Evolution of Colistin Resistance in the Klebsiella pneumoniae Complex Follows Multiple Evolutionary Trajectories with Variable Effects on Fitness and Virulence Characteristics.

The increasing prevalence of multidrug-resistanthas led to a resurgence in the use of colistin as a last-resort drug. Colistin is a cationic antibiotic that selectively acts on Gram-negative bacteria through electrostatic interactions with anionic phosphate groups of the lipid A moiety of lipopolysaccharides (LPSs). Colistin resistance inis mediated through loss of these phosphate groups, their modification by cationic groups, and by the hydroxylation of acyl groups of lipid A. Here, we study theevolutionary trajectories toward colistin resistance in four clinicalcomplex strains and their impact on fitness and virulence characteristics. Through population sequencing duringevolution, we found that colistin resistance develops through a combination of single nucleotide polymorphisms, insertions and deletions, and the integration of insertion sequence elements, affecting genes associated with LPS biosynthesis and modification and capsule structures. Colistin resistance decreased the maximum growth rate of onestrain, but not those of the other threecomplex strains. Colistin-resistant strains had lipid A modified through hydroxylation, palmitoylation, and l-Ara4N addition.strains exhibited cross-resistance to LL-37, in contrast to thesubsp.strain. Virulence, as determined in asurvival assay, was increased in two colistin-resistant strains. Our study suggests that nosocomialcomplex strains can rapidly develop colistin resistance through diverse evolutionary trajectories upon exposure to colistin. This effectively shortens the life span of this last-resort antibiotic for the treatment of infections with multidrug-resistant.