Impact of S. aureus Hypermutators on Chronic Airway Infection in Cystic Fibrosis Patients (Completed 2012)

S. aureus infection levies a heavy toll on human health in hospitals and the community, a problem worsened by its recalcitrance to immune clearance and therapeutic intervention.  The genetic events and host microenvironments that mediate the evolution of S. aureus from a formidable acute pathogen to one optimized for disease persistence are obscure and largely unexplored.  In several pathogens of note, such as Pseudomonas aeruginosa, strains with a constitutively increased mutation frequency, termed hypermutators, are thought to accelerate pathogen fitness during chronic infection by increasing the rate at which beneficial mutations are acquired.  Genotypic characterization of bacterial hypermutators show that mutations in DNA repair and error avoidance systems causes this phenotype. Although, S. aureus hypermutators have been reported from diverse infection types, their role in effecting disease pathogenesis during infection is unknown. 

To investigate the potential of S. aureus hypermutators as catalysts of pathoadaptation during chronic infection, mutations were constructed in two DNA repair pathways:  1) the mismatch repair (MMR; mutS and mutL) and 2) oxidized guanine (GO; mutM, mutY, and mutT) systems.  Inactivation of these DNA repair systems produces a hypermutator phenotype and alters the predominant mutational type and sites that confer rifampin resistance. Further biochemical analysis of the DmutM and DmutY using a DNA glycosylase assay, demonstrates that each mutant is defective in excising DNA lesions associated with 7,8-dihydro-8-oxo-deoxyguanine (8-oxo-dG).  Despite the inability to repair this oxidative DNA lesion, neither GO nor MMR mutants display enhanced sensitivity to H₂O₂ stress.  In vitro analysis of evolution revealed the capacity of GO and MMR mutants to alter the production of virulence factors (staphyloxanthin and a-hemolysin) and antimicrobial resistance.  Genotypic characterization of a-hemolysin defective mutants showed that mutation at the accessory gene regulator (agr) locus contributed to this phenotype and was a mutational hotspot in all strains tested. My work provides evidence that bacterial hypermutation may represent a general mechanism employed by pathogens to promote disease persistence.

Steven Gill, PhD