Detection of Methicillin Resistant Staphylococcus Aureus with Reduced Susceptibility to Vancomycin

Abstract

Background: As there is no molecular-based assays available for the detection of hVISA and VISA. However, increasing amounts of data support a number of methods for the screening and confirmation of heterogeneous vancomycin intermediate S. aureus (hVISA) and vancomycin intermediate S. aureus (VISA) infection. The vancomycin MIC result alone is unable to accurately distinguish hVISA from VSSA isolates, and the use of MIC testingalone will fail to detect hVISA strains that are relatively common among isolates of Staphylococcus aureus (S. aureus) with broth MICs of 2 g per ml. Objective: The aim of the present work was to detect the efficacy of phenotypic and automated methods for detection of MRSA with reduced susceptibility to vancomycin. It aimed also, to determine the best MIC concentration in vancomycin screening agar for detection of VISA among MRSA isolates. Methods: One hundred MRSA isolates were obtained from 100 patients from different departments of Ain Shams University Hospitals during the period from October 2015 to the end of April 2016. They were isolated from different clinical specimens; sputum, wound swabs, blood, pus, urine, and body fluid that were referred to central microbiology laboratory for routine culture and sensitivity. Detection of S. aureus with reduced susceptibility to vancomycin was done by vancomycin screening agar with different concentrations 2,4,6 ug/ml with and without casein, MIC broth microdilution method for vancomycin according to CLSI 2015, and Vitek 2 automated system for determination of vancomycin MIC. Results: Out of 100 MRSA isolates, vancomycin screening agar 2ug/ml with casein showed highest detection rate for VISA isolates (48 %) among other screening agars. Vancomycin screening agar 6 ug/ml without casein gave the lowest detection rate (29%). So, adding casein to vancomycin screening agar did not increase detection of VISA in any of vancomycin screening agar except for that with 2ug/ml vancomycin. Vancomycin screening agar 2ug/ml with casein gave the best sensitivity among all vancomycin screening agar tested. VITEK 2 system failed to detect any isolates with reduced susceptibility to vancomycin. They were sensitive to linezolid (100%) followed by tigecyclin (99%) then Quinupristin-dalfopristin (91%). However, most of the isolates were resistant to tetracycline (85%) followed by gentamicin (80%) then ciprofloxacin (63%). Conclusion: BHI agar with 2ug/ml vancomycin and 16 g/l casein is a reliable, easy to perform, and inexpensive method to screen large number of S. aureus isolates for detection of reduced susceptibility to vancomycin on a daily basis. Applying quadruplicate technique in vancomycin screening agar may increase the yield for detection of VISA isolates. Although vancomycin screening agar 6 ug/ml is recommended by CLSI as a screening method for detection of VISA, yet it did not perform well and underestimated VISA isolates. VITEK 2 system is not an appropriate method for detection of S. aureus with reduced susceptibility to vancomycin (VISA). MRSA isolates with reduced susceptibility to vancomycin can be treated effectively with Linezolid.