Publications
Investigation of Staphylococcus aureus (SA) with Vitek-1
Erythromycin-Intermediate Susceptibilities - Comparison with Vitek-2
and BD Phoenix. P. TANG, K. PIKE, A. SIMOR, S. RICHARDSON, D.E.
LOW, B.M. WILLEY. Toronto Medical Labs/Mount Sinai Hospital, Toronto,
ON, Canada; Sunnybrook and Women's College Health Sciences Centre,
Toronto, ON, Canada; and Hospital for Sick Children, Toronto, ON,
Canada.
Background:
Most laboratories use automated susceptibility testing systems as
their primary method to detect erythromycin (ery) resistance (R)
in SA, and most default clindamycin (clinda) as resistant when ery
R is detected. While inducible MLS resistance is common among ery-R
SA, no studies have examined strains with intermediate (I) susceptibility
to ery.
Methods:
In total, 69 isolates (63 cross-Canada, 6 U.S.) that were ery-I
according to the Vitek-1 system were identified from a diverse collection
of methicillin-resistant SA (MRSA) and methicillin-susceptible SA
(MSSA). Of these, 57 isolates (55 MRSA, 2 MSSA) were available for
further testing: susceptibilities to ery and clinda were determined
by NCCLS broth microdilution and disk diffusion, and the inducible
MLS phenotype was detected by placing the ery and clinda disks 15mm
apart and measuring clinda R induction zones. Results were compared
to those of Vitek-2 and Phoenix systems.
Results:
According to Vitek-1, 44/57 (77%) strains had ery MIC of 1 µg/mL,
5/57 (9%) were 2 µg/mL and 8/57 (14%) were 4 µg/mL;
all had clinda MIC of <0.5 µg/mL. All 57
were found to have ery MIC >8 µg/mL by broth
microdilution and 56/57 (98%) were ery R by disk diffusion (1 MRSA
was ery-I at 16 mm). 54/57 (95%) had an inducible MLS phenotype.
The Phoenix correctly identified 100% and the Vitek-2 identified
55/57 (96%) of strains as ery-R (MIC >8 µg/mL).
2/57 strains had Vitek-2 ery MIC of 1 and 4 µg/mL;
both had the inducible MLS phenotype.
Conclusions:
The ery-I SA identified in this study represent anomalous results
generated by the Vitek-1 system. As 95% of SA exhibited the anticipated
inducible MLS phenotype, it is recommended that all SA identified
with ery-I and clinda-susceptible profiles by the Vitek-1 system
be interpreted and reported as resistant to both ery and clinda.
42nd INTERSCIENCE CONFERENCE ON ANTIMICROBIAL AGENTS AND CHEMOTHERAPY
(ICAAC), San Diego, California, USA, September 27, 2002. 9:30am
to 11:00am.
©Copyright 2002 Last Modified
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