Physician Introduction on Small Intestinal Bacterial Overgrowth (SIBO)
Presented by Darren Brenner, MD
In the past, it has been argued that the body contains more microbial than human cells, but more recent research has debated this argument. In either case, trillions of microbes reside in the human body. The microbiota of the gastrointestinal tract is more diverse in adulthood and much of the growth occurs within the first three years of life. The relationship between human cells and organisms in the gastrointestinal (GI) tract is mostly symbiotic, implying that each provides evolutionary benefits to the other. Less organisms are identified in the stomach than the colon. The types of organisms also change shifting from aerobes (organisms which require oxygen for growth) to anaerobes (organisms which do not require oxygen for growth or may die if oxygen is present). In most cases, a level of balance is achieved between human cells and microbes.
Small intestinal bacterial overgrowth, also known as SIBO, occurs when higher concentrations of colonic bacteria are identified in the small intestine. In some cases, this leads to symptoms, and in others, the symptoms may be absent. Another classification was added called intestinal methanogenic overgrowth, or IMO. It is characterized by overpopulation of methanogens (methane producing organisms) in the GI tract. These organisms are not bacteria but archaea—single celled organisms, the most common of which is Methanobrevibactor smithii. This organism can live the in colon and in many instances is not found in the small intestine. Thus, the terms “small intestinal” and “bacteria” appear inaccurate and justify the addition of IMO.. The incidence and prevalence of SIBO and IMO remain unknown.
There are multiple factors which protect against the development of intestinal overgrowth. Gastric acid can kill bacteria in the stomach and small intestine. Bile acids and pancreatic enzymes created in the liver and pancreas can also digest and destroy bacteria. Motility blocks bacterial movement upstream from the colon to the small intestine. The immune system also blocks colonization of bacteria in the small intestine.
Intestinal overgrowth occurs when one or more of these protective barriers is damaged or reduced. This occurs with disorders or medications that reduce acid production, slow gut motility, or damage to the organs (liver, pancreas) responsible for producing protective enzymes. Intestinal overgrowth can also occur when the anatomy of the GI tract is altered. Bacteria can reside and replicate in areas that remain unexposed to the protective factors. In these situations, the risk for developing symptoms increases.
The most common symptom associated with intestinal overgrowth is bloating. Other common symptoms include abdominal distention (most individuals will endorse looking 6 to 9 months pregnant), abdominal pain or discomfort, nausea, changes in bowel habits (diarrhea or constipation), and increased gas passage. In severe cases of overgrowth, maldigestion and/or fat, carbohydrate, and protein malabsorption may occur leading to increased gas-related symptoms, diarrhea and potentially a loss of fat-soluble vitamins, vitamin B-12 and iron. Newer (but small) observational studies have found relationships between the production of specific gases and increased or decreased intestinal motility. Individuals with increased intestinal production of hydrogen sulfide appear to experience increased diarrhea whereas methanogenic overgrowth is more so associated with constipation.
Two diagnostic tests are most commonly used to identify intestinal overgrowth. The gold standard is a measurement of the concentration of bacteria in the small intestine. This test requires that an upper endoscope be advanced into the distal duodenum with fluid samples collected in a sterile manner. This test can be difficult and expensive, is only available at a few academic centers, and if not performed sterilely, can result in large numbers of inaccurate tests. Most individuals will undergo breath testing by consuming a sugar solution with a collection of breath samples over the course of two hours. Since we do not produce hydrogen, methane, or hydrogen sulfide, these gases can be identified in exhaled breath samples and show overgrowth.
There are few evidence-based therapies available to treat intestinal overgrowth. Antibiotics are the most evidence-based with the strongest data supporting the use of rifaximin for treating SIBO, and a combination of rifaximin and neomycin for treating IMO. This combination should be used with caution since neomycin has been associated with kidney damage and hearing loss, and retreatment may be required. Studies utilizing diet modification, probiotics, and other natural substances are ongoing. Multiple non-evidence-based supplements are being purported by naturopaths to treat intestinal overgrowth, but most are not validated with supported clear evidence.
About Dr. Brenner
Dr. Brenner is an Associate Professor of Medicine and Surgery in the Division of Gastroenterology at Northwestern University and serves as Director of the Neurogastromotility, Co-Director of the Integrated Bowel Dysfunction program, and Director of the Motts Tonelli GI Physiology Laboratory. He is also an active Irene D. Pritzker Research Scholar.
Dr. Brenner focuses his clinical and research pursuits on a wide range of motility topics including irritable bowel syndrome (IBS), constipation, opioid related constipation, fecal incontinence, and scleroderma. He has published more than 150 articles, abstracts, and online materials on these subjects, and has lectured both nationally and internationally in these areas. He acts as a reviewer and editor for multiple gastrointestinal (GI) peer-reviewed journals and is a current associate editor of the American Journal of Gastroenterology. He was a charter board member of the American Gastroenterological Association Academy of GI and Liver Educators, acts as a member of the advisory board for the International Foundation for Gastrointestinal Disorders (IFFGD) and has served on multiple ACG committees (education, MOC, digital publications and communications). Dr. Brenner is a fellow of the American College of Gastroenterology (ACG), American Gastroenterological Association, and Rome Foundation. He has been appointed to the peer perspective board of Helio Gastroenterology and was named to the Helio 200 top innovators in gastroenterology and hepatology and Best Doctors in America lists.
In his free time, Dr. Brenner is an avid practitioner of Shotokan karate, holds a 3rd degree black belt in this art and will be serving as head coach of the US Junior National Karate Team at the World Maccabiah Games in Israel in 2022.