TESTING VANCOMYCIN RESISTANCE WITH DISC DIFFUSION METHOD- Is the disk diffusion technique still a valid technique for studies of antimicrobial susceptibility with Vancomycin?
Dr.T.V.Rao MD TOPIC OF INTEREST TO POST GRADUATES IN MICROBIOLOGY
Since the late 1980s, strains of vancomycin-resistant enterococci (VRE) have emerged as significant causes of nosocomial infections and colonization's with increasing frequency in every part of the world We all at many laboratories test the Vancomycin resistance by Kirby Bauer methods and report as sensitive or resistant, It was way back that a published research by JANA M. SWENSON,*et al JOURNAL OF CLINICAL MICROBIOLOGY, Sept. 1989,reported we noticed that for several strains of the enterococci we had tested, MICs of vancomycin were 8 to 16 ktg/ml, yet the strains looked susceptible with the disk diffusion test, with zones of 17 to 18 mm (unpublished data). Because of this and because of the apparent current increase in vancomycin resistance, we initiated studies to determine whether the standard disk diffusion test yielded accurate results for vancomycin and enterococci. The published research in 1988 stands good today, that we are truy missing the resistance in Vancomycin by doing a disc diffusion testing and reporting, it certainly matters in patients with enterococcal infections producing a Bacterial endocarditis, Staphylococcus too have become much restiance to Vancomycin and we lack facilities to do MIC for all the isolates as it stands it today to think about improvements in antibiograms with testing of Vancomycin
Testing for Vancomycin Susceptibility
The current CLSI recommendation is that MIC tests should be performed to determine the susceptibility of staphylococci to vancomycin. The disk test does not differentiate vancomycin-susceptible isolates of S. aureus from vancomycin-intermediate strains.
Disk diffusion will detect S. aureus isolates containing the VanA vancomycin-resistance gene (VRSA). These isolates will show no zone of inhibition around the disk (zone = 6mm); their identification should be confirmed. Isolates producing vancomycin zones > 7mm should not be reported as susceptible without performing a vancomycin MIC test.
Recommended methods are CLSI Broth Microdilution, Agar Dilution, and Etest® with inoculum prepared to match McFarland 0.5 turbidity standard. The Etest® is considered the most discriminatory of these methods as it allows for visualization of small colonies around zones of inhibition. A pure culture MUST be used. Repeat test for confirmation.
The CLSI recommends that the inoculum should be prepared using the direct suspension method and plates incubated for a full 24 hours in ambient air at 35° C.
Screening for vancomycin resistance in Staphylococci (MIC > 8 µg/mL) can be performed utilizing a vancomycin agar screening plate – BHI (brain heart infusion) agar containing 6 mg/mL vancomycin. However testing on BHI screening agar does not reliably detect all vancomycin intermediate S. aureus strains.
USE OF VITEK 2 We are all much progressing with use Vitek 2 as, Vitek systems have been widely adopted by many microbiology laboratories for the rapid identification and determination of susceptibilities of pathogens, including VRE strains. In order not only to treat VRE-infected patients but also to implement appropriate control measures to prevent the spread of VRE strains, rapid and accurate identification of VRE-colonized patients is essential. N. Yamane et al. evaluated several automated methods, such as the Microscan conventional- and rapid-panel methods and Vitek GPS-TA cards, and found that every automated method tested was shown to yield a high error rate. Recently, H. P. Entdz et al. pointed out that newly developed Vitek GPS-101 cards with 45 wells have been significantly improved and that they successfully detected all of the VRE strains with the vanB genotype tested (15 out of 15 strains)
References and Microbiologists to learn more from
1 Problems with the Disk Diffusion Test for Detection of Vancomycin Resistance in Enterococci JANA M. SWENSON,* BERTHA C. HILL, AND CLYDE THORNSBERRY Antimicrobics Investigation Branch, Hospital Infections Program, Center for Infectious Di JOURNAL OF CLINICAL MICROBIOLOGY, Sept. 1989, p. 2140-2142
2 Online laboratory continuing education for clinical laboratories and med techs
3 Limitations of Vitek GPS-418 Cards in Exact Detection of Vancomycin-Resistant Enterococci with thevanB Genotyp Tadashi Okabe1J. Clin. Microbiol. June 2000 vol. 38 no. 6 2409-2411 Dr.T.V.Rao MD Professor of Microbiology
Dr.T.V.Rao MD TOPIC OF INTEREST TO POST GRADUATES IN MICROBIOLOGY
Since the late 1980s, strains of vancomycin-resistant enterococci (VRE) have emerged as significant causes of nosocomial infections and colonization's with increasing frequency in every part of the world We all at many laboratories test the Vancomycin resistance by Kirby Bauer methods and report as sensitive or resistant, It was way back that a published research by JANA M. SWENSON,*et al JOURNAL OF CLINICAL MICROBIOLOGY, Sept. 1989,reported we noticed that for several strains of the enterococci we had tested, MICs of vancomycin were 8 to 16 ktg/ml, yet the strains looked susceptible with the disk diffusion test, with zones of 17 to 18 mm (unpublished data). Because of this and because of the apparent current increase in vancomycin resistance, we initiated studies to determine whether the standard disk diffusion test yielded accurate results for vancomycin and enterococci. The published research in 1988 stands good today, that we are truy missing the resistance in Vancomycin by doing a disc diffusion testing and reporting, it certainly matters in patients with enterococcal infections producing a Bacterial endocarditis, Staphylococcus too have become much restiance to Vancomycin and we lack facilities to do MIC for all the isolates as it stands it today to think about improvements in antibiograms with testing of Vancomycin
Testing for Vancomycin Susceptibility
The current CLSI recommendation is that MIC tests should be performed to determine the susceptibility of staphylococci to vancomycin. The disk test does not differentiate vancomycin-susceptible isolates of S. aureus from vancomycin-intermediate strains.
Disk diffusion will detect S. aureus isolates containing the VanA vancomycin-resistance gene (VRSA). These isolates will show no zone of inhibition around the disk (zone = 6mm); their identification should be confirmed. Isolates producing vancomycin zones > 7mm should not be reported as susceptible without performing a vancomycin MIC test.
Recommended methods are CLSI Broth Microdilution, Agar Dilution, and Etest® with inoculum prepared to match McFarland 0.5 turbidity standard. The Etest® is considered the most discriminatory of these methods as it allows for visualization of small colonies around zones of inhibition. A pure culture MUST be used. Repeat test for confirmation.
The CLSI recommends that the inoculum should be prepared using the direct suspension method and plates incubated for a full 24 hours in ambient air at 35° C.
Screening for vancomycin resistance in Staphylococci (MIC > 8 µg/mL) can be performed utilizing a vancomycin agar screening plate – BHI (brain heart infusion) agar containing 6 mg/mL vancomycin. However testing on BHI screening agar does not reliably detect all vancomycin intermediate S. aureus strains.
USE OF VITEK 2 We are all much progressing with use Vitek 2 as, Vitek systems have been widely adopted by many microbiology laboratories for the rapid identification and determination of susceptibilities of pathogens, including VRE strains. In order not only to treat VRE-infected patients but also to implement appropriate control measures to prevent the spread of VRE strains, rapid and accurate identification of VRE-colonized patients is essential. N. Yamane et al. evaluated several automated methods, such as the Microscan conventional- and rapid-panel methods and Vitek GPS-TA cards, and found that every automated method tested was shown to yield a high error rate. Recently, H. P. Entdz et al. pointed out that newly developed Vitek GPS-101 cards with 45 wells have been significantly improved and that they successfully detected all of the VRE strains with the vanB genotype tested (15 out of 15 strains)
References and Microbiologists to learn more from
1 Problems with the Disk Diffusion Test for Detection of Vancomycin Resistance in Enterococci JANA M. SWENSON,* BERTHA C. HILL, AND CLYDE THORNSBERRY Antimicrobics Investigation Branch, Hospital Infections Program, Center for Infectious Di JOURNAL OF CLINICAL MICROBIOLOGY, Sept. 1989, p. 2140-2142
2 Online laboratory continuing education for clinical laboratories and med techs
3 Limitations of Vitek GPS-418 Cards in Exact Detection of Vancomycin-Resistant Enterococci with thevanB Genotyp Tadashi Okabe1J. Clin. Microbiol. June 2000 vol. 38 no. 6 2409-2411 Dr.T.V.Rao MD Professor of Microbiology
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