DNA mismatch repair and hypermutability in the physiology and pathogenesis of Haemophilus influenzae

The methyl-directed mismatch repair (MMR) system maintains fidelity in the process of DNA replication, suppressing spontaneous mutations and chromosomal rearrangements, and assisting in the repair of damaged DNA. Hypermutable isolates of bacterial pathogens defective in MMR have been detected in about 1% of natural populations, and are predicted to have selective advantages during times of stress by increasing the genetic diversity of the population.; The prevalence and phenotypes of mismatch-repair defective Haemophilus influenzae were investigated. The H. influenzae MMR genes mutS, mutL, mutH, and dam, were inactivated. Only mutations in mutS, mutL, or mutH , produced a hypermutable phenotype, and MMR inactivation did not confer increased susceptibility to DNA damaging agents such as UV irradiation, or the oxidative effects of hydrogen peroxide. Mutants of mutS had increased adherence and invasion of human cells in culture. In contrast, H. influenzae dam mutants had decreased invasion into human cell culture models, and were attenuated in the infant rat model of infection.; In a screen of 500 H. influenzae isolates, 14 from eight different patient sources were hypermutable identified by frequency of mutation to antibiotic resistance. The majority (12/14) of these hypermutators were from patients with cystic fibrosis (CF). Several mutator isolates had point mutations in mutS, producing defective MutS protein. Six clonal mutator strains isolated from one CF patient over 11 months showed a trend towards increasing MIC values for antibiotics the patient was administered for treatment suggesting mutators may contribute to clinical antibiotic resistance.; MMR in H. influenzae has evolved into a specialized system seemingly best adapted for the relatively sheltered human environment. H. influenzae mutants in MMR genes, including mutS, seem prevalent, particularly in CF sputum, and may contribute to disease pathogenesis with increased virulence and greater rates of antibiotic resistance. Inhibition of Dam methylase interferes with H. influenzae MMR activity without increasing the global mutation rate, and more importantly, results in attenuation of both in vitro and in vivo virulence.