Small Bowel Bacterial Overgrowth

Small bowel bacterial overgrowth (SBBO) also called small intestinal bacterial overgrowth (SIBO) occurs when there is abnormal growth and accumulation of bacteria in the small intestine. Normally, there are relatively few bacteria in the small intestine, especially when compared to the colon (large intestine), which normally harbors large numbers of bacteria. The small intestine is where food and nutrients are absorbed into the body. Excessive numbers of bacteria there can themselves digest the nutrients, leaving less for human absorption.  Also as a consequence of bacterial metabolism, certain breakdown products are released by the bacteria that can cause symptoms for the human host.


Normally, the small intestine is quite active propelling contents (peristalsis) after eating. Later and often during sleep, spontaneously occurring peristaltic motion serves to clean out residual particles and debris.  Anything that causes disruption of this normal activity or otherwise restricts intestinal flow can lead to stasis, allowing bacteria to flourish on the residual intestinal contents and leading to bacterial overgrowth.

Altered intestinal peristalsis can occur due to an effect of bowel surgery, several disease states, a side effect of medications, or develop without any apparent underlying cause. Surgery that bypasses intestinal segments and creates a blind loop (for example, done to treat ulcers or for weight loss) or creates intestinal pouches (Crohn’s disease) or leaves areas of obstruction or narrowing (for example, adhesions) may lead to SBBO. Medical conditions such as diabetes and scleroderma can affect peristalsis. Medications such as narcotics slow bowel motility. Some patients with irritable bowel syndrome have SBBO with no identified reason.


When SBBO causes symptoms, they often include: abdominal bloating, abdominal discomfort, nonbloody diarrhea, and in severe cases weight loss.

SBBO may cause or contribute to Irritable Bowel Syndrome (IBS) in some patients. Many of the symptoms of small bowel bacterial overgrowth are consistent with those of irritable bowel syndrome. An interesting observation is that there are a variety of intestinal bacteria that can predominate in patients with SIBBO. Different bacterial strains may produce different byproducts of their metabolism each of which can elicit different symptoms. For example, some bacteria produce methane, which can slow colonic motion and lead to constipation. Also, the quantity of bacterial overgrowth can wax and wane with time. This variation of bacterial types and quantity in patients may account for the fluctuating and alternating symptoms often seen in IBS.


The diagnosis can sometimes be made based on symptoms and the medical history.

To detect SBBO, the most often utilized test is hydrogen breath testing.  The patient breathes into a device that measures the amount of hydrogen in the breath. A sugar that is not absorbable by humans (usually lactulose is used) is ingested and repeated measurements of breath hydrogen are done over time.  Bacteria in the GI tract will digest the sugar and release hydrogen, which is then absorbed and exhaled. Normally, this does not occur until the sugar reaches the colon where bacteria are present.  In SBBO, the presence of numerous bacteria in the small intestine leads to rapid digestion and release and an early peak of exhaled hydrogen.


The mainstay of treatment of small bowel bacterial overgrowth is antibiotic therapy. Treatment should be addressed at improving the underlying disease when possible but more often than not is not possible, not practical, or not very effective.

Nonsurgical conditions associated with intestinal stasis should be corrected when possible. This may include the elimination or substitution of drugs known to decrease intestinal motility (mainly narcotics) and optimizing treatment of diabetes

In cases of sluggish motility, such as longstanding diabetes and chronic intestinal pseudo-obstruction, methods to enhance motility can be attempted. Prokinetic agents that stimulate intestinal peristalsis including metoclopramide (Reglan), domperidone (Motilium), and erythromycin can be tried in some cases.

Antibiotic therapy is the most often recommended treatment. Many different antibiotics have been used with varying success. Often, rifaxamin (Xifaxan) is used as this antibiotic seems to target the gut bacteria and is not absorbed into the blood stream, limiting potential side effects.  Some patients will be treated with one course of antibiotics, others may require intermittent treatments.

Probiotics (beneficial bacteria) can be tried in an attempt to replace the symptom causing bacterial strains with nonsymptom causing ones.