Fitness barriers to spread of colistin-resistant Klebsiella pneumoniae overcome by establishing niche in patient population with elevated colistin use

There is an urgent need to improve our understanding of how new antibiotic resistant organisms emerge and spread. A high-priority resistance threat is the ST258 lineage of carbapenem-resistant Klebsiella pneumoniae. Here, we studied resistance to the last-line drug colistin among ST258 by tracking its evolution across 21 U.S. hospitals over the course of a year. Phylogenetic analysis supported a significant fitness cost being associated with resistance, as resistance emergence was common but resistance variants were rarely transmitted. Furthermore, several resistance variants that were transmitted had acquired secondary variants that reverted the strain to susceptible. The exceptions to the general pattern of instability associated with resistance were two large clusters of resistant strains in one sublineage (clade IIB) present across Southern California hospitals. Quantification of transmission fitness in the healthcare environment indicated that, while resistant isolates from other clades were less fit than their susceptible counterparts, clade IIB resistant isolates were more fit, despite having similar resistance variants. Additional analyses supported the increased fitness of colistin-resistant clade IIB isolates being driven by a lineage-defining variant that increased clade IIBs association with patient subpopulations with enhanced colistin exposure. These results show that a favorable genetic background and sustained selective pressure led to the emergence and spread of a colistin-resistant ST258 sublineage across a regional healthcare network. These findings highlight the utility of integrating pathogen genomic and corresponding clinical data from regional healthcare networks to detect emerging antibiotic resistance threats and understand the clinical practices and patient populations that drive their spread. Significance StatementSelective pressure in hospitals leads to frequent antibiotic resistance evolution. However, emergent resistance alleles are often not transmitted to other individuals because of fitness costs associated with resistance. Due to the difficulty of studying pathogen fitness in humans, our understanding of how resistant organisms circumvent these costs is limited. We integrate genomic and clinical data to understand the evolutionary trajectories leading to transmissible resistance for the last-line antibiotic colistin. While colistin resistance is generally associated with a fitness cost that hinders transmission, this cost was mitigated in a sublineage that had previously acquired mutations increasing its association with patient populations more likely to receive colistin, suggesting a key role for historical contingency in the emergence and spread of stable resistance.