Evaluation of automated vitek system & conventional method for the detection of heteroresistant vancomycin intermediate staphylococcus aureus

Vancomycin has remained the mainstay for the treatment of MRSA infection for the past four decades. The overuse of the drug has lead to selective pressure overtime leading to the emergence of heteroresistant Vancomycin Intermediate Staphylococcus aureus (hVISA) & VRSA. The evolution of S.aureus to MRSA and now to hVISA and VISA is an important and ongoing public health concern.

1) To evaluate Automated Vitek 2 System & Conventional method for the detection of hVISA. 2) To assess the percentage of hVISA phenotypes amongst Staphylococcus aureus isolates at a tertiary care hospital in Bangalore.

A study was conducted in VIMS & RC for a period of 4 months from October 2015-January 2016. 100 isolates of S.aureus from various clinical samples were identified by conventional methods including direct smear, colony morphology & Gram's stain. Determination of ID & MIC by Vitek 2 testing (AST-628 Gram-positive susceptibility card; bioMérieux, Inc.,) was performed for all isolates according to manufacturer's instructions. MRSA isolates with hVISA phenotypes were detected. Next, a 2 McFarland Standard suspension of MRSA isolates was made, 200 μl of this suspension was pipetted onto BHI plate & swabbed evenly. Vancomycin Etest strip (bioMérieux) was placed and incubated at 35º C for 48 hrs. An MIC of 8μl or even a single mutant growth was taken as heteroresistance.

Out of 100 isolates of S.aureus, 57 isolates (57%) were MRSA by Vitek.

Out of the 57 isolates of MRSA, 17 (29.8%) were detected as hVISA by Vitek-2 system & 6 (10.5%) by conventional modified macrodilution Etest.

Vitek-2 system plays a crucial role in the detection of hVISA compared to conventional modified macrodilution Etest.