The intestine contains a radial gradient of microbes that changes with the distribution of oxygen and nutrients, researchers report in the November issue of Gastroenterology. Further study of this distribution could provide information about activities of the microbiota in the healthy and inflamed intestine.
The bacteria of the intestinal live in a complex community dominated by anaerobic organisms from the Firmicutes and Bacteroidetes phyla. Although the composition of the intestinal microbiota is generally stable in humans, it can change with diet and activity. Furthermore, there are differences in the microbiota along the longitudinal axis of the intestine, and even in different regions of a single fecal sample.
Lindsey Albenberg et al examined the radial distribution of the gut microbiota and factors in the intestinal environment, such as oxygen, that might alter its composition.
Using a specially designed probe to measure oxygen levels in the intestines of mice, they found average PO2 values in the lumen of the cecum to be extremely low (<1 mm Hg). However, they also found that oxygen diffused from intestinal tissue and established a radial gradient that extended from the tissue interface to the lumen.
To investigate whether this change in PO2 affects the composition of the microbiome, Albenberg et al exposed mice to hyperbaric oxygen therapy for 9 days. This increased tissue oxygenation 5-fold and changed the composition of the microbial communities in the intestine. The authors found that mucosa-associated bacteria changed with the O2 tension in the mucosal layer, which might affect the function of the mucosa.
Sequence analyses of 16S ribosomal RNA genes from humans showed that the rectal mucosa had a larger proportion of oxygen-tolerant organisms of the Proteobacteria and Actinobacteria phyla than the feces.
In an editorial that accompanies the article, Thomas M. Schmidt and John Y. Kao state that these findings reveal the potential for O2 to shape the structure of the gut microbiome.
They explain that microbes that use O2 for respiration have a selective advantage in environments containing O2—especially those rich in organic compounds but with few alternative electron acceptors for microbial respiration (any place where fermentation is the primary, energy-yielding form of metabolism) (see figure).
The food taken into the gastrointestinal tracts of most animals support dense communities of microbes that perform fermentation.
Schmidt and Kao write that the mammalian intestinal tract has been considered to be an anoxic environment that contains mostly anaerobic bacteria, but now the potential roles of O2 in this ecosystem are being reexamined.
They state that an estimated 30% of the microbes from the mammalian gastrointestinal tract were found to have the potential to use low, even nanomolar concentrations of O2 in metabolism.
Further experiments are therefore needed to assess the effects of oxygen on the distribution of the microbiota, and whether it is altered in chronic intestinal inflammation or injury.