Small Intestinal Bacterial Overgrowth
(SIBO)
Medical Author: Dennis Lee, MD
Medical Editor: Jay W. Marks, MD
What is small intestinal bacterial overgrowth (SIBO)?
The small bowel, also known as the small intestine, is the
section of the
gastrointestinal tract
that connects the stomach with the colon. The main purpose of the small
intestine is to digest and absorb food into the body. The small intestine is
approximately 21 feet in length and begins in the duodenum (into
which food from the stomach empties), followed by the jejunum, and then the
ileum (which empties the food that has not been digested into the large
intestine or colon).
The entire gastrointestinal tract, including the small
intestine, normally contains bacteria. The number of bacteria is greatest in the
colon (at least 1,000,000,000 bacteria per milliliter (ml) of fluid) and much
lower in the small intestine (less than 10,000 bacteria per ml of fluid). Moreover,
the types of bacteria within the small intestine are different than the types of
bacteria within the colon. Small intestinal bacterial overgrowth (SIBO) refers
to a condition in
which abnormally large numbers of bacteria (at least 100,000 bacteria per ml of
fluid) are present in the small intestine and the types of bacteria in the small
intestine resemble more the bacteria of the colon than the small intestine.
Small intestinal bacterial overgrowth (SIBO) is also known as small bowel
bacterial overgrowth (SBBO).
What causes small intestinal bacterial overgrowth?
The gastrointestinal tract is a continuous muscular tube
through which
digesting food is transported on its way to the colon. The coordinated activity
of the muscles of the stomach and small intestine propels the food from the stomach,
through the small intestine, and into the colon. Even when there is no food in the
small intestine, muscular activity sweeps through the small intestine from the stomach
to the colon.
The muscular activity that sweeps through the small intestine is important for
the digestion of food, but it also is important because it sweeps bacteria out
of the small intestine and thereby limits the numbers of bacteria in the small
intestine. Anything that interferes with the progression of normal muscular activity
through the small intestine can result in SIBO. Simply stated, any condition
that interferes with muscular activity in the small intestine allows the bacteria to
stay longer and multiply in the small intestine. The lack of muscular activity also
may allow bacteria to spread backwards from the colon and into the small
intestine.
Many conditions are associated with SIBO. A few are common.
- Neurologic and muscular diseases can alter the normal activity of the intestinal muscles. Diabetes
mellitus damages the nerves that control the intestinal muscles.
Scleroderma
damages the intestinal muscles directly. In both cases, abnormal muscular
activity in the small intestine allows SIBO to develop.
- Partial or intermittent obstruction of the small intestine interferes with the transport of food and
bacteria through the small intestine and can result in SIBO. Causes of obstruction
leading to SIBO include adhesions (scarring) from previous surgery and
Crohn's
disease.
- Diverticuli (out-pouchings) of the small intestine that allow
bacteria to multiply inside diverticuli.
Next: What are small intestinal bacterial overgrowth symptoms? »
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