Role of Oxacillin Susceptibility Testing Strategy in Changing Scenario of mecA Positive Staphylococcus aureus Isolates (OS-MRSA) Detection

Abstract

Staphylococcus aureus strains that are mecA and PBP2a positive but phenotypically susceptible to oxacillin are becoming more and more abundant, according to research from all around the world. The oxacillin susceptibility of Staphylococcus aureus (OS-MRSA) contributes to consequent treatment-failure due to misidentification by conventional susceptibility tests. Therefore, the objective of the current study was to ascertain the prevalence of OSMRSA in a tertiary care facility located in Mysore, South India. 395 MRSA isolates collected from diverse clinical samples were included in this lab-based prospective investigation. These isolates were tested using an oxacillin 1μg disc phenotypically by standard disc diffusion test, and simultaneously MIC to Oxacillin was determined from Vitek2 systems. Additionally, MRSA specific mecA gene detection was applied to these isolates in order to confirm their MRSA status genotypically. PCR findings demonstrate that 65% of the isolates were MRSA. The vitek2 system detected 4.06% OS-MRSA isolates with an oxacillin MIC of ≤2µg/ml. The disc diffusion method identified a total of 13.75% isolates as oxacillin sensitive and 10% isolates were oxacillin intermediately sensitive. Oxacillin sensitivity was shown for 1.87% of the mecA-positive MRSA isolates using the VITEK2 and disc diffusion techniques. This analysis found isolates with lower oxacillin MICs but relatively reduced OS-MRSA incidence. Using an oxacillin disc for routine laboratory MRSA detection might occasionally produce false negative results, which can result in improper antibiotic administration and treatment failure. In order to distinguish OS-MRSA from MRSA, it is crucial to combine phenotypic and genotypic techniques.