Phenotypic characterization of inducible and constitutive clindamycin resistance in clinical Staphylococcus aureus isolates.

Staphylococcus aureus is a major human pathogen causing infections ranging from mild skin infections to life-threatening systemic conditions. Clindamycin has long been used to treat serious Staphylococcus aureus infections due to its pharmacokinetic profile and ability to inhibit toxin production. However, inducible resistance to clindamycin via erythromycin ribosomal methylases encoded by erm genes poses a significant clinical challenge; it often leads to treatment failure and remains undetected by routine antimicrobial susceptibility testing. This cross-sectional study analyzed 120 non-duplicate clinical Staphylococcus aureus isolates from the University Hospital Center of Fez, Morocco. Identification followed standard microbiological methods, and susceptibility testing adhered to the European Committee on Antimicrobial Susceptibility Testing (EUCAST/CA-SFM) 2024 guidelines. Inducible clindamycin resistance was assessed using the D-test, while methicillin resistance was determined by the cefoxitin disk diffusion method. Of the 120 isolates, 50 (41.67%) were resistant to erythromycin. Constitutive macrolide-lincosamide-streptogramin B (MLSB) resistance was detected in 31 isolates (25.83%), inducible MLSB (iMLSB) resistance in 5 isolates (4.17%), and the macrolide-streptogramin B (MS) phenotype in 14 isolates (11.66%). All isolates with inducible clindamycin resistance or the MS phenotype were methicillin-susceptible Staphylococcus aureus (MSSA), while constitutive MLSB resistance was more frequent among methicillin-resistant Staphylococcus aureus (MRSA). Inducible clindamycin resistance, though less prevalent, remains clinically significant and can be readily detected using the D-test. Routine use of the D-test is essential to guide appropriate antimicrobial therapy and prevent clindamycin treatment failure.