Genetic relationship between methicillin-sensitive and methicillin-resistant Staphylococcus aureus strains from France and from international sources: delineation of genomic groups.

Cluster analysis of the SmaI patterns, generated by pulsed-field gel electrophoresis, of 44 methicillin-resistant (MRSA) and 118 methicillin-sensitive (MSSA) Staphylococcus aureus strains isolated in various French hospitals and 61 MRSA and 48 MSSA strains from 20 other countries revealed 20 genomic groups distributed into four distantly related phylogenic branches. Eighty-three of the 105 MRSA strains (79%) were clustered in the six genomic groups of phylogenic branch I; and 154 of the 166 MSSA strains (92.8%) were clustered in the 14 genomic groups of phylogenic branches II, III, and IV. Agreement between genomic group and two other markers, esterase type and phage group, was obtained, emphasizing the clonal structure of the population. The genomic groups were delineated by esterase type. The distribution of the strains within the genomic groups was independent of their geographical origin; French strains were clustered with strains from other countries. The three types of the staphylococcal cassette chromosome mec (SCCmec) complex were distributed according to genomic groups. Most of the time, type I and type II SCCmec complexes were found in the MRSA strains belonging to the same genomic groups. In contrast, the type III SCCmec complex was specific to the MRSA strains belonging to the three genomic groups characterized by a common esterase type.