Prevalence of Staphylococcus aureus Isolated from Clinical Samples that Exhibits Inducible Clindamycin Resistance: A Cross-sectional Study

Introduction: The resistance to Macrolide-Lincosamide- Streptogramin B (MLSB) in Staphylococcus aureus is often caused by erm genes, which can either be constitutive MLSB (cMLSB) or inducible MLSB (iMLSB). Clindamycin (CLI), a lincosamide, and erythromycin (ERY), a macrolide, both act by binding to the 50S ribosomal subunits of bacterial cells, inhibiting protein synthesis. Methylation of the ribosomal target site in iMLSB-resistant isolates displays resistance to ERY but allows susceptibility to CLI. iMLSB resistance emerges when a strong inducer of the methylase enzyme, such as ERY, is present. Identifying iMLSB resistance is crucial for effectively managing S. aureus. Aim: To detect the prevalence of inducible CLI resistance in S. aureus isolates from various clinical samples and to determine the association between methicillin resistance and inducible CLI resistance in S. aureus isolates. Materials and Methods: The study was a cross-sectional study conducted at Muzaffarnagar Medical College and Hospital, Muzaffarnagar, Uttar Pradesh, India over a period of six months, from January 2024 to June 2024. A total of 100 isolates of S. aureus obtained from various clinical samples were included. Simple random sampling was employed to ensure an unbiased selection of isolates. Only culture-confirmed S. aureus isolates were considered, while duplicate isolates and samples showing polymicrobial growth were excluded from the study. The sample population comprised individuals of varying ages and genders who sought treatment at the hospital during the study period due to symptoms indicating potential infections. The S. aureus isolates were initially identified using standard biochemical techniques and were then tested for susceptibility using the modified Kirby-Bauer disc diffusion method on Mueller-Hinton agar plates, following Clinical and Laboratory Standards Institute (CLSI) guidelines. The CLSI guidelines were also followed to test for inducible resistance to CLI using the ‘D test’ method. Data were entered and analysed using the Statistical Package for the Social Sciences (SPSS) software, version 24.0, with statistical significance considered at a p-value <0.05. Results: Routine disc diffusion testing was employed to examine antibiotic susceptibility in 100 S. aureus strains, revealing that 44 (44%) displayed resistance to ERY. Out of the 100 strains, 22 (22%) showed iMLSB resistance, 27 (27%) were cMLSB-positive, and 15 (15%) had the Macrolide-Streptogramin B (MS) phenotype. In this study, it was discovered that 18 (82%) of the S. aureus isolates that were positive for the D test were Methicillin-Resistant S. aureus (MRSA), while 4 (18%) were sensitive to cefoxitin (MSSA). Conclusion: This study revealed a substantial prevalence of inducible CLI resistance among S. aureus isolates, with a pronounced association in MRSA strains. The strong correlation between methicillin and iMLSB resistance underscores the critical need for routine D-test screening to guide precise antimicrobial therapy, mitigate treatment failures and support effective antimicrobial stewardship practices.