Small Intestinal Bacterial Overgrowth (SIBO) is a condition characterized by an excessive amount of bacteria in the small intestine, a part of the digestive tract that typically contains relatively few microorganisms. These bacteria, when overgrown, ferment carbohydrates and produce gases, primarily hydrogen and methane. This article explores the distinct types of SIBO and addresses whether both hydrogen and methane-producing overgrowths can coexist.
Understanding Individual SIBO Types
SIBO can manifest in different forms based on the type of gas produced by the overgrown microbes. Hydrogen-dominant SIBO occurs when bacteria such as E. coli and Klebsiella produce hydrogen gas. This type is commonly associated with symptoms like diarrhea, bloating, abdominal discomfort, and gas. The excess hydrogen can lead to increased stool frequency and liquidity.
Conversely, methane-dominant SIBO, more accurately termed Intestinal Methanogen Overgrowth (IMO), involves an overgrowth of methane-producing microorganisms called archaea, rather than bacteria. The primary archaea implicated in IMO is Methanobrevibacter smithii. IMO is strongly linked to constipation because methane gas slows down intestinal transit time. Individuals with IMO often experience chronic constipation, bloating, and abdominal distension.
When Both Gases Are Present
It is possible for both hydrogen and methane-producing overgrowths to exist simultaneously in the small intestine, a common presentation often referred to as “mixed SIBO.” This co-occurrence can lead to a combination of symptoms. For instance, individuals might report alternating patterns of diarrhea and constipation, alongside severe bloating or abdominal pain.
The coexistence of hydrogen and methane overgrowths can arise because hydrogen-producing bacteria can provide the hydrogen gas that methane-producing archaea utilize. Archaea consume hydrogen to produce methane, which can reduce hydrogen levels but create its own set of issues.
Identifying Mixed SIBO
Breath testing is the primary non-invasive method used to diagnose SIBO, including cases where both hydrogen and methane are present. During this test, a patient ingests a sugar solution, typically lactulose or glucose. Breath samples are then collected at regular intervals, often every 15 to 20 minutes for up to three hours, to measure the levels of exhaled hydrogen and methane gases.
Clinicians interpret these results by looking for a rise in hydrogen gas of 20 parts per million (ppm) or more from baseline within 90 minutes, or a methane gas level of 10 ppm or more at any point during the test. The simultaneous elevation of both gases confirms the presence of mixed SIBO, guiding targeted treatment.
Managing Combined SIBO
Managing mixed SIBO involves a comprehensive strategy that addresses both the hydrogen and methane-producing microorganisms. Specific antibiotics are commonly used to reduce the bacterial and archaeal populations. Rifaximin is often prescribed for hydrogen-dominant SIBO, while methane-producing overgrowth may require additional antibiotics like neomycin or metronidazole, often used in combination with rifaximin.
Beyond antibiotic therapy, dietary modifications play a supportive role. A low-FODMAP diet, which restricts certain fermentable carbohydrates, can help reduce the substrate available for microbial fermentation, thereby alleviating symptoms. Prokinetics, medications that promote gut motility, may be recommended to improve the small intestine’s cleansing waves and prevent bacterial stasis. Individualized treatment plans developed with professional guidance are important for effective management.