Small intestinal bacterial overgrowth

Small intestinal bacterial overgrowth (SIBO), also termed bacterial overgrowth, or small bowel bacterial overgrowth syndrome (SBBOS), is a disorder of excessive bacterial growth in the small intestine. Unlike the colon (or large bowel), which is rich with bacteria, the small intestine usually has fewer than 100,000 organisms per millilitre. Patients with SIBO typically develop symptoms which may include nausea, bloating, diarrhea, constipation, malnutrition, weight loss, brain fog, and confusion, and malabsorption by various mechanisms. The diagnosis of SIBO is made by several techniques, with the gold standard being an aspirate from the jejunum that grows more than 105 bacteria per millilitre. Risk factors for the development of SIBO include dysmotility; anatomical disturbances in the bowel, including fistulae, diverticula and blind loops created after surgery, and resection of the ileo-cecal valve; gastroenteritis-induced alterations to the small intestine; and the use of certain medications, including proton pump inhibitors. SIBO is treated with an elemental diet or antibiotics, which may be given cyclically to prevent tolerance to the antibiotics, sometimes followed by prokinetic drugs to prevent recurrence if dysmotility is a suspected cause.

Definition SIBO may be defined as an increased number of bacteria measured via exhaled hydrogen and/or methane gas following the ingestion of glucose or lactulose, or via analysis of small bowel aspirate fluid. Nevertheless, as of 2020, the definition of SIBO as a clinical entity lacks precision and consistency; it is a term generally applied to a clinical disorder where symptoms, clinical signs, and/or laboratory abnormalities are attributed to changes in the numbers of bacteria or the composition of the bacterial population in the small intestine. The main obstacle to accurately defining SIBO is limited understanding of the normal intestinal microbial population. Future advances in sampling technology and techniques for counting bacterial populations and their metabolites should provide much-needed clarity.

Methane-dominant SIBO The archaeon Methanobrevibacter smithii has been associated with symptoms of SIBO, which result in a positive methane breath test. In addition to the archaeon, a few bacteria can also produce methane, such as members of the Clostridium and Bacteroides genus. Production of methane, therefore, may not be bacterial, nor limited to the small intestine, and it has been proposed that the condition should be classified as a separate 'intestinal methanogen overgrowth' (IMO).

Signs and symptoms Symptoms traditionally linked to SIBO include bloating, diarrhea, constipation, and abdominal pain/discomfort. Steatorrhea may be seen in more severe cases. SIBO can cause a variety of symptoms, many of which are also found in other conditions, making the diagnosis challenging at times. Many of the symptoms are due to malabsorption of nutrients due to the effects of bacteria, which either metabolize nutrients or cause inflammation of the small bowel, impairing absorption. The symptoms of SIBO include nausea, flatus, constipation, bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness. SIBO also causes an increased permeability of the small intestine. Some patients may lose weight. Children with SIBO may develop malnutrition and have difficulty attaining proper growth. Steatorrhea, a sticky type of diarrhea where fat is not properly absorbed and spills into the stool, may also occur. People with long-term SIBO may develop complications as a result of malabsorption of nutrients. Blood tests may show increased level of folate (vitamin B9). Less commonly, there may be vitamin B12 deficiency or other nutritional deficiencies. The combination of elevated folate and low vitamin B12 is unusual. Anemia may occur from a variety of mechanisms, as many of the nutrients involved in the production of red blood cells are absorbed in the affected small bowel. Iron is absorbed in the more proximal parts of the small bowel, the duodenum and jejunum, and patients with malabsorption of iron can develop a microcytic anemia, with small red blood cells. Vitamin B12 is absorbed in the last part of the small bowel, the ileum, and patients who have malabsorption of vitamin B12 can develop a megaloblastic anemia with large red blood cells.

Causes

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