The small intestinal microflora are usually limited in number, less than 105 colony-forming units per mL of intestinal fluid. This number is somewhat arbitrary, because of the intense reluctance of most intestinal bacteria to grow in culture.
There are many reasons why colonic bacteria should grow up into the intestine and proliferate to produce small intestinal bacterial overgrowth (SIBO).
These often produce stagnation of bacteria, which leads to their overgrowth, both because of lack of normal pushing down of the bacteria and because food is retained for too long a duration
Failure of normal gut movements encourages food stagnation and bacterial proliferation. This may be due to diverse causes including:
Evidence is yet to accumulate as to whether this is a cause or effect of SIBO. Methane production on breath tests are found to be strongly linked with a subgroup of IBS patients who complain of constipation, who also have weak and infrequent MMC. The role of proton pump inhibitors in this association with IBS is yet to be tested, though it is well known that hypochlorhydria contributes to SIBO.
In elderly patients, SIBO is probably caused by a reduction in small intestinal dysmotility, though hypochlorhydria and immune cellular deficiency may play subsidiary roles.
In chronic pancreatitis, there is mucosal inflammation, which results in reduced intestinal movements, besides the dysmotility induced by the narcotic analgesics typically prescribed in this condition. In scleroderma, about half of patients have been found to have SIBO, especially if they have symptoms related to the gut.
Complications of such overgrowth include:
The two most common underlying factors include too little gastric acid and less-than-normal intestinal motility.
Hydrochloric acid in the stomach is capable of killing most organisms that enter the stomach from the oropharynx. Thus, bacteria are rarely found in the stomach and upper small intestine. Hypochlorhydria is a risk factor and is often associated with Helicobacter pylori colonization or with aging, though SIBO itself may cause false-positives for H. pylori due to its reaction with urea on testing. Proton pump inhibitors, such as omeprazole and histamine 2 receptor blockers, may also promote SIBO when used on a long-term basis, such as 4 weeks.
Motility of the small bowel is maintained by propulsive forces created by the circular and longitudinal smooth muscle of the small bowel wall. These form MMCs, which push food down the digestive tract rapidly and prevent bacteria from colonizing the upper small intestine.
Other factors include: