The small bowel, also known as the small intestine, is part of the digestive system that connects the stomach with the large bowel or colon. The main purpose of the small intestine is to digest and absorb food into the body. The small bowel is divided into three parts: the duodenum (which food from the stomach empties into), the jejunum and the ileum (which empties undigested food 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 and much lower in the small intestine. 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 are present in the small intestine, and the types of bacteria found in the small intestine is more like the bacteria found in the colon.

What causes small intestinal bacterial overgrowth?

Disorders that affect one or more of the protective mechanisms can lead to SIBO. In general, SIBO can be divided into categorical causes as follows:

Functional and motility disorders — The small bowel has a mechanism for cleansing the small bowel of debris called the migrating motor complex.  This system can be disrupted at times in patients with irritable bowel syndrome, narcotic use, intestinal pseudo-obstruction, diabetes, patients with a history of radiation therapy to the abdomen, and scleroderma.

Anatomic disorders — Anatomic abnormalities can lead to SIBO by causing stasis. Anatomic disorders associated with SIBO include adhesions from previous surgery; strictures due to radiation, inflammatory bowel disease, or tumors of the small bowel; small intestinal diverticulosis; blind intestinal loops; reversed segments; and gastric bypass for the treatment of obesity.

What are small intestinal bacteria overgrowth symptoms?

The symptoms of SIBO include:

  • Excess flatulence
  • Abdominal bloating
  • Diarrhea
  • Constipation
  • Abdominal pain 

What is the normal relationship between bacteria and the small intestine?

At birth, there are no bacteria present in the gastrointestinal tract. During birth, however, bacteria from the mother’s colon and vagina are swallowed by the baby and within a few weeks or months, they populate the baby’s gastrointestinal tract.

The relationship between normal intestinal bacteria and the human body is complex. Each benefits from the other. The bacteria benefit from the warm, moist environment of the small intestine that is ideal for growing as well as the constant flow of food passing down the gastrointestinal tract. The human body benefits in several ways from normal bacteria:

  • They stimulate the growth of the intestinal lining and the immune system of the intestine.
  • They prevent the growth of disease-causing bacteria within the intestine.
  • They produce vitamin K, which is absorbed and used by the host.
  • They improve the muscular activity of the small intestine.

The gastrointestinal tract, particularly the small intestine, contains an extensive immune system. The immune system protects the intestine from disease-causing viruses, bacteria and parasites. Somehow, the intestine only attacks harmful bacteria. It appears to become tolerant of the normal bacteria and does not mount an attack against them.

How does small intestinal bacterial overgrowth cause symptoms?

When bacteria digest food in the intestine, they produce gas. The gas can accumulate in the abdomen giving rise to abdominal bloating or distension. Distension can cause abdominal pain. This increased amount of gas causes wind or flatulence.  The bacteria are believed to convert food into substances that are irritating or toxic to the cells of the inner lining of the small intestine and colon. These irritating substances produce diarrhea. There is also some evidence that the production of methane gas by the bacteria may cause constipation.

How is SIBO treated?

Dietary manipulation — Dietary manipulation may assist in the treatment of SIBO. The primary goal is to provide a diet that consists of nutrients readily absorbed, thus leaving fewer calories for bacterial metabolism. As carbohydrates are the primary nutritional source for bacteria, diets should be modified to reduce nonabsorbed carbohydrates.

Antibiotic therapy — Most patients with SIBO require treatment with antibiotics. The goal of therapy is to reduce (rather than eradicate) the flora, leading to symptomatic improvement. Recommended antibiotic regimens reflect the predominant organisms associated with SIBO and cover both aerobic and anaerobic enteric bacteria.