Susceptibility testing and resistance phenotypic detection in Staphylococcus aureus by conventional and molecular methods: Importance of automated (Vitek 2) system

Background: Once identified as Staphylococcus aureus, it is imperative to understand their methicillin resistance status for institution of appropriate therapy. Conventional disc diffusion tests may not accurately identify MRSA strains and may not be in concordance with PCR results. Objectives: Relative abilities of phenotypic and molecular methods to detect MRSA strains was evaluated to understand possible causes of disagreement between these tests, if any, by using automated microbial identification and antibiotic sensitivity test system. Methods: 70 clinical isolates of S. aureus from North Karnataka were collected and phenotypically characterized as methicillin resistant or sensitive by disc diffusion test. PCR was done to detect mec A gene positivity. Isolates having disagreement between PCR results and disc-diffusion test were analyzed using Vitek 2 automated system (bioMerieux, France. Software version: 05.02). Results and Conclusion: There was 98.5 % (69/70) agreement between PCR and cefoxitin disc diffusion while it was 80% (56/70) between oxacillin and PCR. 14 isolates (20%) showed disagreement between either of the two disc diffusion tests and PCR. 10 of these were classified as MSSA by Vitek 2 with acquired penicillinase activity while 4 were categorized under MRSA by modification of mec A. Since automated systems provide additional information on likely penicillinase /modified mec A activity, use of these systems help clinicians treat patients with such MSSA infections like cases of MRSA. Keywords––MRSA, non mecA resistance, acquired penicillinase, PBP modification, Vitek 2 system.