The stomach and the upper regions of the small intestine usually don't have many bacteria growing there. For instance, samples of small intestinal fluids usually contain less than 10,000 microorganisms/mL. This makes sense since the small intestine, filled with bile and enzymes, is pretty inhospitable. Also, a lot of bacteria in the small intestine would lead to competition for the nutrients that are absorbed. The large intestine, on the other hand, can contain up to 10 trillion microbes per gram of intestinal content. This is useful because these microbes break down indigestible fibers, produce short-chain fatty acids that fuel the cells of the colon, and train immune system cells.
What happens when bacteria proliferate in regions they’re not supposed to be in? In a disorder known as “small intestinal bacterial overgrowth” (SIBO), the small intestines of affected individuals have higher concentrations of bacteria, relative to people without the disorder.
The excess bacteria seem to decrease the amount of nutrients absorbed from food, damage the lining of the small intestine, and produce toxins that can pass into the blood. Many of the symptoms of SIBO, such as nausea, abdominal pain, and altered bowel habits, are similar to irritable bowel syndrome (IBS), and some people with IBS may be likely to have SIBO.
But unlike IBS, SIBO can be diagnosed using biomarkers. One commonly used method is to administer certain carbohydrates to a patient and measure hydrogen breath levels over a period of time, a process depicted in Figure 1. The gold standard for diagnosis is to take a sample from the small intestine and measure the concentration of bacteria in that sample. However, this is somewhat invasive, so it’s not used as often.
Researchers believe that the factors that may lead to SIBO include gastrointestinal infections, disrupted motility of the small intestine, or low production of stomach acid. Treatments typically involve the use of antibiotic drugs to reduce bacterial counts.
Reference: Kerlin & Wong. Gastroenterology. 1998 Oct.