Resilience of tigecycline heteroresistance phenotype in Acinetobacter baumannii

Background: Heteroresistance, frequently observed in diverse bacterial species, imposes clinical challenges. For this study, we investigated the stability and resilience of tigecycline heteroresistance in Acinetobacter baumannii. Methods: Four tigecycline-heteroresistant (HR) A. baumannii strains and resistant populations (RPs) obtained from them were subjected to laboratory evolution assays for 30 days in antibiotic-free media. The heteroresistance phenotype was determined using a population analysis. Bacterial growth curves and in vitro competitiveness were determined to investigate the fitness cost of heteroresistance. Tigecycline efficacy was evaluated using an in vitro time–killing assay. Genetic mutations were identified using whole genome sequencing, and expression of genes in the two-component systems was also evaluated. Results: Tigecycline heteroresistance was preserved even in antibiotic-free media, and tigecycline-RPs reverted to heteroresistance during serial culture without tigecycline pressure. The tigecycline-RPs showed a higher fitness cost than their respective HR strains, and the HR strains exhibited a survival advantage upon tigecycline treatment. Although the AdeABC efflux pump was overexpressed in the tigecycline-RPs, it was down-regulated in the HR strains. Conclusions: Our data indicate that tigecycline heteroresistance is a highly resilient phenotype in A. baumannii that gives a high fitness advantage to bacteria in terms of competitiveness and response to antibiotic pressure.