Staphylococcus aureus continues to cause significant clinical burden globally and therapeutic approaches must adapt swiftly to meet the emerging challenges of multidrug resistant strains. Detailed understanding of host:pathogen dynamics is paramount in developing optimal control strategies. The first aim of this project was to identify novel virulence traits of S. aureus by screening genetically related isolates from a dominant MRSA clade, ST 239, through the zebrafish embryo systemic model of infection. Virulence differences between strains and bioinformatic analysis of whole genome sequence data revealed single nucleotide polymorphisms in both characterised and putative genes however technical errors in sequencing data prevented further interrogation of S. aureus pathogenomics. The second aim sought to characterise the role of genes required for growth on human blood. A library of transposon mutated non-essential S. aureus genes was screened for growth phenotype on human blood and strains of interest were prioritised by virulence assessment in the zebrafish infection model. Genes involved in purine and folate synthesis were found to be important for growth on human blood. Growth of a folate deficient mutant (pabA) was re-established if exogenous pyrimidines were added to human blood and a key gene in the pyrimidine salvage pathway, tdk, is implicated in this process. Lysis products of RBCs are currently speculated to either inhibit Tdk directly or cause toxicity thereby increasing the need for dTTP against a pyrimidine poor background. Further understanding of the requirements for growth in human blood may highlight novel areas of therapeutic target development. Finally, the need for a large bacterial inoculum to establish S. aureus infection in both zebrafish and mammalian models of infection was interrogated. It was found that other Gram-positive, nonpathogenic bacteria (and even their components) enable S. aureus to initiate infection. The other bacteria themselves cannot cause disease but augment infection by the pathogen. Either live bacteria or purified, particulate cell wall peptidoglycan of Micrococcus luteus or Staphylococcus epidermidis increased host mortality when co-injected with low dose S. aureus. Inert latex beads and solubilised peptidoglycan did not. These findings have important ramifications for human disease as it may begin to explain how initiation of S. aureus infection occurs.