Intra-strain variability of methicillin-resistant Staphylococcus aureus strains ST228-MRSA-I and ST5-MRSA-II

Isolates belonging to two major epidemic strains of methicillin-resistant Staphylococcus aureus (MRSA) from clonal complex 5 were characterised using diagnostic microarrays in order to detect and analyse intra-strain variability. Isolates were sampled from hospitals scattered all over Germany. The study included 56 isolates of ST228-MRSA-I, which is also known as the South German Epidemic Strain, and 40 isolates of ST5-MRSA-II (UK-EMRSA-3, Rhine-Hesse Epidemic Strain, New York/Japan Clone), as well as, for comparison, some control strains and overseas isolates of ST5-MRSA-II. Both strains showed a remarkable variability. This affected plasmid-borne resistance genes (tetK, blaZ/R/I, aacA–aphD, qacA), genes from SCCmec elements (aadD, ermA, merA/B/R/T), toxin gene clusters on pathogenicity islands (sec/l, tst1) or, probably, on plasmids, (sed/j/r), the presence or absence of beta-haemolysin-converting phages (sea, sea-N315, sak, chp, scn), deletions of single chromosomal genes (bbp, clfA) or, occasionally, of rather large clusters of neighbouring genes (seg, sei, sem, sen, seo, seu, lukD/E). Both strains could be split into four major clusters each, based on the presence of a mercury resistance operon (merA/B/R/T) and lukD/E in ST228-MRSA-I or of tst1 and enterotoxin genes seD/J/R in ST5-MRSA-II. The use of this variability for typing purposes as well as its phylogenetic significance are discussed.