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
dru
s. 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.