Multilocus sequence typing for characterization of methicillin-resistant and methicillin-susceptible clones of Staphylococcus aureus

A multilocus sequence typing (MLST) scheme has been developed for Staphyloc occus aureus. The sequences of internal fragments of seven housekeeping gen es were obtained for 155 S. aureus isolates from patients,vith community-ac quired and hospital-acquired invasive disease in the Oxford, United Kingdom , area. Fifty-three different allelic profiles were identified, and 17 of t hese were represented by at least two isolates. The MLST scheme was highly discriminatory and was validated by showing that pairs of isolates with the same allelic profile produced very similar SmaI restriction fragment patte rns by pulsed-field gel electrophoresis. All 22 isolates with the most prev alent allelic profile were methicillin-resistant S. aureas (MRSA) isolates and had allelic profiles identical to that of a reference strain of the epi demic MRSA clone 16 (EMRSA-16). Pour MRSA isolates that were identical in a llelic profile to the other major epidemic MRSA clone prevalent in British hospitals (clone EMRSA-15) were also identified. The majority of isolates ( 81%) were methicillin-susceptible S. aureas (MSSA) isolates, and seven MSSA clones included five or more isolates. Three of the MSSA clones included a t least five isolates from patients with community-acquired invasive diseas e and may represent virulent clones with an increased ability to cause dise ase in otherwise healthy individuals. The most prevalent MSSA clone (17 iso lates) was very closely related to EMRSA-16, and the success of the latter clone at causing disease in hospitals may be due to its emergence from a vi rulent MSSA clone that was already a major cause of invasive disease in bot h the community and hospital settings. MLST provides an unambiguous method for assigning MRSA and MSSA isolates to known clones or assigning them as n ovel clones via the Internet.