Phenotypic and genomic features shaping the activity of a novel T4-like phage against clinical multidrug-resistant Klebsiella pneumoniae

Background The use of bacteriophages holds a great promise in the treatment of multidrug resistant bacteria. The interaction between bacteriophages and multidrug-resistant Klebsiella pneumoniae is shaped by extensive capsular diversity, accessory genome variation and multilayered anti-phage defense systems, resulting in highly variable susceptibility profiles. Here, we describe MB01, a lytic T4-like phage isolated from wastewater in Brazil and characterize its biological properties and activity against MDR K. pneumoniae . Results MB01 showed a short replication cycle, strong bacterial suppression at low MOIs and high stability across physiological temperature and pH ranges. Among clinical isolates, MB-01 was effective for K. pneumoniae isolates. Whole genome sequencing revealed complex resistomes, heterogeneous accessory genes and marked differences in defense systems and prophage content. Defense-enriched strains were fully resistant to MB01, while permissive isolates carried fewer defenses, indicating that genomic background strongly shapes phage outcome. Conclusions These findings highlight the value of integrating phenotypic and genomic analyses for rational phage selection and support the development of targeted phage libraries to address MDR K. pneumoniae infections.