Effect of polymyxin resistance evolution on the fitness costs of Acinetobacter baumannii and the underlying mechanisms of polymyxin resistance

The increasing incidence and emergence of polymyxin-resistant Acinetobacter baumannii pose a significant threat to global public health. The presence of sub-minimal inhibitory concentration (sub-MIC) antibiotics during clinical treatment is inevitable and may drive the development of antibiotic resistance. Therefore, understanding the fitness costs and underlying mechanisms of resistance evolution under sub-MIC polymyxin B (PMB) is crucial for controlling the spread of PMB-resistant strains. In this study, PMB resistance evolved rapidly within 1-2 days in different A. baumannii strains and acquisition of PMB resistance altered the bacterial susceptibility to other antibiotics. All evolved strains suffered varying degrees of fitness costs, manifesting as a reduced growth rate, in vitro competitiveness, motility ability, biofilm formation, and adhesion and invasion to host cells. Proteomic and qRT-PCR analyses revealed upregulation of efflux pumps, two-component systems and outer membrane proteins in resistant strains, along with downregulation of genes involved in lipid A biosynthesis, motility and biofilm formation. Notably, the increasing PMB resistance was attenuated upon co-treatment with efflux pump inhibitor CCCP. This study indicates that evolution of PMB resistance incurs substantial fitness costs in A. baumannii, and the loss of LPS production and overexpression of efflux pumps may represent the key mechanisms underlying this evolutionary process.