Small Intestinal Bacterial Overgrowth (SIBO)
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Small Intestinal Bacterial Overgrowth (SIBO) means an abnormally high number of bacteria in the small intestine. Symptoms include:
- diarrhea
- weight loss
- anemia
- malabsorption
It seems likely to develop in patients with diverticula, stenosis, blind loops, or motor disorders in the gut. It's also highly related to Irritable Bowel Syndrom (IBS).
The stomach is almost sterile, so bacteria count increases as we move further along the gastrointestinal tract. The small bowel flora is mainly Gram positive bacteria. Strict anaerobes are rare. The ileocecal value acts as a barrier between small and large intestines (potential point of dysfunction). Here, Gram negative aerobes predominate and strict anaerobes are common. In the colon, anaerobes outnumber aerobes. See chart below for summary.
| Location | Composition |
|---|---|
| Proximal small intestine | Gram positive >>> strict anaerobes |
| Distal small intestine | Gram negative > strict anaerobes |
| Large intestine | Anaerobes > aerobes |
Gram positive small intestinal bacterial overgrowth is a "frequent" finding in healthy elderly people and has not been correlated with symptoms of SIBO.
Gram negative and strict anaerobe small intestinal bacterial overgrowth is another story. We might call these "colonic type" bacteria. They have the ability to deconjugate bile salts, affect the binding capacity of intrinsic factor (required for vitamin B12 absorption), and reduce the absorptive function of enterocytes.
The normal upper small bowel contains less than 10^4 colony forming bacteria per milliliter of intestinal fluid. Most of these bacteria are Gram positive aerobes. To define SIBO, the count has to be higher and the composition has to be wrong. Therefore, we define SIBO as > 10^5 colonic-type bacteria in the small intestine.
This figure is from Simren & Stotzer, 2006.
Testing
See my other post on testing for SIBO.