Methicillin-resistant Staphylococcus aureus (MRSA) has been a major concern for public health over the past two decades. Along with colleagues from Columbia University Medical Center, the AMNH, the V.A. Medical Center in the Bronx, VCU Medical School, and Trinity College in Dublin, we investigated the molecular and evolutionary basis of the adaptation of a family of epidemic MRSA strains called USA300 in a paper published in Dec 2013 in mBio, the Open Access journal of the American Society for Microbiology.

USA300 strains are distinguished by a region in their genome totaling 31,000 bp, known as a genomic island containing the arginine catabolic mobile element (ACME). Given that non-USA300 strains seldom contain ACME-like regions, it has been hypothesized that ACME was transferred from another Staphylococcus species, S. epidermidis.

Phylogenetic reconstruction of ACME. (A) Reconciled gene genealogy for arcA (green), aliD (red), and speG (black), which depicts the smallest number of reticulation events based on each gene tree. Numbers on branches are Bayesian clade credibility values for the speG locus phylogeny. The polytomy indicated by the green bar represents ACME loci from US300 strains and their very close relatives from S. epidermidis and includes all strains found in the tree in panel B. Note that arcA, speG, and aliD coalesced in the same genome just prior to the putative transfer (HGT) to USA300 strains. (B) Bayesian chronogram of ACME loci from the polytomy depicted in panel A, calibrated using the dates of strain isolation. The node bars indicate the uncertainty (95% highest posterior density) for the divergence times. Branch values are the posterior probabilities of clade credibility. (B) Inset shows the distribution of sampled dates for calculation of divergence times from the Bayesian analysis.

Abbreviations: Se, S. epidermidis; Sa, S. aureus; Sp, Staphylococcus pettenkoferi; Sc, Staphylococcus capitis.