SIBO, or small intestinal bacterial overgrowth, happens when bacteria that normally live in your large intestine migrate into your small intestine and multiply there in abnormally high numbers. A healthy small intestine contains relatively few bacteria, typically around 10,000 organisms per milliliter in the middle sections. In SIBO, that count can skyrocket to as high as one billion organisms per milliliter, disrupting digestion and nutrient absorption in the process.
Which Bacteria Are Involved
SIBO isn’t caused by a single “bad” bacterium. It involves an overgrowth of species that are perfectly normal residents of your large intestine but cause problems when they set up camp in the small intestine, where they don’t belong in large numbers. The most commonly identified organisms include Escherichia coli, Klebsiella, and Enterococcus, along with anaerobic species like Bacteroides, Clostridium, Fusobacterium, and Bifidobacterium. Multiple species typically coexist at once, which is part of what makes SIBO tricky to treat.
These bacteria ferment carbohydrates that arrive in the small intestine, producing gases as a byproduct. They also consume proteins and vitamin B12 that your body needs, and they break down bile salts that are essential for digesting fats. The result is a cascade of digestive problems that can range from mild bloating to significant malnutrition.
Three Subtypes Based on Gas Production
Not all overgrowth looks the same. The type of gas produced by the overgrown organisms determines which subtype you have, and each one tends to cause different symptoms.
Hydrogen-dominant SIBO is the classic form. Bacteria like E. coli, Klebsiella, and Enterococcus ferment carbohydrates and release excess hydrogen gas. This subtype is more closely linked to diarrhea, bloating, and abdominal pain.
Methane-dominant overgrowth (now called IMO) involves a different class of organism entirely. The main culprit is Methanobrevibacter smithii, which is technically an archaeon, not a bacterium. Archaea are single-celled organisms that are structurally distinct from bacteria. M. smithii feeds on the hydrogen produced by other microbes and converts it into methane gas, which slows intestinal movement. This is why methane-dominant overgrowth is strongly associated with constipation. Because it can occur in the large intestine as well as the small intestine, experts now call it “intestinal methanogen overgrowth” (IMO) rather than methane-SIBO.
Hydrogen sulfide-dominant SIBO involves species like Desulfovibrio, Fusobacterium, Citrobacter, Prevotella, and Proteus, which produce hydrogen sulfide gas. This subtype correlates with diarrhea and symptoms resembling diarrhea-predominant IBS.
Why Bacteria Overgrow in the Small Intestine
Your body has several built-in defenses to keep bacterial counts low in the small intestine. The most important is the migrating motor complex, or MMC, a pattern of wave-like muscle contractions that sweeps through your gut between meals. Think of it as a self-cleaning cycle. It activates roughly every 90 minutes to two hours during fasting, pushing food residue and stray bacteria down toward the large intestine where they belong.
When the MMC is sluggish or infrequent, food waste and bacteria sit in the small intestine longer than they should, creating the conditions for overgrowth. Most people with SIBO have some degree of MMC dysfunction. Anything that disrupts gut motility, from food poisoning to abdominal surgery to certain neurological conditions, can impair this cleaning wave.
Other risk factors include long-term use of acid-suppressing medications (proton pump inhibitors), which can nearly triple the risk of developing SIBO by reducing the stomach acid that normally kills bacteria before they reach the small intestine. Structural issues like surgical removal of the ileocecal valve (the one-way gate between the small and large intestine), intestinal adhesions from surgery, and conditions that slow the gut, such as diabetes or hypothyroidism, also raise risk.
Symptoms and Nutritional Consequences
The hallmark symptoms of SIBO are bloating, gas, and abdominal pain, often within an hour or two of eating. Depending on the subtype and severity, you may also experience diarrhea, constipation, or an alternating mix of both. Nausea, indigestion, fatigue, and unintentional weight loss are common. Some people notice their stool becomes oily, foul-smelling, or floats, which signals that fats aren’t being properly absorbed.
The nutritional impact can be significant over time. Because the overgrown bacteria consume vitamin B12 before your body can absorb it, B12 deficiency is one of the most common consequences. This can lead to nerve problems, brain fog, and anemia. The bacteria also break down bile salts needed to digest fats, leading to poor absorption of fat-soluble vitamins (A, D, E, and K) and calcium. Long-term calcium malabsorption raises the risk of osteoporosis and kidney stones. Interestingly, folate levels often run high in SIBO because the bacteria themselves produce it.
How SIBO Is Diagnosed
The standard non-invasive test is a breath test. You drink a sugar solution (either glucose or lactulose), then blow into collection tubes at regular intervals over 90 minutes to two hours. The bacteria in your small intestine ferment the sugar and release gases that travel through your bloodstream to your lungs, where they can be measured in your breath.
A rise in exhaled hydrogen of at least 20 parts per million above your baseline within 90 minutes points to hydrogen-dominant SIBO. For methane, a reading of 10 ppm or higher at any point during the test indicates methanogen overgrowth. Hydrogen sulfide testing is newer and less widely available but follows a similar principle.
Treatment and Symptom Management
The primary treatment for hydrogen-dominant SIBO is a targeted antibiotic called rifaximin, which works mostly inside the gut and has minimal systemic side effects. Treatment courses typically last 7 to 14 days. For methane-dominant overgrowth, a second antibiotic is often added because archaea don’t respond to rifaximin alone. Recurrence is common, particularly if the underlying cause (like impaired motility) hasn’t been addressed.
Diet plays an important supporting role. A low-FODMAP diet, which limits certain fermentable carbohydrates that feed gut bacteria, reduces symptoms in up to 86% of people with SIBO or IBS. The approach works by starving the overgrown organisms of their preferred fuel, which can help bring down abnormally high bacterial levels in the small intestine. It’s typically used alongside antibiotics rather than as a standalone cure, and it’s meant to be temporary. After a restriction phase, foods are gradually reintroduced to identify individual triggers.
Because MMC dysfunction is central to most SIBO cases, spacing meals at least four to five hours apart gives the cleaning wave time to activate between eating. Frequent snacking keeps the MMC suppressed, which allows bacteria to linger in the small intestine. Addressing the root cause of impaired motility, whether it stems from a prior gut infection, medication side effects, or another condition, is key to preventing relapse.