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).

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. The most common include:

  • 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.

The symptoms of SIBO include:

  • excess gas
  • abdominal bloating and distension
  • diarrhea
  • abdominal pain

A small number of patients with SIBO have chronic constipation rather than diarrhea. When the overgrowth is severe and prolonged, the bacteria may interfere with the digestion and/or absorption of food and deficiencies of vitamins and minerals may develop. Weight loss also may occur. Patients with SIBO sometimes also report symptoms that are unrelated to the gastrointestinal tract, symptoms such as body aches or fatigue. The symptoms of SIBO tend to be chronic. A typical patient with SIBO can experience symptoms that fluctuate in intensity over months, years, or even decades before the diagnosis is made.

Culturing bacteria from the small intestine One method of diagnosing bacterial overgrowth is culturing (growing) the bacteria from a sample of fluid taken from the small intestine. The culturing must be quantitative, meaning that the actual number of bacteria must be determined. Essentially, the bacteria in a known quantity of fluid are counted. Culturing requires a long flexible tube to be passed through the nose, down the throat and esophagus, and through the stomach under x-ray guidance so that fluid can be obtained from the small intestine.

There are several problems with diagnosing SIBO by culturing. Passage of the tube is uncomfortable and expensive, and the skill necessary to pass the tube is not commonly available. The quantitative culturing of intestinal fluid is not a routine procedure for most laboratories, and, therefore, the accuracy of the cultures is questionable. Finally, with the tube only one, or at most a few, locations of the small intestine can be sampled. Usually it is the duodenum. It is possible that the overgrowth involves just the jejunum or ileum, and overgrowth may be missed if only the duodenum is sampled. Because of all these potential problems, quantitative culturing for intestinal bacteria usually is utilized only for research purposes.

Hydrogen breath test (HBT) Bacteria that live in the colon are capable of digesting and using sugars and carbohydrates as food. When the bacteria normally present in the colon digest sugars and carbohydrates, they produce gas, most commonly carbon dioxide, but also smaller amounts of hydrogen and methane. (The types of bacteria normally found in the esophagus, stomach, and small intestine produce little gas.) Most of the sugars and carbohydrates that we eat are digestible and are digested and absorbed in the small intestine, never reaching the colonic bacteria. Moreover, more than 80% of the gas that is produced by bacteria in the colon is used up by other bacteria within the colon. As a result, relatively little of the gas that is produced remains in the colon, and it is eliminated as flatus (farts). Although the overwhelming majority of the hydrogen and methane produced by colonic bacteria is used up by other bacteria, small amounts of these gases are absorbed through the lining of the colon and into the blood. The gases circulate in the blood and go to the lungs, where they are eliminated in the breath. The gases can be measured in the breath with special analyzers (usually a gas chromatograph).

There are several limitations to the hydrogen breath test for the diagnosis of SIBO. Hydrogen breath test with lactulose may be able to diagnose only 60% of patients with SIBO, and glucose may be only slightly better. Since glucose is absorbed completely before it completes passage through the small intestine, it may not be able to diagnose SIBO of the distal small intestine (ileum). A major problem is that there is no “gold standard” for the diagnosis of SIBO since culture of the bacteria has its own limitations, as discussed previously. Without a testing gold standard, it is difficult to know just how good hydrogen breath test is for the diagnosis of SIBO.