Small intestinal bacterial overgrowth, also termed bacterial overgrowth, or Small bowel
bacterial overgrowth syndrome, is a disorder of excessive bacterial growth in the small
intestine.
Unlike the colon, 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, gastroenteritis induced alterations to the
small intestine 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.
Signs and symptoms
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.
SIBO also causes an increased permeability of the small intestine.
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 fats are not properly absorbed 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 significantly lower serum albumin concentration.
Pathophysiology
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.
There is an overlap in findings between tropical sprue, post-infectious irritable bowel syndrome
and small intestinal bacterial overgrowth in the pathophysiology of the three conditions
and also SIBO can similarly sometimes be triggered by an acute gastrointestinal infection.
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: disordered motility or movement of the small
bowel or anatomical changes that lead to stasis, disorders in the immune system and 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, 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.
Proton pump inhibitors, a class of medication that are used to reduce stomach acid, is associated
with an increased risk of developing SIBO.
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 criticized by some authors for being an imperfect test for SIBO,
with multiple known false positives.
Proposed link with Irritable bowel syndrome
Some studies reported up to 80% of patients with irritable bowel syndrome have SIBO.
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.
Proposed link with Fibromyalgia
Fibromyalgia 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.
Proposed link with Rosacea
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.
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.
Diagnosis
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.
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.
Some experts however, consider aspiration of more than 103 positive if the flora is
predominately 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 percent.
Breath tests have their own reliability problems with a high rate of false positive.
Some doctors factor in a patients' response to treatment as part of the diagnosis.
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.
Treatment Bacterial overgrowth is usually treated with
a course of antibiotics although whether antibiotics should be a first line treatment is a matter
of debate.
Some experts recommend probiotics as first line therapy with antibiotics being reserved
as a second line treatment for more severe cases of SIBO.
Prokinetic drugs are other options but research in humans is limited.
A variety of antibiotics, including tetracycline, amoxicillin-clavulanate, fluoroquinolones,
metronidazole, neomycin, cephalexin, trimethoprim-sulfamethoxazole, and nitazoxanide 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.
Probiotics are bacterial preparations that alter the bacterial flora in the bowel to
cause a beneficial effect.
Animal research has demonstrated that probiotics have barrier enhancing, antibacterial, immune
modulating and anti-inflammatory effects which may have a positive effect in the management
of SIBO in humans.
Lactobacillus casei has been found to be effective in improving breath hydrogen scores after
6 weeks of treatment presumably by suppressing levels of a small intestinal bacterial overgrowth
of fermenting bacteria.
The multi-strain preparation VSL#3 was found to be effective in suppressing SIBO.
Lactobacillus plantarum, lactobacillus acidophilus, lactobacillus casei have all demonstrated
effectiveness in the treatment and management of SIBO.
Conversely lactobacillus fermentum and saccharomyces boulardii have been found to be ineffective.
A combination of lactobacillus plantarum and lactobacillus rhamnosus has been found to
be effective in suppressing bacterial overgrowth of abnormal gas producing organisms in the
small intestine.
Probiotics are superior to antibiotics in the treatment of SIBO.
A combination of probiotic strains has been found to produce better results than therapy
with the antibiotic drug metronidazole and probiotics have been found to be effective
in treating and preventing secondary lactase deficiency and small intestinal bacteria overgrowth
in individuals suffering from post-infectious irritable bowel syndrome.
Probiotics taken in uncomplicated cases of SIBO can usually result in the individual
becoming symptom free.
Probiotic therapy may need to be taken continuously to prevent the return of overgrowth of gas
producing bacteria.
Probiotic yogurt may also be effective in treating SIBO with evidence of reduced inflammation
after 4 weeks of treatment.
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 feeding on indigestible
carbohydrate rich foods, following a FODMAP restriction diet may help.
References
External links