Discriminatory Information and Courses from MediaLab, Inc.
These are the MediaLab courses that cover Discriminatory and links to relevant pages within the course.
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|The bacterial cells shown in the image were observed in a smear prepared from the colony shown before. Which of the following tests will help to affirm the identification of Staphylococcus aureus?||View Page|
|Most strains of S. anginosus (milleri) carry the F antigen (see image). Rare strains that carry the group A antigen can be differentiated from S. pyogenes by which of the following laboratory tests:||View Page|
|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 sones > 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 hrs in ambient air at 35° C. Screening for vancomycin resistance in Staphylococci (MIC's > 8 ug/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.