DISSEMINATION AMONG STAPHYLOCOCCI OF DNA-SEQUENCES ASSOCIATED WITH METHICILLIN RESISTANCE

DNA probes consisting of pUC19 containing cloned Staphylococcus aureus chromosomal fragments were constructed from two methicillin-resistant S. aureus strains with different DNA sequences 5' to mecA, the gene that mediates methicillin resistance. The probe from one strain, BMS1, contained a portion of the regulatory sequences (the terminal 641 bp of mecRI and all of mecI associated with the induction and repression of mecA transcription (pG0195). The Second probe, from strain COL (pG0198), contained DNA not found in strain BMS1. This DNA was within the sequences added at the site of a mecRI deletion. Genomic digests of 14 S. aureus isolates recovered between 1961 and 1969 all hybridized with pG0198. In contrast, 78% (36 of 46) of the S. aureus organisms isolated since 1988 hybridized with pG0195 but not with pG0198; the remainder hybridized,vith pG0198. No S. aureus isolates hybridized with both probes. Staphylococcus epidermidis digests hybridized with pG0198 (46%), pG0195 (14%), or both probes (35%); all 20 Staphylococcus haemolyticus isolates hybridized with pG0198. The restriction fragment length polymorphism patterns of all pGO198-hybridizing regions in S. aureus were identical to those in strain COL. In addition, the mecRI deletion junction nucleotide sequences of eight S. aureus and six S. epidermidis isolates were identical. However, 21 of 23 S. epidermidis and all 20 S. haemolyticus isolates had from 5 to more than 20 additional chromosomal bands that hybridized with pGO198; none of 21 S. aureus isolates had additional hybridizing bands. These data suggest that the additional DNA responsible for the mecRI deletion was part of a repetitive, and possibly mobile, element resident in coagulase-negative staphylococci but not in S. aureus. These data also support a hypothesis that the deletion event occurred in a coagulase-negative staphylococcus with subsequent acquisition of the interrupted sequences by S. aureus.