Comparison between Cefoxitin Disc Diffusion and Phoenix Automated System with mecA/mecC PCR for Determination of Methicillin Resistance in Staphylococcus aureus Isolates and Investigation of the Presence of PVL Gene

Introduction: A fast and accurate determination of methicillin resistance in Staphylococcus aureus strains is vital. This study aimed to compare the sensitivity and specificity of the cefoxitin disc diffusion (CDD) test and BD Phoenix automated system considering mecA/mecC positivity as the gold standard and to investigate the presence of Panton-Valentine leukocidin (PVL) toxin gene, a crucial virulence factor of S. aureus strains. Materials and Methods: Overall, 179 Staphylococcus aureus strains from various clinical samples were included. Antibiotic sensitivity was tested using the Phoenix automated system and by applying the Kirby-Bauer disc diffusion method for cefoxitin (30 μg). The mecA, mecC, and PVL presence was determined using the conventional multiplex polymerase chain reaction method. mecA/mecC positivity was considered as the gold standard. Statistical analysis was performed using SPSS 15.0 for Windows (Chicago, IL, USA). Results: Overall, 91 strains (50.8%) were mecA positive and identified as methicillin resistant Staphylococcus aureus (MRSA). No isolates containing the mecC gene were detected. The Phoenix automated system falsely identified six methicillin-sensitive S. aureus (MSSA) isolates, which were mecA and mecC negative as MRSA. The sensitivity and specificity of the CDD test were found to be 100% in determining MRSA, and the sensitivity and specificity the Phoenix automated system were 100% and 93.2%, respectively. The PVL positivity rate in MRSA and MSSA strains was 6.5% and 7.4%, respectively. All PVL-positive strains were isolated from the skin and soft tissues. Conclusion: The CDD test is a reliable method for routine procedures. Methicillin-sensitive strains can be determined as MRSA via the Phoenix automated system. Nevertheless, mecC-controlled MRSA should not be excluded from methods used for determining methicillin resistance. Panton-Valentine leukocidin toxin gene should be determined to enable clinicians to understand the infection severity.