Small intestinal bacterial overgrowth
|Small intestine bacterial overgrowth syndrome|
|Classification and external resources|
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 bowel usually has fewer than 104 organisms per millilitre. Patients with bacterial overgrowth typically develop symptoms including nausea, bloating, vomiting, diarrhea, malnutrition, weight loss and malabsorption 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 dysmotility, anatomical disturbances in the bowel, including fistulae, diverticula and blind loops created after surgery, and resection of the ileo-cecal valve as well as the use of certain medications, including proton pump inhibitors. Small bowel bacterial overgrowth syndrome is treated with an elemental diet or else antibiotics, which may be given in a cyclic fashion to prevent tolerance to the antibiotics sometimes followed by prokinetic drugs to prevent recurrence if dysmotility is a suspected cause.
Bacterial overgrowth 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 bacterial overgrowth include nausea, flatus, constipation, bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness. Some patients may lose weight. Children with bacterial overgrowth may develop malnutrition and have difficulty attaining proper growth. Steatorrhea, a sticky type of diarrhea where lipids are malabsorbed and spill into the stool, may also occur.
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. In older adults, small bowel bacterial overgrowth is associated with a higher frequency of diarrhea, a lower body mass index, and a significanly lower serum albumin concentration.
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.
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. 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 and possibly celiac disease 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.
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. The use of immunosuppressant medications to treat other conditions can cause this, as evidenced from animal models. Other causes include inherited immunodeficiency conditions, such as combined variable immunodeficiency, IgA deficiency, and hypogammaglobulinemia.
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.
In recent years, several proposed links between SIBO and other disorders have been made. However, the usual methodology of these studies involves the use of breath testing as an indirect investigation for SIBO. Breath testing has been critizised by some authors for being an imperfect test for SIBO, with multiple known false positives.
Some studies reported up to 80% of patients with irritable bowel syndrome (IBS) have SIBO (using the hydrogen breath test). Subsequent studies demonstrated statistically significant reduction in IBS symptoms following therapy for SIBO.
There is a lack of consensus however, regarding the suggested link between IBS and SIBO. Other authors concluded that the abnormal breath results so common in IBS patients do not suggest SIBO, and state that "abnormal fermentation timing and dynamics of the breath test findings support a role for abnormal intestinal bacterial distribution in IBS."  There is general consensus that breath tests are abnormal in IBS, however the disagreement lies in whether this is representative of SIBO. More research is needed to clarifiy this possible link.
Fybromyalgia is a poorly understood pain condition. Lactulose breath testing has shown that patients with fibromyalgia have a more pronounced degree of abnormal results compared to both IBS patients and the general population. This study also demonstrated positive correlation between the amount of pain and the degree of abnormality on the breath test. A subsequent study also demonstrated increased prevalence of intestinal hyperpermeability, which some believe occurs commonly with SIBO.
Intestinal bacteria may play an etiological role in the dermatological condition Rosacea. A recent study subjected patients to a hydrogen breath test to detect the occurrence of SIBO. It was found that significantly more patients were hydrogen-positive than controls indicating the presence of bacterial overgrowth (47% v. 5%, p<0.001).
Hydrogen-positive patients were then given a 10-day course of rifaximin, a non-absorbable antibiotic that does not leave the digestive tract and therefore does not enter the circulation or reach the skin. 96% of patients experienced a complete remission of rosacea symptoms that lasted beyond 9 months. These patients were also negative when retested for bacterial overgrowth. In the 4% of patients that experienced relapse, it was found that bacterial overgrowth had returned. These patients were given a second course of rifaximin which again cleared rosacea symptoms and normalized hydrogen excretion.
In another study, it was found that some rosacea patients that tested hydrogen-negative were still positive for bacterial overgrowth when using a methane breath test instead. These patients showed little improvement with rifaximin, as found in the previous study, but experienced clearance of rosacea symptoms and normalization of methane excretion following administration of the antibiotic metronidazole, which is effective at targeting methanogenic intestinal bacteria.
These results suggest that optimal antibiotic therapy may vary between patients and that diverse species of intestinal bacteria appear to be capable of mediating rosacea symptoms.
This may also explain the improvement in symptoms experienced by some patients when given a reduced carbohydrate diet. Such a diet would restrict the available material necessary for bacterial fermentation and thereby reduce intestinal bacterial populations.
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 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).
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.
Breath tests have been developed to test for bacterial overgrowth, based on bacterial metabolism of carbohydrates to hydrogen and/or methane, or based on the detection of by-products of digestion of carbohydrates that are not usually metabolized. The hydrogen breath test involves having the patient fast for a minimum of 12 hours then having them drink a substrate usually glucose or lactulose, then measuring expired hydrogen and methane concentrations typically over a period of 2-3 hours. It compares well to jejunal aspirates in making the diagnosis of bacterial overgrowth. 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.
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.
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.
Bacterial overgrowth is usually treated with a course of antibiotics. A variety of antibiotics, including tetracycline, amoxicillin-clavulanate, fluoroquinolones, metronidazole, neomycin, cephalexin, trimethoprim-sulfamethoxazole have been used; however, the best evidence is for the use of rifaximin.
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.
An elemental diet taken for two weeks is an alternative to antibiotics for eliminating SIBO. 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. For example if the symptoms are caused by bacterial overgrowth on complex carbohydrate rich foods (for example bread), eating light food consisting of fruits and green leafy vegetables may substantially lessen the symptoms.
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