Inhibition of a conserved DNA repair complex promotes susceptibility of Staphylococcus aureus to host defences

Abstract

Staphylococcus aureus is a leading cause of serious infections in humans, but treatment can be ineffective due to the emergence of antibiotic-resistant strains. During infection, the most important host defence mechanism against Staphylococcus aureus is the oxidative burst of neutrophils, which involves the rapid generation and release of reactive oxygen species. However, whilst it has been hypothesised that DNA is an important target of the oxidative burst, this had not been tested. Work described in this thesis revealed that the neutrophil oxidative burst triggers the SOS DNA repair pathway in phagocytosed S. aureus, indicating that staphylococcal DNA is a target of the host immune response. A screen of a panel of mutants defective in various DNA repair processes revealed that the RexAB complex was essential for maximal staphylococcal survival in whole human blood, during incubation with purified human neutrophils and during invasive murine infection. This repair complex also conferred a survival benefit to Streptococcus gordonnii and Enterococcus faecalis in human blood, indicating that DNA is a conserved target of neutrophil-mediated killing. Using recombinant S. aureus RexAB, it was demonstrated that this complex is a functional member of the AddAB family of helicase-nuclease enzymes, required for the repair of DNA double-strand breaks. In an effort to develop inhibitors of RexAB for use as potential therapeutics, subsequent work identified a compound that potentiated the activity of DNA-damaging antibiotics. However, it was not possible to demonstrate inhibition of RexAB. In summary, these data demonstrate that RexAB is required for staphylococcal survival during infection by repairing DNA damage caused by the neutrophil oxidative burst. Future work will focus on the discovery and development of small-molecule inhibitors of RexAB that could be used therapeutically to sensitise S. aureus to host immune defences.