Michael Coyne and Laurie E. Comstock, PhD

A research team including Michael Coyne, BWH Division of Infectious Diseases, first study author, and Laurie E. Comstock, PhD, BWH Department of Medicine, senior study author, provides evidence for the presence of unique microbial ecosystems and strains within the human gut of every individual due to extensive DNA transfer between co-resident Bacteroidales strains. Bacteroidales include more than 25 species that colonize the human gut and represent as much as half of the gut-resident bacteria in many individuals.

Researchers found that horizontal transfer, or the exchange of DNA between co-resident Bacteroidales species, occurs frequently within the human gut. These data provide evidence that the genomes of our gut bacteria are substantially modified by other, co-resident members of the ecosystem, resulting in highly personalized Bacteroidales strains likely unique to each individual.

The team was able to identify five large chromosomal regions, encompassing in total more than 140 kilobase (kb) of DNA, each present in a minimum of three of an individual’s co-resident strains. These chromosomal regions were nearly identical at the DNA sequence level between these strains, but were not present in other non-resident Bacteroidales strains at this level of identity. Analyses for similar regions between mock communities of Bacteroidales strains revealed that unique regions with this level of identity occur very rarely, lending statistical significance to these findings.

In addition, the researchers identified specific regions of DNA that are prone to transfer between co-resident strains. Furthermore, the team found that human gut Bacteroidales genomes are continually modified by the movement of small, mobilizable DNA regions within each of these genomes.

It is thought that interspecies transfer of these large regions of DNA confers beneficial properties to the recipient, resulting in increased fitness and competitive advantages within its particular personalized gut microbial ecosystem. The team is now investigating the benefit that these DNA transfers have conferred to the recipient strain and whether these regions of DNA have since been transferred to other Bacteroidales members of the particular human gut community.

The study was published June 17, 2014 in mBio.