Investigation of Staphylococcus aureus isolates identified as erythromycin intermediate by the Vitek-1 System: comparison with results obtained with the Vitek-2 and Phoenix systems.

ABSTRACT

We identified 69 Staphylococcus aureus isolates that were erythromycin intermediate as reported by the Vitek-1 system using the GPS-105 card. Of the 57 strains that were available for further testing, all were erythromycin resistant by broth microdilution and the Phoenix system, while the Vitek-2 system identified 55 of 57 strains (96%) as erythromycin resistant. The majority of isolates (54 of 57 [95%]) exhibited the inducible MLS (macrolide-lincosamide-streptogramin family) phenotype, as shown by the double-disk test. We recommend that all S. aureus strains determined as erythromycin intermediate by the Vitek-1 system be interpreted as resistant to erythromycin.