Methicillin Resistant Staphylococcus Aureus : Phenotypic and genotypic characterization and detection of glycopeptide and oxazolidinone resistance - Report from a tertiary care centre in South India

Background: : Methicillin Resistant Staphylococcus Aureus (MRSA) is an important nosocomial pathogen causing significant mortality and morbidity. There is a growing body of evidence showing that glycopeptide and oxazolidinone Minimum Inhibitory Concentration (MIC) against Staphylococcus aureus strains are increasing incrementally (MIC creep).This is a cause for concern and needs proactive detection and prevention

Objective:

To characterize health care associated MRSA (HA-MRSA) by phenotypic and genotypic methods; to determine MIC creep and presence of emerging resistance for the drugs of choice for MRSA in India  namely vancomycin , teicoplanin and linezolid.

Methods:

One hundred and twenty HA-MRSA (Blood stream -22, skin and soft tissue infections -94, respiratory infections-3, cerebrospinal fluid-1) isolated during the period September 2009 to August 2010 were included in the study.Methicillin  resistance was screened by disc diffusion method using oxacillin (1µg) and cefoxitin (30µg )disks. MIC was determined to cefoxitin, vancomycin, teicoplanin and linezolid in accordance with CLSI guidelines using suitable controls. Detection of Penicillin binding Protein 2a (PBP2a) was done by latex agglutination and mec A gene by Polymerase Chain Reaction (PCR)

Results:

The prevalence of MRSA during the study period was 45.5%.All the isolates expressed PBP2a and carried mec A gene .MIC ₉₀ to vancomycin , linezolid  and teicoplanin were 1µg, 4µg and 0.5µg /ml respectively.There were two isolates of vancomycin intermediate Staphylococcus aureus (VISA ) with MIC of 4 µg/ml.and three isolates resistant to linezolid (MIC-8µg/ml).All isolates had Teicoplanin MIC within the susceptible range and cefoxitin MIC in the resistant range

Conclusions:

MRSA is a significant threat in developing countries. Coexistent resistance to glycopeptides is a reality and increasing MICs to the drugs of choice may lead to treatment failures. Appropriate antimicrobial prescribing by health care providers to reduce suboptimal exposures to drugs, strict adherence to infection control and treatment guidelines and control of MRSA infections by active screening is the need of the hour