficacy of fecal microbiota transplantation for patients with small intestinal bacterial overgrowth: a randomized, placebo-controlled clinic study

Background

Small intestinal bacterial overgrowth (SIBO) is characterized by the condition that bacteria overgrowth in the small intestine. Fecal microbiota transplantation (FMT) has been applied as an effective tool for reestablishing the structure of gut microbiota. However, whether FMT could be applied as a routine SIBO treatment has not been investigated.

Methods

In this trial, 55 SIBO patients were enrolled. All participants were randomized in two groups, and were given FMT capsule or placebo capsules once a week for 4 consecutive weeks. Measurements including the lactulose hydrogen breath test gastrointestinal symptoms, as well as fecal microbiota diversity were assessed before and after FMT therapy.

Results

Gastrointestinal symptoms significantly improved in SIBO patients after treatment with FMT compared to participants in placebo group. The gut microbiota diversity of FMT group had a significant increase, while placebo group showed none.

Conclusions

This study suggests that applying FMT for patients with SIBO can alleviate gastrointestinal symptoms, indicating that FMT may be a promising and novel therapeutic regimen for SIBO.

Trial registry

This study was retrospectively registered with the Chinese Clinical Trial registry on 2019.7.10 (ID: ChiCTR1900024409, http://www.chictr.org.cn).

Chronic diarrhoea and risk of rheumatoid arthritis: findings from the French E3N-EPIC Cohort Study

Objectives

To assess the relationship between gastrointestinal disorders and the risk of further development of RA.

Methods

The Etude Epidémiologique auprès des femmes de la Mutuelle générale de l’Education Nationale-European Prospective Investigation into Cancer and Nutrition Study is a French prospective cohort including 98 995 healthy women since 1990. Participants completed mailed questionnaires on their lifestyles and health-related information. Gastrointestinal disorders were assessed in the third questionnaire (sent in 1993). Hazard ratios and 95% CIs for incident RA were estimated using Cox proportional hazards regression models with age as the time scale. Models were age adjusted, and then additionally adjusted for known risk factors of RA such as smoking, and for potential cofounders.

Results

Among 65 424 women, 530 validated incident RA cases were diagnosed after a mean (s.d.) of 11.7 (5.9) years after study baseline. In comparison with no gastrointestinal disorder, chronic diarrhoea was associated with an increased risk of developing RA during follow-up (hazard ratio = 1.70, 95% CI 1.13, 2.58), independently of dysthyroidism or dietary habits. The association was stronger among ever-smokers (hazard ratio = 2.21, 95% CI 1.32, 3.70). There was no association between RA risk and constipation or alternating diarrhoea/constipation.

Conclusion

Chronic diarrhoea was associated with an increased risk of subsequent RA development, particularly among ever-smokers. These data fit with the mucosal origin hypothesis of RA, where interaction between intestinal dysbiosis and smoking could occur at an early stage to promote emergence of autoimmunity, followed years later by clinical disease.

Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome - An Update

Small intestinal bacterial overgrowth (SIBO) is one manifestation of gut microbiome dysbiosis and is highly prevalent in IBS (Irritable Bowel Syndrome). SIBO can be diagnosed either by a small bowel aspirate culture showing ≥10³ colony-forming units (CFU) per mL of aspirate, or a positive hydrogen lactulose or glucose breath test. Numerous pathogenic organisms have been shown to be increased in subjects with SIBO and IBS, including but not limited to Enterococcus, Escherichia coli, and Klebsiella. In addition, Methanobrevibacter smithii, the causal organism in a positive methane breath test, has been linked to constipation predominant irritable bowel syndrome (IBS-C). As M. smithii is an archaeon and can overgrow in areas outside of the small intestine, it was recently proposed that the term intestinal methanogen overgrowth (IMO) is more appropriate for the overgrowth of these organisms. Due to gut microbiome dysbiosis, patients with IBS may have increased intestinal permeability, dysmotility, chronic inflammation, autoimmunity, decreased absorption of bile salts, and even altered enteral and central neuronal activity. As a consequence, SIBO and IBS share a myriad of symptoms including abdominal pain, distention, diarrhea, and bloating. Furthermore, gut microbiome dysbiosis may be associated with select neuropsychological symptoms, although more research is needed to confirm this connection. This review will focus on the role of the gut microbiome and SIBO in IBS, as well as novel innovations that may help better characterize intestinal overgrowth and microbial dysbiosis.