Identification of Evolutionary Trade-Offs Associated with High-Level Colistin Resistance in Acinetobacter baumannii

Colistin (COL) belongs to the polymyxin group of drugs which possesses a positive charge and interacts with lipopolysaccharide (LPS) of Gram-negative bacterial outer membrane. Additionally, it can penetrate the cell membrane and disrupt the integrity of the phospholipid leading to cell death. Resistance against COL has been reported in Acinetobacter baumannii , a bacteria in the ‘ESKAPE’ group of ‘priority pathogens’ of the World Health Organization. Multiple and pan-drug-resistant strains of A. baumannii are rising in many medical facilities and they are becoming resistant to last-resort antibiotics including colistin. Though plasmid-encoded acquisition of mcr -1 has been associated with clinical resistance, drug efflux, complete loss of LPS by inactivation of the biosynthetic pathway ( lpx ACD), and modifications of target LPS by-products of chromosomal pmr CAB genes has been ascribed with resistance evolution against COL. To further delve into the evolutionary dynamics of COL-resistance here we report experimental evolution of extreme COL-resistance by adaptive evolution of the reference strain A. baumannii ATCC 19606. Phenotypic characterization has been performed for the evolved strains, which demonstrated hyperbiofilm phenotype and striking decrease in fitness. A comprehensive antibiotic susceptibility profiling has been performed on the evolved strains. Whole genome sequencing of the resistant strains led to the identification of mutations prospectively associated with COL resistance. Phenotypic characterization of three COL-resistant clinical isolates of A. baumannii revealed similarity with experimentally evolved resistant mutants at least in one of the isolates.