9.3.3 Vancomycin-resistant Enterococcus

When dealing with strains of Enterococcus resistant to vancomycin (VRE) the important thing is to distinguish between species that are intrinsically resistant to glycopeptide antimicrobials such as vancomycin and have no public health significance, and those which harbour acquired resistance genes.

For acquired resistance genes there are two types:

  • the vanA phenotype, which are both vancomycin- and teicoplanin resistant
  • the vanB phenotype which are just vancomycin-resistant.

E. faecium is the most common species to have acquired resistance and the pattern of vancomycin and teicoplanin resistance in this organism can be used to infer the presence of vanA or vanB. Gradient diffusion test strips to measure the MIC are a reliable and straightforward method to use to confirm vanA and vanB(Figure 17).

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Figure 17  E-test for VRE ‒ E. faecium with a) teicoplanin and b) vancomycin. The breakpoint for teicoplanin and vancomycin are 2 mg/L and 4 mg/L respectively.
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To interpret the E-test you read the figure where the zone of inhibition hits the test strip. There is no zone at all for the vancomycin and for teicoplanin there is only a small zone ‒ it is many dilutions higher than the breakpoint.

Figure 17  E-test for VRE ‒ E. faecium with a) teicoplanin and b) vancomycin. The breakpoint for teicoplanin and vancomycin are 2 mg/L ...

  • In Figure 17, is the test organism of phenotype vanA or vanB?

  • The organism is type vanA because it is resistant to both vancomycin and teicoplanin.

Note that disk diffusion testing is relatively unreliable for many other antimicrobials in Enterococci. It is essential to follow the guidelines very closely. Most antimicrobials do not have breakpoints for these species defined by EUCAST so it is only recommended to test the antimicrobials that do, otherwise you will end up with results that cannot be interpreted.

  • Which of the following species would need further testing for acquired resistance if they had a raised vancomycin MIC indicating resistance to this antimicrobial: E. gallinarum, E. casseliflavus, E. faecium, or E. faecalis?

  • As you learned in Section 3.2.3, E. faecium and E. faecalis would both need further testing. The others have intrinsic chromosomally encoded resistance to glycopeptides. They only rarely cause infections, for example in immunosuppressed humans. That is why these organisms are not included in the surveillance schemes.

This example illustrates how important it is to identify the Enterococcal species. However, it is not easy to tell the difference between species on biochemical tests alone. E. faecium and E. faecalis tend to be non-motile while the other species are motile and most isolates of E. casseliflavus/E. flavescens have a distinct yellow pigment, which can be observed by collecting growth from an agar plate on a swab. However, unless the laboratory has access to MALDI-TOF it may be necessary to send isolates to the reference laboratory.

9.3.2 Vancomycin-resistant S. aureus

9.3.4 Penicillin-resistant Streptococcus pneumoniae