Evolution in Fast Forward: a Potential Role for Mutators in Accelerating Staphylococcus aureus Pathoadaptation

Abstract

Pathogen evolution and subsequent phenotypic heterogeneity during chronic infection are proposed to enhance Staphylococcus aureus survival during human infection. We tested this theory by genetically and phenotypically characterizing strains with mutations constructed in the mismatch repair (MMR) and oxidized guanine (GO) system, termed mutators, which exhibit increased spontaneous-mutation frequencies. Analysis of these mutators revealed not only strain-dependent increases in the spontaneous-mutation frequency but also shifts in mutational type and hot spots consistent with loss of GO or MMR functions. Although the GO and MMR systems are relied upon in some bacterial species to prevent reactive oxygen species-induced DNA damage, no deficit in hydrogen peroxide sensitivity was found when either of these DNA repair pathways was lost in S. aureus. To gain insight into the contribution of increased mutation supply to S. aureus pathoadaptation, we measured the rate of ␣-hemolysin and staphyloxanthin inactivation during serial passage. Detection of increased rates of ␣-hemolysin and staphyloxanthin inactivation in GO and MMR mutants suggests that these strains are capable of modifying virulence phenotypes implicated in mediating infection. Accelerated derivation of altered virulence phenotypes, combined with the absence of increased ROS sensitivity, highlights the potential of mutators to drive pathoadaptation in the host and serve as catalysts for persistent infections.

Bypass of genetic constraints during mutator evolution to antibiotic resistance

“…Their emergence is an eventual consequence of the genetic structure of asexuals, which allows mutator alleles to hitchhike with beneficial mutations occurring in the same genome [16,17]. Mutators pose a serious concern in clinical infections because they can readily evolve further adaptations, such as those promoting evasion of the immune system [18], increasing resistance to antibiotics [19] or alleviating the resistance fitness cost [20]. Different mutators, however, display a characteristic tendency to elevate only some types of transitions, transversions or frameshifts.…”

Pharmacokinetics and Pharmacodynamics of Aerosolized Antibacterial Agents in Chronically Infected Cystic Fibrosis Patients

“…Furthermore, growth of S. aureus in CF lungs is associated with hypermutability (22,266), in vitro exposure of hypermutators to gentamicin resulted in an increased emergence of SCVs (268), and antibacterials other than gentamicin have not been studied. S. aureus hypermutators were found to be more virulent (27). Thus, SCV colonial morphotype, heteroresistance, and hypermutability serve as catalysts for persistent S. aureus infections in CF patients.…”

“…Such data are considered to provide important information about the risk of resistance emerging in a treated patient. However, pathoadaptive processes favor the emergence of bacteria displaying a hypermutator phenotype (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27), exhibiting significantly increased rates of mutation, or a small-colony morphotype (30)(31)(32)(33)(34), facilitating recurrent and persistent infections. Both adaptive responses confer antibiotic resistance, which, however, is not analyzed by routine procedures.…”

“…In addition to the poor penetration of antibacterials into the focus of infection, antibacterial therapy of CF patients is aggravated by several factors, such as biofilm formation, which impairs drug penetration and protects the pathogens from immune defense (7)(8)(9)(10)(11)(12)(13)(14); the development of hypermutator strains, characterized by high mutation rates, which result in pronounced resistance against antimicrobials and pathoadaptation ( [15][16][17][18][19][20][21][22][23][24][25][26][27]; drug adsorption to sputum components (28,29), which limits antibiotic activities; and slow growth as small-colony morphotypes (30)(31)(32)(33)(34), which facilitates recurrent and persistent infections and reduces the antibacterial activities of several antimicrobials (35,36). The result of the multiplicity of factors impairing the activities of antibacterials is that the pathogen cannot be eliminated, and infection persists in a chronic state.…”

Optimization of Polymyxin B in Combination with Doripenem To Combat Mutator Pseudomonas aeruginosa

“…The mutation frequency was determined by plating an overnight bacterial culture on LB agar containing 100 mg of rifampin/liter (36,37). The mutation frequency was defined as the proportion of bacterial cells that grew on drug containing agar divided by the total population.…”