The Search for a Practical Method for Colistin Susceptibility Testing: Have We Found It by Going Back to the Future?

Polymyxins are relied upon for the treatment of carbapenem-resistant Gram-negative bacterial infections, but polymyxin resistance is increasing. Only broth microdilution is recommended for polymyxin susceptibility testing, but this method is impractical for most clinical microbiology laboratories. An article in this issue of the Journal of Clinical Microbiology (P. J. Simner, Y. Bergman, M. Trejo, A. A. Roberts, R. Marayan, T. Tekle, S. Campeau, A. Kazmi, D. Bell, S. Lewis, P. D. Tamma, R. Humphries, and J. A. Hindler, J Clin Microbiol 57:e01163-18, 2019, https://doi.org/10.1128/JCM.01163-18) found that colistin broth disk elution, a method that requires only colistin disks and broth, had excellent performance compared to broth microdilution for all strains except mcr -positive Escherichia coli strains.