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Journal of Clinical Microbiology, January 1998, p. 52-57, Vol. 36, No. 1
Laboratoire de Bactériologie, CHU de
Rouen, Hôpital Charles Nicolle, 76031 Rouen Cedex, France
Received 23 June 1997/Returned for modification 22 August
1997/Accepted 15 October 1997
The performance characteristics of the E-test (AB Biodisk, Solna,
Sweden), the ATB Staph, the Rapid ATB Staph, and the Vitek GPS-503 card
(bioMérieux, La Balme Les Grottes, France) methods for the
detection of oxacillin resistance in a collection of staphylococci with
a high proportion of troublesome strains were evaluated. Sixty-four
Staphylococcus aureus strains and 76 coagulase-negative staphylococcal strains were tested. All strains were mecA
positive and were characterized by the oxacillin agar screen plate
test; 75 (53.6%) were found to be heterogeneous by a large-inoculum oxacillin disk diffusion assay, and oxacillin MICs for 89 (63.6%) were
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Comparison of ATB Staph, Rapid ATB Staph, Vitek, and E-Test
Methods for Detection of Oxacillin Heteroresistance in
Staphylococci Possessing mecA
32 µg/ml. Three (4.7%) S. aureus strains and 25 (32.9%) coagulase-negative strains were classified as susceptible by
the E-test, as defined by the National Committee for Clinical
Laboratory Standards (NCCLS) oxacillin breakpoint (MIC
2 µg/ml). The ATB Staph method failed to detect oxacillin resistance in
7 (11%) S. aureus isolates and 32 (42.1%)
coagulase-negative isolates. The MICs for all but six of these
discrepant isolates were
16 µg/ml. The Rapid ATB Staph method was
tested against S. aureus strains only and yielded 15 (23.4%) false-susceptible results for strains for which the MICs were
32 µg/ml. The Vitek system was the best-performing system, since it
failed to detect oxacillin resistance in only 3 (4.7%) S. aureus strains and 15 (19.7%) coagulase-negative strains, the
MICs for all of which were
2 µg/ml. These data indicate that (i)
the performance of the two ATB Staph systems can be limited when the
prevalence of borderline-heteroresistant staphylococci is high and (ii)
the unreliability of the E-test and the Vitek methods for detecting
resistant coagulase-negative strains might be reduced by the potential
revision of the oxacillin breakpoint currently recommended by the
NCCLS.
*
Corresponding author. Mailing address: C.H.U. de Rouen,
Hôpital Charles Nicolle, Laboratoire de Bactériologie, 1, rue de Germont, 76031 Rouen Cedex, France. Phone: 33 2 32 88 80 52. Fax: 33 2 32 88 80 24. E-mail: bacteriologie{at}chu-rouen.fr.
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