The human jejunum has an endogenous microbiota that differs from those in the oral cavity and colon

The upper half of the human small intestine, known as the jejunum, is the primary site for absorption of nutrient-derived carbohydrates, amino acids, small peptides, and vitamins. In contrast to the colon, which contains 10¹¹-10¹² colony forming units of bacteria per ml (CFU/ml), the normal jejunum generally ranges from 10³ to 10⁵ CFU per ml. Because invasive procedures are required to access the jejunum, much less is known about its bacterial microbiota. Bacteria inhabiting the jejunal lumen have been investigated by classical culture techniques, but not by culture-independent metagenomics.

The lumen of the upper jejunum was sampled during enteroscopy of 20 research subjects. Culture on aerobic and anaerobic media gave live bacterial counts ranging from 5.8 × 10³ CFU/ml to 8.0 × 10⁶ CFU/ml. DNA from the same samples was analyzed by 16S rRNA gene-specific quantitative PCR, yielding values from 1.5 × 10⁵ to 3.1 × 10⁷ bacterial genomes per ml. When calculated for each sample, estimated bacterial viability ranged from effectively 100% to a low of 0.3%. 16S rRNA metagenomic analysis of uncultured bacteria by Illumina MiSeq sequencing gave detailed microbial composition by phylum, genus and species. The genera Streptococcus, Prevotella, Veillonella and Fusobacterium, were especially abundant, as well as non-oral genera including Escherichia, Klebsiella, and Citrobacter. The jejunum was devoid of the genera Alistipes, Ruminococcus, Faecalibacterium, and other extreme anaerobes abundant in the colon. In patients with higher bacterial loads, there was no significant change in microbial species composition.

The jejunal lumen contains a distinctive bacterial population consisting primarily of facultative anaerobes and oxygen-tolerant obligate anaerobes similar to those found in the oral cavity. However, the frequent abundance of Enterobacteriaceae represents a major difference from oral microbiota. Although a few genera are shared with the colon, we found no evidence for retrograde movement of the most abundant colonic microbes to the jejunum. Some individuals had much higher bacterial loads, but this was not correlated with decreases in bacterial species diversity or other evidence of dysbiosis.