الفهرس 
S.Aureus with Reduced Susceplibility to VancomycinOnly 14 pages are availabe for public view
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Abstract
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 hVISA and VISA infection. According to CLSI, broth microdilution is considered the gold standard to determine vancomycin MIC. However, because it is time consuming, a considerable number of clinical laboratories do not use it as routine methodology. Other techniques have been widely used, with variable sensitivity and specificity, such as E-test and automated systems. The only CLSI vancomycin screen agar method in place for clinical isolates for the detection of vancomycin resistant S. aureus (VRSA) and possibly VISA is BHI agar containing 6 mg/liter vancomycin (BHIA6V), a method originally established for detection of vancomycin resistance in enterococci , however this method have a very low sensitivity for the detection of h-VISA. The CDC recommends a supplemental test for VISA detection, with the caveat that strains with vancomycin MICs of 4 ug/ml will not be reliably identified and screen agar plates with a lower concentration of 3 mg/l vancomycin have a very high false positive rate.
In our study, One hunderds MRSA isolates were subjected for detection of S.aureus with reduced susceptibility to vancomycin by vancomycin screening agar with different concentrations 2, 4, 6 ug/ml with and without casein, Vitek 2 automated system for determination of vancomycin MIC (Biomerieux, France) and broth microdilution method for detection of vancomycin MIC according to CLSI 2015 as reference method. 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. Vancomycin screening agar 2ug/ml with casein detected 30/30 of isolates that were VISA by BMD with 100% sensitivity and 74.3% specificity. Vancomycin screening agar 2ug/ml without casein detected 20/30 0f isolates that were VISA by BMD with 66.7% sensitivity and 85.7% specificity. Vancomycin screening agar 4ug/ml with casein detected 20/30 0f isolates that were VISA by BMD with 66.7% sensitivity and 84.3% specificity. Vancomycin screening agar 4ug/ml without casein detected 20/30 0f isolates that were VISA by BMD with 66.7% sensitivity and 85.7% specificity. Both vancomycin screening agar 4 ug/ml with and without casein failed to detect 10 isolates out of 30 (33.3%) that were VISA positive by BMD. Vancomycin screening agar 6ug/ml with casein detected 20/30 0f isolates that were VISA by BMD with 66.7% sensitivity and 85.7% specificity. Vancomycin screening agar 6ug/ml without casein detected 19/30 0f isolates that were VISA by BMD with 63.3% sensitivity and 85.7% specificity. VITEK 2 system failed to detect any isolates with reduced susceptibolity to vancomycin. They were sensitive to linezolid (100%) followed by tigecyclin (99%) then Quinupristin-dalfopristin (91%). However, most of the isolates were resistant to tetracyclin (85%) followed by gentamicin (80%) then ciprofloxacin (63%).