Emergence of mutHV76G among longitudinal carbapenem resistant Klebsiella pneumoniae causing long-term colonization and recurrent infection disrupts DNA mismatch repair and results in a hypermutator phenotype

Although hypermutation due to Mut protein mutations that disrupt DNA mismatch repair has been characterized in some bacteria, its mechanisms and consequences in Klebsiella pneumoniae remain poorly defined. We analyzed 11 longitudinal KPC-3 carbapenemase-producing, ST258 K. pneumoniae isolates collected over [~]4 years from an immunocompromised patient with chronic colonization and recurrent infections. After [~]3.3 years, isolates developed ceftazidime-avibactam (CZA)-resistance with restored carbapenem susceptibility, coinciding with emergence of a V76G substitution in a highly-conserved motif in the core of MutH endonuclease. Compared with earlier isolates, mutHV76G-carrying isolates showed greater within-host genomic diversification (69-179 vs. 2-12 SNP differences) and acquired blaKPC-3L169P, encoding an KPC {Omega}-loop substitution that mediates CZA resistance and re-establishes carbapenem susceptibility. mutHV76G isolates exhibited stepwise increases in meropenem-vaborbactam (MVB) and cefiderocol minimum inhibitory concentrations, plausibly linked to substitutions in KPC, OmpK36 porin, CirA iron transporter and/or EnvZ kinase. Clinical mutHV76G isolates and CRISPR-engineered mutHV76G mutants were hypermutators based on rifampin mutational frequencies. Using isogenic mutant and parent strains, we confirmed that mutHV76G accelerated evolution of CZA and MVB resistance in vitro and in vivo, promoted transfer and uptake of resistance plasmids, and improved bacterial fitness during mouse infections. Resistance evolution in mice recapitulated clinical trajectories, with blaKPC-3 and ompK36 mutations emerging under CZA and MVB exposure, respectively. Phenotypes of mutHV76G and mutH-null strains were comparable, indicating that the V76G substitution largely abrogates MutH function. Our findings reveal MutH-mediated hypermutation as an adaptive mechanism in K. pneumoniae, enabling rapid antibiotic resistance development and plasmid acquisition without fitness cost. ImportanceHypermutator bacteria pose a formidable clinical threat by rapidly evolving antibiotic resistance and adapting within the human host. Klebsiella pneumoniae is a major cause of multidrug-resistant infections, yet the contribution of hypermutation to its evolution remains poorly characterized. Analyzing K. pneumoniae isolates collected over [~]4 years from a chronically infected/colonized patient, we demonstrate that emergence of a mutation in mutH (mutHV76G), a DNA mismatch repair gene, results in hypermutation phenotypes and rapid accumulation of gene mutations. Both clinical and lab-engineered mutHV76G mutant strains rapidly acquire resistance or reduced susceptibility to new antibiotics like ceftazidime-avibactam, meropenem-vaborbactam and cefiderocol, due to mutations in carbapenemase (blaKPC-3), porin (ompK36) and other genes. mutHV76G-driven hypermutation also enhances horizontal transfer of resistance plasmids and improves K. pneumoniae fitness during mouse infections. This study is important for understanding K. pneumoniae hypermutation as a potent mediator of antibiotic resistance and other phenotypes relevant to human infections.