Investigation of the molecular pathogenesis of the multi-host bacterial pathogen Staphylococcus aureus
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Date
31/01/2023Author
Sargison, Fiona Anne
Metadata
Abstract
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.