Dear colleague, I have a small question concerning colistin resistance, I would like to know if an inhibition diameter between 6 mm and 9 mm around a 25 ug colistin disc by agar diffusion method can be due to the presence of a mcr gene, and what will be its value in MIC ?
thank you
I think it's not possible/accurate to relay on inhibition zone diameter to infere the presence of mcr gene.
Kindly check this article
https://jcm.asm.org/content/57/2/e01163-18
Dear Chahinez Boutefnouchet
The disk diffusion method is not recommended to assess antimicrobial susceptibility to colistin.
You can find details on how to determine MIC of colistin by using the broth microdilution method according to the recommendations from the European Committee on Antimicrobial Susceptibility Testing (EUCAS T)
The European Committee on Antimicrobial Susceptibility Testing. Breakpoint tables for interpretation of MICs and zone diameters. Version 10.0, 2020.
Thank you
Tamara
The disk diffusion method is actually not recommended. The reference method is the detemination of MIC by broth dilution. Other techniques are validated by CLSI like colisitn broth disk elution or colistin agar test. See page 132 of M100 CLSI 2020 attached.
The abtibiotic disc has a very low difusion on agar plates so the MIC in broth will give a correct result in addition only the pcr can proof the presence of mcr gene according to the strain you're working on.
Actually, the disk diffusion method is not recommended for colistin resistance as there is low diffusion of colistin antibiotic on agar plates. A better way is the determination of MIC in cation adjusted mueller broth, according to EUCAST guidelines a reference method. Further, identification of mcr genes, PCR with specific primers for mcr gene determinants will be used.
Thank you very much for your answers, I think I will test the MICs by the method : Colistin Agar Test (CAT) according to the CLSI recommendation.
my greetings
- Badges
- Science topic