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.