Correlation between the resistance genotype determined by multiplex PCR assays and the antibiotic susceptibility patterns of Staphylococcus aureus and Staphylococcus epidermidis

Clinical isolates of Staphylococcus aureus (a total of 206) and S. epidermi dis (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 oxa cillin (mecA), gentamicin [aac(6')-aph(2 ")], and erythromycin (ermA, ermB, ermC, and msrA). We found a nearly perfect correlation between genotypic a nd 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) ei ght 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 erythromy cin resistance genes targeted but resistant to the corresponding antibiotic . In order to demonstrate in vitro that the eight susceptible strains harbo ring the resistance gene may become resistant, we subcultured the susceptib le strains on media with increasing gradients of the antibiotic. We were ab le to select cells demonstrating a resistant phenotype for all of these eig ht strains carrying the resistance gene based on disk diffusion and MIC det erminations. The four oxacillin-resistant strains negative for mecA were PC R positive for blaZ and had the phenotype of p-lactamase hyperproducers, wh ich could explain their borderline oxacillin resistance phenotype. The eryt hromycin resistance for the two strains found to be negative by PCR is prob ably associated with a novel mechanism. This study reiterates the usefulnes s of DNA-based assays for the detection of antibiotic resistance genes asso ciated with staphylococcal infections.