Abstract
Methicillin-resistance in staphylococci results from expression of mecA, which occurs in a larger region of DNA (the mec region) lacking counterpart in susceptible cells. The mec region harbors in addition a highly polymorphic element, the dru (direct repeat unit) segment, which in an early S. aureus strain, BB270, was found to contain 10 imperfect 40 base-pair repeats. We have explored the utility of direct sequencing of dru segments for discriminating among strains of methicillin-resistant S. aureus (MRSA) and coagulase-negative staphylococci (MRCNS). We sequenced dru segments of 24 clinical isolates of MRSA, and 15 of MRCNS, and reexamined strain BB270. Six S. aureus and 2 S. epidermidis isolates were found to have deletions which removed all drus. The other strains were found to have multiple contiguous dru repeats of precisely 40 bp. Analysis of these strains plus dru segment sequence from 4 recent reports yielded 18 unique dru segment sequences (designated "dru types") differing in numbers of repeats and/or sequences of particular repeats. Dru typing was more discriminating than sequencing of non-mec region genes, including a repeat-containing segment (spa Xr) of the S. aureus protein A gene. Yet dru type was sufficiently stable to register epidemiological clusters. Dru sequencing is a useful tool for tracking methicillin-resistant lineages of S. aureus and CNS.
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