Investigation of the molecular pathogenesis of the multi-host bacterial pathogen Staphylococcus aureus

Staphylococcus aureus is a major human and animal bacterial pathogen that causes an array of diseases. The main aim of this thesis is to investigate the key host-pathogen interactions that underpin S. aureus infection. S. aureus abundantly secretes two isoforms of the enzyme lipase into the extracellular milieu, where they scavenge upon polymeric triglycerides. It has previously been suggested that these lipases may interfere with the function of innate immune cells, such as macrophages and neutrophils, but the impact of lipases on phagocytic killing mechanisms remains unknown. We showed that there were no differences in the survival of S. aureus USA300 LAC wild type and its lipase-deficient isogenic mutant after incubation with human whole blood or neutrophils. Furthermore, there was no detectable lipase-dependent effect on phagocytosis, intracellular survival, or escape from both human primary and immortalised cell line macrophages, even upon supplementation with exogenous recombinant lipases. Therefore, we showed that S. aureus lipases do not inhibit bacterial killing mechanisms of human macrophages, neutrophils, or whole blood. Furthermore, the capacity of S. aureus to adapt to distinct host-species ecologies is a major public health and economic concern. Approximately 60 years ago, a human-to-poultry host jump and adaptation of S. aureus belonging to the widespread CC5 clade, led to the avian-adaptation and global expansion of S. aureus in broiler poultry. Our research aims to combine transposon (Tn) mutagenesis of S. aureus with experimental models of infections to identify the immune cell repertoire and bacterial genes involved in avian host-adaptation. To determine the avian immune cell tropism for S. aureus, mCherry-integrated clones from common avian (CC385, CC5) and human (CC8) S. aureus clonal lineages were screened in blood extracted from the transgenic chicken line Runx1-eGFP. We demonstrate that monocytes and heterophils generate the first-line response to S. aureus infection, with avian strains exhibiting differential uptake by heterophils compared to a human strain. Furthermore, our analysis demonstrated that avian S. aureus strains may have adapted to the avian host through the inhibition of degranulation of heterophils as a novel survival mechanism. TraDIS analysis of genes involved in the fitness of S. aureus during infection of peripheral blood leukocytes demonstrated that regulators of the Type VII secretion system and Spl-proteases were required for survival of S. aureus in the face of the innate immune response of PBLs. Taken together, these studies provide new insights into the evolution of S. aureus and the key host-pathogen interactions underpinning S. aureus infections.