Bacterial Overgrowth Syndrome

What is Bacterial Overgrowth Syndrome?

Also known as BOS, this is a term that illustrates the clinical materializations that happens when the usually low number of bacteria inhabiting the stomach, jejunum, duodenum, and proximal ileum increase considerably or are overtaken by other pathogens.

This upper intestinal tract was at one time believed to be an environment that was sterile; but, low levels of various bacteria are currently widely believed to exist within or be attached to its surface. These bacteria are believed to be present from the time of birth thru adulthood, living in a close, long-term relationship with the human host. This relationship is believed to be essential for the typical digestive processes, immunity protection, as well as intestinal development. Bacterial varieties typically present include enterococci, lactobacilli, oral streptococci, and other gram-positive aerobic or anaerobes.


There are no precise symptoms for Bacterial Overgrowth Syndrome. However, a variety of vague GI symptoms are not uncommon in individuals who are affected. Physicians need to have a suspicion for BOS in individuals who make an appointment because of the following:

  • Flatulence
  • Bloating
  • Diarrhea
  • Abdominal pain
  • Dyspepsia
  • Weight loss

Cases of advanced of BOS may also develop as malabsorption findings, as follows:

  • Large cell anemia caused by deficiency of folate/vitamin B-12
  • Small cell anemia caused by deficiency in iron
  • Neurological disorder caused by deficiency of vitamin B-12
  • Excess fat in feces due to malabsorption of lipids
  • Skin inflammation due to deficiency of selenium
  • Involuntary contraction due to low serum calcium levels
  • Night blindness due to deficiency of vitamin A
  • BOS has been linked to rosacea
  • Atrophy due to malnutrition of protein-energy


Structural abnormalities or disorders that disturb the mechanisms that are protective and safeguard against rising bacterial trouble may lead to BOS.

Individuals with the following medical problems are at an increased risk for Bacterial Overgrowth Syndrome:

  • Irritable bowel syndrome
  • History of surgery in the upper intestinal tract
  • Celiac disease
  • Liver cirrhosis
  • Immune deficiency for example AIDS, severe malnutrition, IGA deficiency
  • End stage renal disease
  • Short bowel syndrome
  • Gastrojejunal Anastomosis
  • Antral resection
  • Pancreatic exocrine insufficiency

Atypical small intestinal movement due to the below may result in BOS:

  • Diabetic autonomic neuropathy
  • Scleroderma
  • Pseudo-obstruction
  • Amyloidosis
  • Neurological diseases such as Myotonic dystrophy, Parkinson disease
  • Crohn disease
  • Hypothyroidism

Partial obstruction caused by the following might result in BOS:


  • Strictures
  • Abdominal masses
  • Adhesions
  • Leiomyosarcoma

Prevalence of BOS rises with age.


Treatment in BOS needs to include the correction of the main causal infection if any, with antibiotic therapy as well as support nutritionally. The main approach needs to be the management of any anatomic defect or disease that promotes BOS. Many of the conditions linked with BOS are not reversible readily, and treatment is supported on antibiotics focused at rebalancing normal flora. Consideration should be taken to stop the complete elimination of protective micro-bacterium. The goal needs to be directed at the reduction of symptoms. Initial antibiotic therapy is normally empiric and should be broad and cover both aerobic and anaerobic microorganisms. Resistance patterns in the community need to be considered as well.

Tetracycline was the foundation of therapy but it being used as a single drug alone has fallen as the mainstay in adults especially given the community increases in bacterial resistance.

Bacterial sensitivities for BOS support using of clavulanate- amoxicillin. Clavulanate-amoxicillin seems to be 75% effective in those individuals who have diabetes.

Studies have shown that rifaximin eliminates BOS in up to 80% of individuals. Doses that are higher (1200 or 1600 mg/d) are more successful then standard doses (600 or 800 mg/d). Long-term favorable results have been attained with rifaximin in those individuals with irritable bowel as well as BOS.

Clindamycin and metronidazole have had good results in elderly individuals with idiopathic BOS.

Cholestyramine decreases diarrhea in neonates and infants and neonates with inflexible diarrhea. Infants with 10 days to 25 days of persistent and severe diarrhea for which a no cause is found in spite of an extensive immunologic and infectious workup were treated with gentamicin or metronidazole and cholestyramine. Gentamicin and cholestyramine drastically reduced the weight of stools within 4-5 days of therapy but had mild detrimental results on nitrogen and fat absorption.

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