Clindamycin Resistance among Staphylococcus aureus Clinical Isolates in Alexandria

Background: Macrolides, lincosamides, streptogramins-B (MLSB) resistance can be constitutive or inducible.Staphylococcal strains with constitutive MLSB (cMLSB) phenotype are easily detected using routine disk diffusion technique. Contrarily, inducible MLSB (iMLSB) phenotypeneeds a special double disk diffusion test, D-testto be detected. Misdiagnosis of iMLSB can lead to therapy failure with clindamycin. The aim of this study was to detect the prevalence of erythromycin and clindamycin resistance among Staphylococcus aureus clinical isolates and to detect the resistance encoding genes ermA and ermC. Materials and Methods: One hundred non-repetitive S. aureus isolates were included in this study. Antimicrobial susceptibility was detected to all isolates. The molecular identification of S. aureus and methicillin resistance was performed by detecting femA and mecA genes respectively. iMLSB was detected by D-test assay. The presence of ermA and ermC genes was detected by PCR. Results: Methicillin resistance was detected in 54% of isolates. Most of isolates (73%) were susceptible to erythromycin and clindamycin. Out of the 27 isolates resistant to erythromycin, 10(37%) showed cMLSB phenotype. The remaining 17 (63%) isolates resistant to erythromycin but susceptible to clindamycin were clindamycin were classified into 14 (82%) iMLSB resistance phenotype that showed positive D-test and 3 (18%) macrolide-streptogramin (MSB) resistance phenotype that showed negative D-test. ermA and ermC were detected among the 27 S. aureus isolates resistant to clindamycin and/or erythromycin. Both genes were detected together in 2 isolates (7.5%), ermA was detected alone in 5 isolates (18.5 %) and ermC was detected alone in 20 isolates (74%). Conclusion: From the previous results, we can conclude that D test is a simple, reliable method to detect clindamycin resistance in erythromycin resistant isolates. Nevertheless, the incidence of true susceptibility to clindamycin among erythromycin resistant S. aureus isolates is low.