Various Resistance Patterns and their Correlation with Minimal Inhibitory Concentrations against Clindamycin among Methicillin Resistant Staphylococcus Aureus Isolates from Critical Care Units of a Rural Tertiary Care Hospital in Uttarakhand State, India
Background: The macrolide lincosamide streptogramin B (MLSB) family of antibiotics is among the important treatment options for skin and soft tissue infections caused by methicillin resistant Staphylococcus aureus (MRSA). However due to their widespread use resistance has emerged to this group as well. Objective: To determine the various resistance patterns among MRSA isolates from critically ill patients and to compare them with minimal inhibitory concentrations (MIC) of clindamycin (CL).
Materials and Methods: A total of 67 MRSA isolates were studied by disc approximation test (D-test) to detect inducible resistance to CL and MIC by agar dilution technique. Results: Of the total 67 MRSA isolates, 38 were sensitive to both CL and erythromycin (ER). Twenty nine isolates showed different resistance patterns; 13 were positive for D-test indicating inducible resistance (MLSBi), 11 were positive for constitutive resistance (MLSBc) and five isolates showed possible efflux mechanism for macrolide resistance and were D-test negative (MS phenotype). Out of 38 sensitive isolates, seven showed subpopulations inside the CL-sensitive zone. When tested further six belonged to ‘R phenotype’ and one to ‘D phenotype’. Out of 38 sensitive isolates, nine isolates showed high MIC values (≥4μg/ml) for CL. All the 11 isolates of MLSBc phenotype also showed very high MIC values.
Conclusion: Differentiating the various resistance patterns and detecting MLSBi resistance among MRSA helps to avoid any treatment failure with CL, thus preventing any pharmacotherapeutic and pharmacoeconomic losses from patient’s perspective. Also testing the subpopulation inside the CL-sensitive zone raises the question about the existence of hetero-resistance or some other resistance mechanisms.