Occult Colistin Heteroresistance as a Cause of Treatment Failure

Resistance to last-line drugs continues to emerge in clinical gram-negative bacilli. Although colistin resistance is rare, colistin treatment failures are not uncommon, raising concern that colistin heteroresistance may be a basis for treatment failure. Heteroresistance is defined as a subpopulation of the bacterial population demonstrating phenotypic resistance without any genotypic changes compared with the parent population.

Researchers evaluated two colistin-heteroresistant Klebsiella pneumoniae isolates from patients that yielded a fully colistin susceptible phenotype when tested by standard methods. Broth microdilution incubated at 48 hours rather than the standard 24 hours detected heteroresistance, but agar-based testing with Etest w…