Detection of Inducible and Constitutive Clindamycin Resistance in Staphylococcus aureus and Its Association with Methicillin Resistance

Abstract

Background: Inducible clindamycin resistance (iMLSB) in Staphylococcus aureus appears susceptible on routine testing and it is detected throgh D-test. Local data on its frequency in methicillin resistant Staphylococcus aureus (MRSA) and methicillin sensitive Staphylococcus aureus (MSSA) guide the safe use of clindamycin.
Methods: This cross-sectional study was conducted in the Department of Microbiology, Teerthanker Mahaveer Medical College and Research Centre, Moradabad, from March 2025 to February 2026. Non-duplicate S. aureus isolates from clinical specimens were isolated by standard methods. Methicillin resistance was detected by the cefoxitin disk (30 µg), and antimicrobial susceptibility by Kirby-Bauer disk diffusion according to CLSI 2025. The D-test was done on erythromycin-resistant, clindamycin-susceptible or intermediate isolates. Data were analysed in SPSS version 27 using the chi-square test (p < 0.05).
Results: Of 150 isolates, 91 (60.7%) were MRSA and 59 (39.3%) were MSSA. The MS phenotype was found in 68 (45.3%), constitutive MLSB in 57 (38.0%) and iMLSB in 24 (16.0%), and 81 isolates (54.0%) were clindamycin-resistant overall. Phenotypes differed significantly between MRSA and MSSA (p = 0.0008), with iMLSB in 19.8% versus 10.2% and cMLSB in 47.3% versus 25.4%. Phenotype also differed by specimen (p = 0.001): iMLSB occurred in 12 blood (20.3%) and 12 pus (15.6%) isolates. There was no significant association with age, gender, IPD/OPD or ICU location. All isolates were susceptible to vancomycin and teicoplanin, and only one was resistant to linezolid (0.7%). Resistance to fluoroquinolones exceeded 87%.
Conclusion: One in six isolates showed inducible clindamycin resistance that routine testing would have missed. Resistance was higher in MRSA and in blood isolates. The D-test should be included in routine susceptibility testing.