Phenotypic and genotypic expression of clindamycin resistance Among clinical isolates of staphylococcus aureus

Resistance to antimicrobial agents among Staphylococcus aureus is an increasing problem. Emergence of Methicillin resistance in hospital acquired and also among community acquired Staphylococcus aureus has resulted with very few therapeutic options to treat staphylococcal infections. The Macrolide Lincosamide Streptogramin B [MLSB] family of antibiotics is one such alternative with clindamycin being the preferred drug due to its excellent pharmacokinetic properties. Hence clindamycin is commonly used to treat serious infections including skin and soft tissue infections produced by drug resistant Staphylococcus aureus including MRSA. As clindamycin is a safe drug to use in serious MRSA infections, it was continuously misused resulting in increased resistance to the drug. Clindamycin resistance may be constitutive or inducible. Two common genes responsible for resistance to macrolide, lincosamide and streptogramin B (MLSB) antibiotics are the ermA and ermC genes. In vitro routine tests for clindamycin susceptibility may fail to detect inducible clindamycin resistance due to erm genes resulting in treatment failure; thus necessitating the need to detect such resistance by a simple disc approximation test [D test] on a routine basis. The present study was undertaken to know the rate of inducible clindamycin resistance and erm genes among clinical isolates of Staphylococcus aureus in our hospital. Materials and Methods: 355 Staphylococcal species were isolated from various clinical specimen in the department of microbiology over a period of 6 months. Of which, 81 S. aureus isolated and identified by standard protocol were included in the present study. MRSA & MSSA were detected using cefoxitin [30 µg] disc as per CLSI criteria. Antibiotic sensitivity to routine antimicrobial agents was done by Kirby Bauer’s disk diffusion method. D test was performed on all erythromycin resistant MRSA and MSSA isolates to detect phenotypic expression of clindamycin resistance. ermA and ermC genome identified by PCR on D test positive isolates. Results: Out of 81 isolates of S. aureus, 18 (22%) were found to be MRSA & 63(78%) MSSA. Erythromycin resistance was seen in 70(86%) & 11(14%) were erythromycin sensitive of S aureus. Out of the total 70 erythromycin resistant strains of Staph aureus, 22(31%) isolates showed inducible clindamycin resistance, 26(37%) showed constitutive clindamycin resistance and 22(31%) were MS phenotypes. Hence D test was positive among 16(73%) MSSA & 06(27%) MRSA isolates. D test positive strains (22) were tested for ermA and ermC genes. 4/22(18.2%) ermA genes and 15/22(68.2%) ermC in erythromycin resistant S aureus strains respectively. Conclusion: Clindamycin is a very safe and effective drug which can be used against CA & HA MRSA infection. Development of in vivo therapeutic failure can be prevented by doing a D test routinely in microbiology lab.