Isolation and characterization of a novel K3-type capsule-targeting phage for the treatment of carbapenem-resistant Acinetobacter baumannii.

Acinetobacter baumannii (A. baumannii) is a prevalent pathogen associated with nosocomial infections, particularly carbapenem-resistant A. baumannii (CRAB), which poses a significant global public health concern. The rising prevalence of CRAB underscores the urgent need for novel therapeutic strategies, including phage-based interventions. Here, we isolated and characterized novel phage P11B exhibiting a potent lytic activity specifically against K3 capsule-type CRAB with an optimal multiplicity of infection of 0.0001, an exceptionally short latent period (~10 min), and a high burst size (~188 PFU/cell). P11B further demonstrated superior stability across wide temperature/pH ranges, enhancing therapeutic potential and surpassing many previously reported K3-specific phages. Phage-host co-cultivation experiments generated resistant mutants showing markedly attenuated virulence compared to wild-type strains. Whole-genome sequencing identified three key mutations in pgi, algC, and galU genes encoding metabolic enzymes and extracellular proteoglycan biosynthesis regulators. Genetic complementation restored phage susceptibility, confirming that these mutations mediate phage adsorption through capsular modification. Our study identifies the core genes that link phage receptor functionality with virulence in CRAB, demonstrating an evolutionary trade-off between phage resistance and pathogenicity. These findings elucidate the molecular mechanisms underlying phage-host interactions and the core genetic determinants that connect phage receptor modulation to virulence attenuation in CRAB, revealing an evolutionary trade-off that supports the use of capsule-targeted phage therapy against K3-type A. baumannii infections. IMPORTANCE: The global spread of MDR-AB, particularly carbapenem-resistant Acinetobacter baumannii (CRAB) strains with various capsule types, necessitates the development of innovative antimicrobial strategies. In this study, we identify phage P11B as a potent lytic agent against KL3 CRAB and reveal an evolutionary trade-off: mutations in capsule biosynthesis genes (pgi, algC, galU) that confer phage resistance also directly reduce virulence. This biological constraint addresses a significant limitation of phage therapy by ensuring that resistant mutants remain less pathogenic. Our findings mechanistically link modifications in phage receptors to a reduction in virulence, thereby advancing capsule-targeted phage engineering as a sustainable approach to combat CRAB. This study clarifies fundamental genetic factors that connect the modulation of phage receptors with the reduction of virulence in CRAB, highlighting an evolutionary compromise that enhances the therapeutic promise of phages aimed at the capsule in treating KL3-type A. baumannii infections.