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Small bowel (intestinal) bacterial overgrowth

The gut is full of organisms with which we share our bodies, but normally most of the organisms in the alimentary tract are found in the mouth and in the large intestine. There are approximately 10 trillion bacteria in the large bowel, with concentrations of bacteria of billions per ml, but the small bowel is usually relatively free from organisms with perhaps only 1000 bacteria per ml in the ileum and 100 organisms per ml in the jejunum. The colonic bacteria metabolise and ferment any residue left after the food has been digested, but overgrowth of bacteria within the small bowel leads to fermentation before food is properly digested and absorbed. This can lead to symptoms of bloating, belching, abdominal pain and loose stools.


Small intestinal bacterial overgrowth often arises as a result of other abnormalities within the gut. Abnormalities in metabolism or motility can give ingested bacteria the opportunity to grow in parts of bowel where normal gut function and immune surveillance would prevent them from establishing significant colonisation. Common scenarios in which SIBO arises include people on long-term  proton pump inhibitors, where the reduction in gastric acid  means bacteria that would normally be killed before reaching the small bowel survive and then are able to establish bacterial communities in the jejunum and ileum. Abnormalities in motility, as arises in  conditions like scleroderma and connective tissue disease ( such as Ehlos Danlos syndrome) may also predispose people to small intestinal bacterial overgrowth.


Small intestinal bacterial overgrowth can be diagnosed by hydrogen and methane breath testing. It is often treated with a combination of dietary intervention and antibiotics. Dietary intervention involves avoidance of foods that promte bacterial fermentation. In those where predisposing factors remain overgrowth may of course recur and require further treatment, but repeated or longterm antibiotic courses should not be undertaken without careful consideration because of the potential impact on the rest of the microbiome and the development of bacterial resistance to antibiotics.

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