Abstract
Background: Colistin is a last-resort antibiotic used for the treatment of infections caused by multidrug-resistant gram-negative pathogens. The emergence of plasmid-borne colistin resistance mediated by the mcr genes, poses a risk of its spread and its occurrence should be monitored.
The aim of this study is to discuss possible detection methods and their reliability in screening for this type of resistance. Material and methods: The reliability of the disk diffusion, the Rapid Polymyxin NP test, and two types of gradient tests for the screening of mcr-mediated colistin resistance was evaluated on 16 human and two reference isolates of Escherichia coli (colistin-susceptible and colistin-resistant with chromosomally-mediated resistance or harbouring the mcr-1 gene).
The broth microdilution was the reference method used for the determination of colistin resistance. Results: For the targeted screening of colistin-resistant strains, a selective agar supplemented with colistin can be used and colistin resistance can be confirmed in cultured isolates.
The gradient methods and the Rapid Polymyxin NP test agreed 100% with the reference broth microdilution method. Using the disk diffusion with 10μg and 50μg disks, only 11% and 33% of results were correct, respectively.
Therefore, disk diffusion is an inappropriate method for the screening of colistin resistance. Conclusion: Prospective prevalence studies of intestinal carriage of colistin resistant Enterobacterales among Czech hospitalized patients and travellers do not yet indicate the spread of strains with mcr-mediated plasmid-borne colistin resistance.
However, the high prevalence of strains carrying the mcr genes in raw meat products poses a risk of exposure to their consumers. The detection of mcr-harbouring colistin-resistant strains cannot be expected during routine microbiological testing, without using reliable but expensive methods.
The active screening cultures for monitoring of colistin resistance can be performed on selective agar with colistin followed by verification of the resistance in the obtained isolates.