Small Intestinal Bacterial Overgrowth and Intestinal Methanogen Overgrowth: Are They Overdiagnosed?

Small intestinal bacterial overgrowth was originally described as a cause of maldigestion and malabsorption, and its diagnosis was based on the culture of aspirates from the upper small intestine. The application of hydrogen breath tests to assess suspected small intestinal bacterial overgrowth increased the frequency of this diagnosis dramatically, and expanded its clinical context from maldigestion to include a number of non-specific gastrointestinal complaints and even some systemic ills. However, a critical review of recent literature suggests that reliance on breath tests, particularly those using lactulose as the substrate, has led to overdiagnosis of small intestinal bacterial overgrowth. Many aspects of the current concept of its diagnosis are contentious and unresolved. These include diagnostic thresholds, response to antibiotic therapy, as well as the roles of methane, hydrogen sulfide, and other gases emanating from bacterial fermentation in the gastrointestinal tract. We discuss these issues, paying particular attention to the concept of intestinal methanogen overgrowth and exploring the role of methanogens and methane in gastrointestinal homeostasis. We advocate a critical and cautious approach to the diagnosis of small intestinal bacterial overgrowth and intestinal methanogen overgrowth, and that antibiotics are prescribed are with great care. We also look forward to the application of modern molecular microbiology to the study of the small intestinal microbiome and metabolome, including a more complete understanding of methane biology.