Small intestinal bacterial overgrowth

SIBO may be defined as an increased number of bacteria measured via exhaled hydrogen and/or methane gas following the ingestion of glucose, or via analysis of small bowel aspirate fluid. 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. 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 ==

can occur in SIBO. 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. 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==

'', shown in this electron micrograph, is commonly isolated in patients with SIBO. Certain people are more predisposed to the development of SIBO because of certain risk factors. These factors can be grouped into four categories: (1) motility disorders, impaired movement of the small bowel, or anatomical changes that lead to stasis (a state in which the normal flow of a body liquid stops); (2) disorders of the immune system; (3) interference with the production of proteolytic enzymes, gastric acid, or bile; and (4) conditions that cause more bacteria from the colon to enter the small bowel. Problems with motility may either be diffuse or localized to particular areas. MMC impairment may be a result of post-infectious IBS, drug use, or intestinal pseudo-obstruction among other causes. There is an overlap in findings between tropical sprue, post-infectious IBS and SIBO in the pathophysiology of the three conditions and also SIBO can similarly sometimes be triggered by an acute gastrointestinal infection. As of 2020, there is still controversy about the role of SIBO in the pathogenesis of common functional symptoms such as those considered to be components of IBS. cause diffuse slowing of the bowel, leading to increased bacterial concentrations. More commonly, the small bowel may have anatomical problems, such as out-pouchings known as diverticula that can cause bacteria to accumulate. After surgery involving the stomach and duodenum (most commonly with Billroth II antrectomy), a blind loop may be formed, leading to stasis of flow of intestinal contents. This can cause overgrowth, and is termed blind loop syndrome. Systemic or metabolic disorders may lead to conditions allowing SIBO as well. For example, diabetes can cause intestinal neuropathy, pancreatitis, leading to pancreatic insufficiency can impair digestive enzyme production, and bile may be affected as part of cirrhosis of the liver. The use of proton pump inhibitors, a class of medication used to reduce stomach acid, is associated with an increased risk of developing SIBO. Finally, abnormal connections between the bacteria-rich colon and the small bowel can increase the bacterial load in the small bowel. Patients with Crohn's disease or other diseases of the ileum may require surgery that removes the ileocecal valve connecting the small and large bowel; this leads to an increased reflux of bacteria into the small bowel. After bariatric surgery for obesity, connections between the stomach and the ileum can be formed, which may increase bacterial load in the small bowel. Related conditions In recent years, several proposed links between SIBO and other disorders have been made. Usually, such research uses breath testing as an indirect investigation for SIBO. Irritable bowel syndrome Some studies reported that up to 80% of patients with irritable bowel syndrome (IBS) have SIBO (using the hydrogen breath test). IBS-D is associated with elevated hydrogen numbers on breath tests, while IBS-C is associated with elevated methane numbers on breath tests. Subsequent studies demonstrated statistically significant reduction in IBS symptoms following therapy for SIBO. There is consensus that breath tests are abnormal in IBS; however, the disagreement lies in whether this is representative of SIBO. ==Diagnosis==

is the gold standard for diagnosis. A bacterial load of greater than 105 bacteria per milliliter is diagnostic for SIBO. D-xylose absorption test Malabsorption can be detected by a test called the D-xylose absorption test. Xylose is a sugar that does not require enzymes to be digested. The D-xylose test involves having a patient drink a certain quantity of D-xylose, and measuring levels in the urine and blood; if there is no evidence of D-xylose in the urine and blood, it suggests that the small bowel is not absorbing properly (as opposed to problems with enzymes required for digestion). Small bowel aspirate culture The gold standard for detection of SIBO is aspiration and culture of fluid from the jejunum. More than 105 colony-forming units (more than 100,000 bacteria) per milliliter from the small bowel suggests SIBO. The normal small bowel has less than 104 bacteria per millilitre. However, some experts consider aspiration of more than 103 positive if the flora is predominantly colonic-type bacteria, as these types of bacteria are considered pathological in excessive numbers in the small intestine. The reliability of aspiration in the diagnosis of SIBO has been questioned, as SIBO can be patchy, and the reproducibility can be as low as 38%. Some doctors factor in a patient's response to treatment as part of the diagnosis. This may affect results from hydrogen-methane breath testing. Breath tests give multiple false positives (a positive test result when in reality the person does not have the condition). On the other hand, breath tests are commonly used because they are non invasive and not expensive. There is insufficient evidence to support the use of inflammatory markers, such as fecal calprotectin, to detect SIBO. ==Treatment==

Treatment strategies should focus on identifying and correcting the root causes, resolving nutritional deficiencies, and administering antibiotics. This is especially important for patients with indigestion and malabsorption. A variety of antibiotics, including tetracycline, amoxicillin-clavulanate, metronidazole, neomycin, cephalexin and trimethoprim-sulfamethoxazole have been used; however, the best evidence is for the use of rifaximin, a poorly-absorbed antibiotic. A combination of probiotic strains has been found to produce better results than therapy with the antibiotic drug metronidazole. An elemental diet has been shown to be highly effective for eliminating SIBO with a two-week diet demonstrating 73% efficacy in normalizing breath test levels. An elemental diet works via providing nutrition for the individual while depriving the bacteria of a food source. Additional treatment options include the use of prokinetic drugs such as 5-HT4 receptor agonists or motilin agonists to extend the SIBO free period after treatment with an elemental diet or antibiotics. A diet void of certain foods that feed the bacteria can help alleviate the symptoms. == Epidemiology ==

According to breath testing, SIBO may be present in 34% of people with gastrointestinal symptoms. SIBO affects males and females in equal proportion. Race does not affect the risk of SIBO. == See also ==