Prevalence and Phenotypic Detection of Erythromycin-Induced Resistance to Clindamycin in MRSA Isolates 1195-1198
VELVIZHI G, M.D. (Micro),
Assistant Professor, Department of Microbiology,
Tirunelveli Medical College,
Tirunelveli District, 627 011 Tamil Nadu.
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Background and objectives: The present study was undertaken to determine the prevalence of MRSA in clinical samples in a tertiary care hospital and to demonstrate the in vitro ability of erythromycin to induce clindamycin resistance in clinical isolates of Staphylococci.
Materials and Methods: A total number of 112 Staphylococcus aureus strains were isolated from clinical specimens and MRSA were detected by the routine antibiotic susceptibility testing methods including the oxacillin disk method, the Cefoxitin disc diffusion test and the Oxacillin screen agar method and the results were interpreted as per the standard guidelines. The clinical isolates of erythromycin-resistant (ER-R), clindamycinsusceptible Staphylococci (CL-S) were examined for inducible clindamycin resistance (ICR) by the erythromycin induction test by using the double disc susceptibility test (D-test). Strains which produced ICR showed flattening of the clindamycin disc zone which was adjacent to the erythromycin disc.
Results: Out of the 112 isolates, 29 (25.9%) Methicillin Sensitive Staphylococcus aureus (MSSA) and 83 Methicillin Resistant Staphylococcus aureus (MRSA) were identified by the Cefoxitin disc diffusion test and the Oxacillin screen agar method. Among the 112 Staphylococcus aureus strains which were studied, 67 (32.4%) were erythromycin resistant. These isolates, when they were subjected to the D test, showed 36 (32 %) constitutive MLSB (cMLSB) phenotypic strains which were resistant to both erythromycin and clindamycin and 31 isolates showed clindamycin sensitivity. Out of these,16 (14.2%) strains were D-zone positive i.e. of the inducible MLSB (iMLSB) phenotype, which were resistant to erythromycin and sensitive to clindamycin, while 15 were negative for the D test, thus indicating that they were of the MS phenotype. Of the 36 cMLSB phenotypic strains, 24 isolates were MRSA and 12 were MSSA, while all the iMLSB phenotype strains were MRSA.
Conclusions: We conclude that a significant number of ER-R CL-S strains were positive for ICR, among the MRSA isolates. These isolates should be reported as clindamycin resistant. Given the high rate of inducible resistance to clindamycin in the staphylococcal isolates, the test for inducible resistance to clindamycin should be included in the routine antibiotic susceptibility tests, as it will help in guiding the therapy.