8.2 Screening for resistance mechanism in practice
Activity 9 Examples in your workplace
How might you initially screen isolates for the presence of possible resistance mechanisms? Can you give any examples from your own laboratory?
Make notes in the text box and then compare with the example answer.
Discussion
Using a specific indicator antimicrobial during routine AST of samples is a common approach to determine whether a resistance mechanism is present or not (see Section 5.1).
Examples of where this approach would be useful include:
- cefpodoxime resistance to screen for ESBL in Enterobacterales
- nalidixic acid or pefloxacin to screen for quinolone resistance in Salmonella species
- meropenem to screen for carbapenemase production in Enterobacterales (CPE).
Another approach would be to use selective media. Commercially available chromogenic media are useful especially for screening samples such as swabs or faecal samples for carriage of resistant organisms.
The choice of screening method depends on what sort of samples you are looking at, for example selective media are used for faecal samples from healthy animals, or as part of routine AST for clinical samples.
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Chromogenic media are particularly useful if the specimen is likely to be contaminated with normal flora, for example faecal samples. Why is this?
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Selective chromogenic media are useful if the sample has normal flora as they save time. Because only the resistant ones will be growing after overnight incubation, they can be worked on directly without having to subculture and do AST first. The test is less likely to miss the resistant ones which might otherwise be hidden among the normal flora. This method is also useful to screen large numbers of samples, and the higher cost of media is offset by savings in time.
For more information about how specific screening tests work in combination with confirmatory tests in practice, see Section 9.
8 Screening for resistance mechanisms
