Microflora of the human small intestine

Area of Science:

• Microbiology
• Gastroenterology
• Human physiology

Background:

• The human small intestine's microbial composition is not fully understood.
• Establishing baseline bacterial levels is crucial for diagnosing dysbiosis.

Purpose of the Study:

• To determine the normal bacterial flora of the human small intestine (duodenum, jejunum, ileum).
• To provide baseline data for interpreting bacterial overgrowth in postoperative disorders.

Main Methods:

• Aspiration and culture of duodenal, jejunal, and ileal samples.
• Identification of aerobic and anaerobic bacteria.
• Quantification of bacterial counts.

Main Results:

• The small intestine is sterile in 45% of North American subjects.
• Sterility rates were 82% (duodenum), 69% (jejunum), and 55% (ileum).
• Gram-positive cocci were most common; E. coli, Enterobacter, and Klebsiella increased distally. Significant bacterial overgrowth ( > 10^5/ml) occurred in the mid-jejunum (2 patients) and mid-ileum (7 patients, 23%). Anaerobes, particularly strict anaerobes like Bacteroides, were rare.

Conclusions:

• The small intestine exhibits a gradient of bacterial colonization, increasing from the duodenum to the ileum.
• Baseline data on small intestinal microbiota are essential for diagnosing bacterial overgrowth syndromes.
• Further research is needed to understand the role of specific bacteria in postoperative conditions.