Background: Coagulase negative staphylococcus (CNS) spp. is a major pathogenic organism of nosocomial and community-acquired urinary tract infections, and causes infections in the immunocompromised host, and, in particular, bloodstream infections in patients with indwelling devices. High prevalence of methicillin resistance has been noticed in CNS which also have been recognized as an important multidrug resistant pathogen. The optimal phenotypic method for detecting methicillin resistance still remains controversial, and new guidelines for detecting methicillin resistance of CNS was proposed by NCCLS in January 1999. We evaluated the relationship between mecA gene by PCR method and antimicrobial susceptibility tests according to the new NCCLS guidelines.

Methods: A total of 82 CNS isolates were examined for oxacillin MICs and penicillin MICs by disk diffusion and agar dilution method according to NCCLS guidelines, and detection of mecA gene by PCR.

Results: In disk diffusion method, 66 strains (80.5%) and 63 strains (76.8%) showed resistance to penicillin and oxacillin, respectively, and in agar dilution method, 71 strains (86.6%) and 53 strains (64.6%), respectively. In PCR method, mecA genes were detected in 49 strains (59.8%). Comparing with mecA gene detection by PCR method, the sensitivity of disk diffusion and agar dilution method was 95.8% and 89.8%, respectively. However, the sensitivity of disk diffusion and agar dilution method was 65.3% and 75.5%, respectively using previous NCCLS criteria.

Conclusion: The new criteria of NCCLS detects the methicillin resistance induced by mecA gene more sensitively than the previous one. (Korean J Clin Microbiol 2000;3(1):57-61)