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Background: Staphylococcus aureus is the leading pathogen responsible for hospital-acquired pneumonia (HAP). Vancomycin is the primary antimicrobial choice for methicillin-resistant S. aureus (MRSA) HAP worldwide, but S. aureus isolates have been reported susceptible or even resistant to vancomycin.

Objectives: This study compares automated and non-automated susceptibility patterns for MRSA isolates to vancomycin to determine comparability of testing methods for this drug.  

Methods: Respiratory samples submitted to Clinical Microbiology from patients in the ICU were plated onto sheep blood agar and chocolate agar media and visually inspected at 12-24 hours. The presence of S. aureus was determined serologically.  S. aureus isolates were subcultured for susceptibility testing to detect MRSA respiratory isolates. Consecutive MRSA respiratory isolates were analyzed for susceptibility and minimum inhibitory concentration (MIC) using E-test strips and BD Phoenix automated system testing for vancomycin and the vancomycin alternative agents teicoplanin, linezolid and tigecycline. 

Results: Ninety-five MRSA respiratory isolates were collected. Three were obtained by bronchoalveolar lavage and the remainder by protected alveolar lavage. There was no heteroresistance detected by E-test at 24 hours, but there were 4 isolates at 48 hours demonstrating elevated vancomycin MIC values of 6 ug/ml.

Conclusions: Vancomycin heteroresistance was not a problem in this series of isolates. Agreement between vancomycin automated MICs and E-test MICs was not universal. Although under-calls were few based upon automated MIC, several instances were indeed identified.