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Antimicrobial Agents and Chemotherapy, February 2000, p. 231-238, Vol. 44, No. 2
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Correlation between the Resistance Genotype Determined by Multiplex PCR Assays and the Antibiotic Susceptibility Patterns of Staphylococcus aureus and Staphylococcus epidermidis

Francis Martineau,1,2 François J. Picard,1 Nicolas Lansac,1,2 Christian Ménard,1 Paul H. Roy,1,3 Marc Ouellette,1,2 and Michel G. Bergeron1,2,*

Centre de Recherche en Infectiologie de l'Université Laval,1 Division de Microbiologie,2 Faculté de Médecine, Université Laval, Québec G1V 4G2, and Département de Biochimie, Université Laval, Québec G1K 7P4,3 Canada

Received 22 March 1999/Returned for modification 27 June 1999/Accepted 22 October 1999

Clinical isolates of Staphylococcus aureus (a total of 206) and S. epidermidis (a total of 188) from various countries were tested with multiplex PCR assays to detect clinically relevant antibiotic resistance genes associated with staphylococci. The targeted genes are implicated in resistance to oxacillin (mecA), gentamicin [aac(6')-aph(2")], and erythromycin (ermA, ermB, ermC, and msrA). We found a nearly perfect correlation between genotypic and phenotypic analysis for most of these 394 strains, showing the following correlations: 98% for oxacillin resistance, 100% for gentamicin resistance, and 98.5% for erythromycin resistance. The discrepant results were (i) eight strains found to be positive by PCR for mecA or ermC but susceptible to the corresponding antibiotic based on disk diffusion and (ii) six strains of S. aureus found to be negative by PCR for mecA or for the four erythromycin resistance genes targeted but resistant to the corresponding antibiotic. In order to demonstrate in vitro that the eight susceptible strains harboring the resistance gene may become resistant, we subcultured the susceptible strains on media with increasing gradients of the antibiotic. We were able to select cells demonstrating a resistant phenotype for all of these eight strains carrying the resistance gene based on disk diffusion and MIC determinations. The four oxacillin-resistant strains negative for mecA were PCR positive for blaZ and had the phenotype of beta -lactamase hyperproducers, which could explain their borderline oxacillin resistance phenotype. The erythromycin resistance for the two strains found to be negative by PCR is probably associated with a novel mechanism. This study reiterates the usefulness of DNA-based assays for the detection of antibiotic resistance genes associated with staphylococcal infections.

* Corresponding author. Mailing address: Centre de Recherche en Infectiologie, CHUQ (Pavillon CHUL), 2705 Blvd. Laurier, Ste-Foy, Québec, Canada G1V 4G2. Phone: (418) 654-2705. Fax: (418) 654-2715. E-mail: Michel.G.Bergeron{at}crchul.ulaval.ca.

Antimicrobial Agents and Chemotherapy, February 2000, p. 231-238, Vol. 44, No. 2
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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