Dear colleague, I would like to have your opinion on a test I have carried out concerning vancomycin resistance,
I prepared a Mueller hinton medium with 16µg of vancomycin (MIC according to CLSI 2020), no satphylococcus colony grew on this medium, while this same strain proved to be resistant to vancomycin (30 µg disc) on antibiogram.
how to explain this result please, thank you.
Hi,
First of all you should include a positive control that has been proven to grow in MH2 plus vancomycin. Then it could also be that your inoculum of staphylococcus cells was too small. I would also double-check the concentration calculation. In general, I would always prefer MICs over agar disc diffusion methods, so you’re on the right track.. good luck!
Hi
It is true I should have included a positive control, but still I was sure of the viability of my strain and the integrity of my midium. If not what about BHI- agar with 6mg/l vancomycin.
Thank you for your response. @khatharina Shaufler
Yeah, I mean agar plates containing the desired antibiotic can also be used, however, this only allows screening and won't tell you the exact MIC. Once you've determined the MIC for your strain you can use the agar plates in follow-up experiments. Good luck!
so if I have strains growing on a BHI-6V medium I can consider them intermediate or resistant to vancomycin? or not.
As I understand it, the only way to determine the exact MIC is through E-test.
thank you very much for your answer
greetings Katharina Schaufler
Etests are gradient diffusion tests and are subject to the same limitations as disc diffusion testing. Vancomycin is a very large molecule and the disc potency you have used is extremely large. The gold standard testing for MIC is either broth dilution or agar dilution. The best way to determine MIC is to perform a series of consecutive double dilutions. It is advised to use at least 2 strains (one high and one low) with known MICs to demonstrate that the media is at the desired concentration. The minimum inhibitory concentration (MIC) occurs at the concentration of no growth.
From EUCAST S. aureus: "Disk diffusion is unreliable and cannot distinguish between wild type isolates and those with non-vanA-mediated glycopeptide resistance". We had the same problem with enterococci and disk diffusion testing. So the only way to determine the "real" or exact MIC (and resistance) is to use a broth dilution method, e.g. according to ISO 20776-1. E-test is better than disk diffusion, but not as good as broth microdilution.
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