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ABSTRACT Objectives: Staphylococcus aureus is one of the most important bacterial pathogens in clinical practice and a primary diagnostic focus for the routine microbiology laboratory. The aim of this study was to find out the phenotypic and genotypic variations in Staphylococcus aureus isolates at a tertiary care center in Lucknow. Methods: 140 clinical isolates of S. aureus were taken in the study. Kirby–Bauer disc diffusion method was performed to identify antibiotic susceptibility testing, phenotypically methicillin-resistant Staphylococcus aureus (MRSA) were identified by using cefoxitin disc (30 μg), and inducible clindamycin resistance was identified by the presence of D-shaped zone around clindamycin and by using conventional PCR method mecA and ermA genes were identified. Results: Out of 140 clinical isolates S. aureus, 93 (66.4%) were MRSA, and 47 (33.6%) were methicillin-sensitive Staphylococcus aureus (MSSA). Phenotype iMLSb was 41 (29.3%), cMLSb phenotype was 37 (26.4%), mecA gene was present in 84 (60%), and none of the samples showed ermA gene positivity. Conclusion: As we know, the presence of the mecA gene is the major evidence for the detection of MRSA isolates. Their presence in low numbers opens the door to search for other mechanisms that may compete with mecA gene in producing resistance phenomenon. The absence of ermA gene in strains S. aureus with iMLSb and cMLSb phenotypes concluded that some other erm gene is responsible for this MLS type of resistance. Due to the frequency of MRSA strains showing the iMLSb phenotype, the use of clindamycin in erythromycin-resistant strains cannot be recommended due to the high possibility of failure in treatment with this antibiotic. J Microbiol Infect Dis 2021 11(4):6-11.

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