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Journal of Clinical Microbiology, Feb 1996, 249-253, Vol 34, No. 2
MK York, L Gibbs, F Chehab and GF Brooks
Approximately 75% of coagulase-negative staphylococci are resistant to
methicillin, but it is suspected that even more resistance exists that is
not detected by standard susceptibility assays. To determine the most
accurate assay for measuring resistance, we compared the detection of mecA
by PCR with detection by National Committee for Clinical Laboratory
Standards methods using oxacillin as the class drug. Strains from 11
species of coagulase-negative staphylococci were selected such that 84%
were susceptible by the broth microdilution method. Of 45 mecA- positive
strains, 1 strain was unable to express the mecA gene product after
induction and was not included in further analyses. For microdilution with
2% NaCl, the disk test without salt, and agar screen containing 4% NaCl
plus-6 micrograms of oxacillin per ml, the sensitivities in detecting the
44 mecA-positive strains were 50, 84, and 70%, respectively, at 24 h and
77, 82, and 100%, respectively, at 48 h. The specificities of
microdilution, disk, and agar screen in detecting the 97 strains lacking
mecA were 100, 89, and 100%, respectively, at 24 h. Only the disk test
proved to be less specific at 48 h (81%). Furthermore, for 10 of the
mecA-positive strains plus an additional 8 strains subsequently added to
the analyses, the MICs were 2 micrograms/ml at 24 h by the broth
microdilution method; all 18 strains were positive for mecA by PCR. Thus,
an oxacillin MIC of > or = 2 micrograms/ml indicated resistance and is
probably a more appropriate breakpoint than the current National Committee
for Clinical Laboratory Standards breakpoint of 4 micrograms/ml for
coagulase-negative staphylococci. Strains for which MICs are < 2
micrograms/ml may be methicillin resistant and should be verified as
susceptible by oxacillin agar screening with incubation for 48 h.
Copyright © 1996 by the American Society for Microbiology. All rights reserved.
Comparison of PCR detection of mecA with standard susceptibility testing methods to determine methicillin resistance in coagulase- negative staphylococci
Department of Laboratory Medicine, University of California, San Francisco 94143, USA. york@pangloss.ucsf.educ
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