Antimicrobial susceptibility profiles of meticillin-susceptible and -resistant Staphylococcus aureus: focus on daptomycin minimum inhibitory concentrations at a tertiary care centre in Mumbai, India

Abstract

In this study, daptomycin minimum inhibitory concentrations (MICs) for Staphylococcus aureus isolates were determined using Etest strips and were correlated with staphylococcal cassette chromosome mec (SCC mec) types and Panton–Valentine leukocidin (PVL) gene positivity. In total, 60 meticillin-resistant S. aureus (MRSA) and 60 meticillin-susceptible S. aureus (MSSA) isolates from clinical samples were subjected to antimicrobial susceptibility testing by disk diffusion according to Clinical and Laboratory Standards Institute guidelines, polymerase chain reaction (PCR) detection of PVL and mecA genes, and SCC mec typing. Daptomycin MICs were determined using Etest strips. The mecA gene was present in all MRSA isolates and was absent in 59 MSSA isolates. The PVL gene was present in 41 MRSA isolates and 25 MSSA isolates. Amongst the MRSA isolates, 10 were SCC mec type III, 27 were SCC mec IV and 23 were SCC mec V. Moreover, 26 SCC mec IV and 15 SCC mec V isolates were PVL-positive. All SCC mec III isolates were multidrug-resistant and PVL-negative. Daptomycin MICs ranged from 0.047 μg/mL to 1 μg/mL for MRSA and from 0.19 μg/mL to 1 μg/mL for MSSA. All MRSA and MSSA isolates were susceptible to daptomycin. Although SCC mec III MRSA and PVL-positive MSSA were resistant to more antimicrobial classes than SCC mec IV and V MRSA and PVL-negative MSSA, there does not appear to be a significant correlation between SCC mec types, PVL positivity and daptomycin MICs. MICs were not as low as expected for a newly introduced drug.