Small bowel bacterial overgrowth syndrome

Small bowel bacterial overgrowth syndrome

Small bowel bacterial overgrowth syndrome (SBBOS), or small intestinal bacterial overgrowth (SIBO), also termed bacterial overgrowth; is a disorder of excessive bacterial growth in the small intestine. Unlike the colon (or large bowel), which is rich with bacteria, the small bowel usually has less than 104 organisms per millilitre. cite journal | author = Quigley E, Quera R | title = Small intestinal bacterial overgrowth: roles of antibiotics, prebiotics, and probiotics | journal = Gastroenterology | volume = 130 | issue = 2 Suppl 1 | pages = S78–90 | year = 2006 | pmid = 16473077 | doi = 10.1053/j.gastro.2005.11.046] Patients with bacterial overgrowth typically develop symptoms including nausea, bloating, vomiting and diarrhea, which is caused by a number of mechanisms. The diagnosis of bacterial overgrowth is made by a number of techniques, with the gold standard diagnosis being an aspirate from the jejunum that grows in excess of 105 bacteria per millilitre. Risk factors for the development of bacterial overgrowth include the use of medications including proton pump inhibitors, anatomical disturbances in the bowel, including fistulae, diverticula and blind loops created after surgery, and resection of the ileo-cecal valve. Small bowel bacterial overgrowth syndrome is treated with antibiotics, which may be given in a cyclic fashion to prevent tolerance to the antibiotics.

Clinical presentation

Bacterial overgrowth can cause a variety of symptoms, many of which are also found in other conditions, making the diagnosis challenging at times. cite journal | author = Teo M, Chung S, Chitti L, Tran C, Kritas S, Butler R, Cummins A | title = Small bowel bacterial overgrowth is a common cause of chronic diarrhea | journal = J Gastroenterol Hepatol | volume = 19 | issue = 8 | pages = 904–9 | year = 2004 | pmid = 15242494 | doi = 10.1111/j.1440-1746.2004.03376.x] 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 bacterial overgrowth include nausea, bloating, flatus, and chronic diarrhea. Some patients may develop abdominal discomfort and lose weight. Children with bacterial overgrowth may develop malnutrition have difficulty attaining proper growth. Steatorrhea is a sticky type of diarrhea, where lipids are malabsorbed and spill into the stool. cite journal | author = Kirsch M | title = Bacterial overgrowth | journal = Am J Gastroenterol | volume = 85 | issue = 3 | pages = 231–7 | year = 1990 | pmid = 2178395]

Patients with bacterial overgrowth that is longstanding can develop complications of their illness as a result of malabsorption of nutrients. Anemia may occur from a variety of mechanisms, as many of the nutrients involved in 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 malabsorb vitamin B12 can develop a megaloblastic anemia with large red blood cells.


Certain species of bacteria are more commonly found in aspirates of the jejunum taken from patients with bacterial overgrowth. The most common isolates are "Escherichia coli", "Streptococcus", "Lactobacillus", "Bacteroides", and "Enterococcus" species. cite journal | author = Bouhnik Y, Alain S, Attar A, Flourié B, Raskine L, Sanson-Le Pors M, Rambaud J | title = Bacterial populations contaminating the upper gut in patients with small intestinal bacterial overgrowth syndrome | journal = Am J Gastroenterol | volume = 94 | issue = 5 | pages = 1327–31 | year = 1999 | pmid = 10235214 | doi = 10.1111/j.1572-0241.1999.01016.x]

Soon after birth, the gastrointestinal tract is colonized with bacteria, which, on the basis of models with animals raised in a germ-free environment, have beneficial effects on function of the gastrointestinal tract. There are 500-1000 different species of bacteria that reside in the bowel. cite journal | author = Hao W, Lee Y | title = Microflora of the gastrointestinal tract: a review | journal = Methods Mol Biol | volume = 268 | issue = | pages = 491–502 | year = | pmid = 15156063] However, if the flora of the small bowel is altered, inflammation or altered digestion can occur, leading to symptoms. Many patients with chronic diarrhea have bacterial overgrowth as a cause or a contributor to their symptoms. While the consensus definition of chronic diarrhea varies, in general it is considered to be an alteration in stool consistency or increased frequency, that occurs for over three weeks. Various mechanisms are involved in the development of diarrhea in bacterial overgrowth. First, the excessive bacterial concentrations can cause direct inflammation of the small bowel cells, leading to an "inflammatory" diarrhea. The malabsorption of lipids, proteins and carbohydrates may cause poorly digestible products to enter into the colon. This can cause diarrhea by the osmotic drive of these molecules, but can also stimulate the secretory mechanisms of colonic cells, leading to a "secretory diarrhea".

Risk factors and causes

Certain patients are more predisposed to the development of bacterial overgrowth because of certain risk factors. These factors can be grouped into three categories: (1) disordered motility or movement of the small bowel or anatomical changes that lead to stasis, (2) disorders in the immune system and (3) 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. Diseases like scleroderma cite journal | author = Rose S, Young M, Reynolds J | title = Gastrointestinal manifestations of scleroderma | journal = Gastroenterol Clin North Am | volume = 27 | issue = 3 | pages = 563–94 | year = 1998 | pmid = 9891698 | doi = 10.1016/S0889-8553(05)70021-2] and possibly celiac disease cite journal | author = Tursi A, Brandimarte G, Giorgetti G | title = High prevalence of small intestinal bacterial overgrowth in celiac patients with persistence of gastrointestinal symptoms after gluten withdrawal | journal = Am J Gastroenterol | volume = 98 | issue = 4 | pages = 839–43 | year = 2003 | pmid = 12738465 | doi = 10.1111/j.1572-0241.2003.07379.x] 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. cite journal | author = Kongara K, Soffer E | title = Intestinal motility in small bowel diverticulosis: a case report and review of the literature | journal = J Clin Gastroenterol | volume = 30 | issue = 1 | pages = 84–6 | year = 2000 | pmid = 10636218 | doi = 10.1097/00004836-200001000-00017 ] 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". cite journal | author = Isaacs P, Kim Y | title = Blind loop syndrome and small bowel bacterial contamination | journal = Clin Gastroenterol | volume = 12 | issue = 2 | pages = 395–414 | year = 1983 | pmid = 6347463]

Disorders of the immune system can cause bacterial overgrowth. Chronic pancreatitis, or inflammation of the pancreas can cause bacterial overgrowth through mechanisms linked to this. [cite journal | author = Trespi E, Ferrieri A | title = Intestinal bacterial overgrowth during chronic pancreatitis | journal = Curr Med Res Opin | volume = 15 | issue = 1 | pages = 47–52 | year = 1999 | pmid = 10216811] The use of immunosuppressant medications to treat other conditions can cause this, as evidenced from animal models. cite journal | author = Marshall J, Christou N, Meakins J | title = Small-bowel bacterial overgrowth and systemic immunosuppression in experimental peritonitis | journal = Surgery | volume = 104 | issue = 2 | pages = 404–11 | year = 1988 | pmid = 3041643] Other causes include inherited immunodeficiency conditions, such as combined variable immunodeficiency, IgA deficiency, and hypogammaglobulinemia. cite journal | author = Pignata C, Budillon G, Monaco G, Nani E, Cuomo R, Parrilli G, Ciccimarra F | title = Jejunal bacterial overgrowth and intestinal permeability in children with immunodeficiency syndromes | journal = Gut | volume = 31 | issue = 8 | pages = 879–82 | year = 1990 | pmid = 2387510 | doi = 10.1136/gut.31.8.879]

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 ileo-cecal valve connecting the small and large bowel; this leads to an increased reflux of bacteria into the small bowel. cite journal | author = Kholoussy A, Yang Y, Bonacquisti K, Witkowski T, Takenaka K, Matsumoto T | title = The competence and bacteriologic effect of the telescoped intestinal valve after small bowel resection | journal = Am Surg | volume = 52 | issue = 10 | pages = 555–9 | year = 1986 | pmid = 3767143] After bariatric surgery for obesity, connections between the stomach and the ileum can be formed, which may increase bacterial load in the small bowel. cite journal | author = Abell T, Minocha A | title = Gastrointestinal complications of bariatric surgery: diagnosis and therapy | journal = Am J Med Sci | volume = 331 | issue = 4 | pages = 214–8 | year = 2006 | pmid = 16617237 | doi = 10.1097/00000441-200604000-00008] Proton pump inhibitor medications that decrease acid in the stomach cause bacterial overgrowth by a similar mechanism, as they prevent the anti-bacterial effects of acid in the stomach. The clinical significance of this in causing symptoms is unclear. cite journal | author = Laine L, Ahnen D, McClain C, Solcia E, Walsh J | title = Review article: potential gastrointestinal effects of long-term acid suppression with proton pump inhibitors | journal = Aliment Pharmacol Ther | volume = 14 | issue = 6 | pages = 651–68 | year = 2000 | pmid = 10848649 | doi = 10.1046/j.1365-2036.2000.00768.x] cite journal | author = Williams C, McColl K | title = Review article: proton pump inhibitors and bacterial overgrowth | journal = Aliment Pharmacol Ther | volume = 23 | issue = 1 | pages = 3–10 | year = 2006 | pmid = 16393275 | doi = 10.1111/j.1365-2036.2006.02707.x]


The diagnosis of bacterial overgrowth can be made by physicians in various ways. Malabsorption can be detected by a test called the "D-xylose" test. Xylose is a sugar that does not require enzymes to be digested. The D-xylose test involves having a patient to 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). cite journal | author = Craig R, Atkinson A | title = D-xylose testing: a review | journal = Gastroenterology | volume = 95 | issue = 1 | pages = 223–31 | year = 1988 | pmid = 3286361]

The gold standard for detection of bacterial overgrowth is the aspiration of more than 105 bacteria per millilitre from the small bowel. The normal small bowel has less than 104 bacteria per millilitre. cite journal | author = Corazza G, Menozzi M, Strocchi A, Rasciti L, Vaira D, Lecchini R, Avanzini P, Chezzi C, Gasbarrini G | title = The diagnosis of small bowel bacterial overgrowth. Reliability of jejunal culture and inadequacy of breath hydrogen testing | journal = Gastroenterology | volume = 98 | issue = 2 | pages = 302–9 | year = 1990 | pmid = 2295385]

Breath tests have been developed to test for bacterial overgrowth, based on bacterial metabolism of carbohydrates to hydrogen, or based on the detection of by-products of digestion of carbohydrates that are not usually metabolized. The hydrogen breath test involves giving patients a load of carbohydrate (usually in the form of rice) and measuring expired hydrogen concentrations after a certain time. It compares well to jejunal aspirates in making the diagnosis of bacterial overgrowth. cite journal | author = Kerlin P, Wong L | title = Breath hydrogen testing in bacterial overgrowth of the small intestine | journal = Gastroenterology | volume = 95 | issue = 4 | pages = 982–8 | year = 1988 | pmid = 3410238] 13C and 14C based tests have also been developed based on the bacterial metabolism of D-xylose. Increased bacterial concentrations are also involved in the deconjugation of bile acids. The glycocholic acid breath test involves the administration of the bile acid 14C glychocholic acid, and the detection of 14CO2, which would be elevated in bacterial overgrowth. cite journal | author = Donald I, Kitchingmam G, Donald F, Kupfer R | title = The diagnosis of small bowel bacterial overgrowth in elderly patients | journal = J Am Geriatr Soc | volume = 40 | issue = 7 | pages = 692–6 | year = 1992 | pmid = 1607585]

Some patients with symptoms of bacterial overgrowth will undergo gastroscopy, or visualization of the stomach and duodenum with an endoscopic camera. Biopsies of the small bowel in bacterial overgrowth can mimic those of celiac disease, making the diagnosis more challenging. Findings include blunting of villi, hyperplasia of crypts and an increased number of lymphocytes in the lamina propria. cite journal | author = Toskes P, Giannella R, Jervis H, Rout W, Takeuchi A | title = Small intestinal mucosal injury in the experimental blind loop syndrome. Light- and electron-microscopic and histochemical studies | journal = Gastroenterology | volume = 68 | issue = 5 Pt 1 | pages = 1193–203 | year = 1975 | pmid = 1126607]

However, some physicians suggest that if the suspicion of bacterial overgrowth is high enough, the best diagnostic test is a trial of treatment. If the symptoms improve, an empiric diagnosis of bacterial overgrowth can be made. cite journal | author = Singh VV, Toskes PP | title = Small Bowel Bacterial Overgrowth: Presentation, Diagnosis, and Treatment | journal = Curr Treat Options Gastroenterol | volume = 7 | issue = 1 | pages = 19–28 | year = 2004 | pmid = 14723835 | doi = 10.1007/s11938-004-0022-4]


Bacterial overgrowth is usually treated with a course of antibiotics. A variety of antibiotics, including neomycin, rifaximin, amoxicillin-clavulanate, fluoroquinolone antibiotics and tetracycline have been used; however, the best evidence is for the use of norfloxacin and amoxicillin-clavulanate. cite journal | author = Attar A, Flourié B, Rambaud J, Franchisseur C, Ruszniewski P, Bouhnik Y | title = Antibiotic efficacy in small intestinal bacterial overgrowth-related chronic diarrhea: a crossover, randomized trial | journal = Gastroenterology | volume = 117 | issue = 4 | pages = 794–7 | year = 1999 | pmid = 10500060 | doi = 10.1016/S0016-5085(99)70336-7]

A course of one week of antibiotics is usually sufficient to treat the condition. However, if the condition recurs, antibiotics can be given in a cyclical fashion in order to prevent tolerance. For example, antibiotics may be given for a week, followed by three weeks off antibiotics, followed by another week of treatment. Alternatively, the choice of antibiotic used can be cycled.

The condition that predisposed the patient to bacterial overgrowth should also be treated. For example, if the bacterial overgrowth is caused by chronic pancreatitis, the patient should be treated with coated pancreatic enzyme supplements.

Probiotics are bacterial preparations that alter the bacterial flora in the bowel to cause a beneficial effect. Their role in bacterial overgrowth is somewhat uncertain.


External links

* [ IBS and small intestinal bacterial overgrowth (SIBO)]

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