Flow of bacterial genes in the environment analyzed

January 2012


The human body is a complex biological network that relies on microbes to help with metabolism and other essential functions. The microbes living in humans outnumber human cells 10 to 1. Their interaction with microbes in the environment often leads to horizontal gene transfer (HGT), the acquisition of genetic material from non-parental lineages, an important feature of bacterial evolution. In particular, HGT provides bacteria with rapid access to genetic innovations, allowing traits such as virulence and antibiotic resistance to spread through the human microbiome. Recent studies — like the one in 2010 that identified the so-called sushi gene, which had been transferred from a marine microbe to a bacteria living in the human gut — provide snapshots of active gene flow. These studies, along with growing concern that antibiotic resistance is being transferred from bacteria in livestock to bacteria in humans, highlight the need to determine the frequency of such transfers and the forces that govern them.


Using computational biology to determine the evolutionary ancestry of the genes in 2,235 complete bacteria genomes, Professor Eric Alm and graduate students Chris Smillie and Mark Smith first identified genes that had recently been transferred between human-associated bacteria and non-human associated bacteria via HGT. (Their heuristic locates blocks of nearly identical DNA in distantly related bacteria taken from different human subjects or environments, often on different continents.) The researchers then determined whether the HGT networks in play were influenced more by phylogeny (the evolutionary history of organisms), geographic proximity or similarity in ecological niche. Phylogeny cou