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 was to discuss possible detection methods and their reliability in screening for this type of resistance.
Material and methods: The reliability of the disk diffusion method, 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 harboring the mcr-1 gene). Broth microdilution was the reference method used for the determination of colistin resistance.
Results: Targeted screening for colistin-resistant strains is best performed with a selective agar medium supplemented with colistin. In cultured isolates, suspected colistin resistance should always be confirmed by reliable methods. There was 100 percent agreement between both the gradient methods and the Rapid Polymyxin NP test and the reference broth microdilution method. When using the disk diffusion method with 10µg and 50µg disks, only 11 % and 33 % of results were correct, respectively. Therefore, disk diffu-sion is inappropriate for colistin resistance screening.
Conclusion: Prospective prevalence studies of intestinal carriage of colistin-resistant Enterobacterales among Czech hospitalized patients and travelers 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 for their consumers. The detection of mcr-harbouring colistin-resistant strains cannot be expected during routine microbiological testing, without using reliable but expensive methods. A suitable alternative method for active monitoring of colistin resistance is the use of selective agar media with colistin followed by verification of the resistance in the obtained isolates.