Phenotypic and Genotypic Study of Inducible Clindamycin Resistance in Clinical Isolates of Staphylococcus aureus in Tabriz, Northwest Iran

Abstract

Background: Resistance to macrolide, lincosamide, and streptogramin B (MLSB) antibiotics is mediated by erm and msrA genes in Staphylococcus aureus. The expression of these genes can lead to three phenotypes, namely constitutive resistance (cMLSB), inducible resistance (iMLSB), which are resistant to macrolide, lincosamide, and streptogramin B antibiotics, and MSB phenotype, which is resistant only to macrolide and streptogramin B. Inducible clindamycin resistance is an important concern because it is not detected in routine laboratory tests. Objectives: The aim of this study was to determine the frequency of MLSB phenotypes and genotypes among 215 clinical isolates of S. aureus and then, examine their resistance to antibacterial agents, which is recommended for methicillin-resistant S. aureus (MRSA) isolates. Methods: Two hundred and fifteen non-repetitive clinical isolates of S. aureus were collected. Resistance to antibacterial agents was determined by disk diffusion and E-test methods. Susceptibility to clindamycin and erythromycin was tested by D-test. All isolates were screened by PCR for the presence of nucA, mecA, ermA, ermB, ermC, and msrA genes. Results: The prevalence of iMLSB, cMLSB and MSB phenotypes among all the isolates was determined as 10.69%, 34.42%, and 0%, respectively. In our study, iMLSB was prevalent more in methicillin susceptible S. aureus (MSSA) (11.71%) than MRSA (9.19%) isolates (P = 0.557). In contrast, the rate of cMLSB was significantly higher in MRSA (79.31%) than MSSA (3.90%) isolates (P = 0.000). No MSB phenotype was detected in our study. The most prevalent genes were ermC and ermA with 39% and 21.5% frequencies, respectively. Six isolates showed D phenotype, while the PCR results of erm genes were negative. All 215 isolates of S. aureus were negative for the presence of ermB and msrA genes. Conclusions: The rate of iMLSB in S. aureus isolates is relatively high in the Northwest Iran. Since isolates with inducible resistance may mutate and change to constitutive resistance, to prevent clinical treatment failure, D-test should be performed along with routine antibiotic susceptibility tests. In this study, ermC gene was the predominant genetic determinant for the expression of MLSB resistance. This predominance is probably due to the spread of distinctive clones (which carry ermC gene) in our region.