Evaluation of different phenotypic methods to detect methicillin resistance in Staphylococcus aureus isolates recovered from cystic fibrosis patients.

Methicillin-resistant Staphylococcus aureus (MRSA) detection in cystic fibrosis (CF) is challenging. We compared different phenotypic methods among 157 S. aureus from 136 CF-patients: cefoxitin (FOX) and oxacillin (OXA) broth-microdilution; MicroScan-WalkAway®; FOX and OXA disk-diffusion (DD), and PBP2a-latex agglutination. PCR detection of mecA/mecC was the gold standard. Growth on ChromID-MRSA agar was evaluated and compared with that of 157 blood culture (BC) isolates. ChromID-MRSA was also tested on sputa from 111 CF-patients. 32 isolates (20%) were mecA-positive. Both FOX DD and MicroScan-WalkAway® (FOX/OXA) showed the highest sensitivity and specificity (100% and 100%, 96.9% and 99.2%, 96.9% and 100%). ChromID-MRSA showed an excellent sensitivity for BC and CF-isolates (100% and 96.9%) but a poorer specificity for CF ones (95.5% vs. 73.7%), which was also observed when samples were seeded on this medium. FOX DD and MicroScan-WalkAway® are suitable for MRSA detection among CF-isolates and should be used to confirm ChromID-MRSA positive CF-cultures.