Main Bacteria Species Involved in Small Intestinal Bacterial Overgrowth (SIBO)
The main bacteria species involved in SIBO include Streptococcus (71%), Escherichia coli (69%), and Lactobacillus (75%), along with other colonic and oropharyngeal bacteria that abnormally colonize the small intestine. 1
Bacterial Composition in SIBO
SIBO is characterized by an abnormally high bacterial population level in the upper gut, exceeding 10^5 organisms/ml (5 log colony-forming unit/ml) 1
The predominant microaerophilic bacteria found in SIBO include:
The predominant anaerobic bacteria found in SIBO include:
Normal Small Intestinal Microbiota vs. SIBO
The normal small intestinal microbiota has considerably fewer bacteria compared to the colon, with a marked gradient from duodenum to distal ileum 2
In the normal small intestine, bacteria are typically Gram-positive aerobes proximally and Gram-negative and Gram-positive anaerobes and facultative anaerobes in the terminal ileum 2
In SIBO, there is a disruption of this normal pattern, with an average of 4.6 bacterial genera present in abnormal quantities 1
Methane-Producing Organisms in SIBO
When methane-producing organisms are predominant, the condition may be referred to as intestinal methanogen overgrowth (IMO), which is often associated with constipation rather than diarrhea 3
Methane-producing organisms are particularly difficult to eradicate and often require more aggressive treatment approaches 4
Pathophysiological Implications
The contaminating flora in SIBO includes commonly identified oropharyngeal and colonic flora, but at different levels from those usually found in their original locations 1
SIBO can significantly alter the intestinal microbiome, with a decrease in beneficial bacteria such as Bifidobacteria and Lactobacillus 5
In the small intestine, Streptococcus species play an important role in the rapid uptake and conversion of available simple carbohydrates 2
Clinical Significance
The bacterial overgrowth leads to symptoms such as bloating, abdominal pain, diarrhea, and malabsorption that significantly affect patients' quality of life 6
In methane-dominant SIBO, constipation is more common than diarrhea 3
SIBO can complicate up to 92% of cases of chronic pancreatitis with pancreatic exocrine insufficiency 2
Diagnostic Considerations
Jejunal aspirate culture showing >10^5 CFU/ml has traditionally been considered the gold standard for diagnosis, though this cutoff was established from samples following surgical diversion 2
Some studies suggest that a lower cutoff of >10^3 CFU/ml may be more appropriate for diagnosing SIBO in certain clinical contexts 2
Breath tests with detection of exhaled H2 are the most accessible diagnostic technique but require rigorous performance and interpretation 3