Vancomycin, linezolid and daptomycin susceptibility pattern among clinical isolates of methicillin-resistant...

The efficacy of vancomycin, drug of choice for methicillin-resistant Staphylococcus aureus (MRSA), has become questionable due to the emergence of MRSA isolates with reduced susceptibility. The present study was conducted to determine the vancomycin, linezolid, and daptomycin susceptibility pattern in clinical isolates of MRSA and to observe minimum inhibitory concentration (MIC) creep over 2 years if any.

MIC of vancomycin, linezolid, and daptomycin were determined by E-test in 198 MRSA isolates and their MIC 50, MIC 90, and geometric mean MIC were calculated.

While all isolates were sensitive to vancomycin, linezolid, and daptomycin, MIC 90 of vancomycin increased from 1.5 μg/ml in 2015 to 2 μg/ml in 2016. The percentage of isolates with vancomycin MIC >2 μg/ml doubled in 2016 (12.9%) as compared to 2015 (6.1%). MIC 90 for linezolid remained steady as 3 μg/ml, but geometric mean MIC increased from 2.20 μg/ml in 2015 to 2.29 μg/ml in 2016, and more than 40% isolates showed MIC 3 μg/ml. MIC 90 and geometric mean MIC of daptomycin decreased from 0.75 μg/ml to 0.5 μg/ml and 0.50 μg/ml to 0.36 μg/ml in 2015 and 2016, respectively.

MIC creep was observed with vancomycin. Although linezolid MIC was within the susceptible zone, more than 40% strains showing MIC 3 μg/ml may herald the future development of either resistant or heteroresistant. Daptomycin showed good sensitivity against MRSA isolates. Therefore, it could be considered as an alternative agent for the treatment of infections caused by MRSA. However, it should be reserved where this class has a clear therapeutic advantage over other anti-MRSA drugs.