Molecular epidemiology of methicillin-resistant and methicillin-susceptible Staphylococcus aureus isolates from global clinical trials.

Determining the genetic characteristics of Staphylococcus aureus is important for better understanding of the global and dynamic epidemiology of this organism as we witness the emergence and spread of virulent and antibiotic-resistant clones. We genotyped 292 S. aureus isolates (105 methicillin resistant and 187 methicillin susceptible) using a combination of pulsed-field gel electrophoresis, multilocus sequence typing, and SCCmec typing. In addition, S. aureus isolates were tested for the presence of the Panton-Valentine leukocidin (PVL) genes. Isolates were recovered from patients with uncomplicated skin infections in 10 different countries during five phase III global clinical trials of retapamulin, a new topical antibiotic agent. The most common methicillin-resistant clone had multilocus sequence type 8, pulsed-field type USA300, and SCCmec type IV and possessed the PVL genes. This clone was isolated exclusively in the United States. The most common PVL-positive, methicillin-susceptible clone had multilocus sequence type 121 and pulsed-field type USA1200. This clone was found primarily in South Africa and the Russian Federation. Other clones were found at lower frequencies and were limited in their geographic distribution. Overall, considerable genetic diversity was observed within multilocus sequence type clonal complexes and pulsed-field types.

A geographic variant of the Staphylococcus aureus Panton-Valentine leukocidin toxin and the origin of community-associated methicillin-resistant S. aureus USA300.

BACKGROUND: The majority of recent community-associated methicillin-resistant Staphylococcus aureus (MRSA) infections in the United States have been caused by a single clone, USA300. USA300 secretes Panton-Valentine leukocidin (PVL) toxin, which is associated with highly virulent infections. METHODS: We sequenced the PVL genes of 174 S. aureus isolates from a global clinical sample. We combined phylogenetic reconstruction and protein modeling methods to analyze genetic variation in PVL. RESULTS: Nucleotide variation was detected at 12 of 1726 sites. Two PVL sequence variants, the R variant and the H variant, were identified on the basis of a substitution at nt 527. Of sequences obtained in the United States, 96.7% harbor the R variant, whereas 95.6% of sequences obtained outside the United States harbor the H variant; 91.3% of MRSA isolates harbor the R variant, and 91.3% of methicillin-susceptible strains harbor the H variant. A molecular model of PVL shows 3 mechanisms by which the amino acid substitution may affect PVL function. CONCLUSIONS: All sampled PVL genes appear to share a recent common ancestor and spread via a combination of clonal expansion and horizontal transfer. US isolates harbor a variant of PVL that is strongly associated with MRSA infections. Protein modeling reveals that this variant may have functional significance. We propose a hypothesis for the origin of USA300.