Quantitative Pharmacodynamic Characterization of Resistance versus Heteroresistance of Colistin in E. coli Using a Semimechanistic Modeling of Killing Curves.

Heteroresistance corresponds to the presence, in a bacterial isolate, of an initial small subpopulation of bacteria characterized by a significant reduction in their sensitivity to a given antibiotic. Mechanisms of heteroresistance versus resistance are poorly understood. The aim of this study was to explore heteroresistance in -positive and -negative Escherichia coli strains exposed to colistin by use of modeling killing curves with a semimechanistic model. We quantify, for a range of phenotypically (susceptibility based on MIC) and genotypically (carriage of -1 or -3 or -negative) different bacteria, a maximum killing rate () of colistin and the corresponding potency (EC), i.e., the colistin concentrations corresponding to /2. Heteroresistant subpopulations were identified in both -negative and -positive E. coli as around 0.06% of the starting population. Minority heteroresistant bacteria, both for -negative and -positive strains, differed from the corresponding dominant populations only by the maximum killing rate of colistin (differences for by a factor of 12.66 and 3.76 for -negative and -positive strains, respectively) and without alteration of their ECs. On the other hand, the resistant -positive strains are distinguished from the -negative strains by differences in their EC, which can reach a factor of 44 for their dominant population and 22 for their heteroresistant subpopulations. It is suggested that the underlying physiological mechanisms differ between resistance and heteroresistance, with resistance being linked to a decrease in the affinity of colistin for its site of action, whereas heteroresistance would, rather, be linked to an alteration of the target, which will be more difficult to be further changed or destroyed.