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Virulence in Australian community-associated methicillin-resistant staphylococcus aureus
thesisposted on 14.02.2017, 02:46 by Chua, Kyra Yu Lin
The global phenomenon of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections is a key public health issue. These CA-MRSA strains can cause severe, fulminant sepsis in previously healthy individuals. Unlike other regions, in Australia, the dominant CA-MRSA strain is the Queensland ST93-IV [2B] clone, a unique strain that is genetically distinct from other S. aureus. This project aimed to characterise a representative isolate from this clone (JKD6159) in comparison with other Australian strains and also the USA300 epidemic CAMRSA strain. Initially, functional virulence analysis using an invertebrate Galleria killing assay and a mouse skin infection assay demonstrated that JKD6159 was the most virulent strain tested. Whole genome sequencing was then used to uncover molecular mechanisms for this increased virulence. The complete genome of JKD6159 was determined, and annotated. Comparative genomics demonstrated that although there were unique regions of difference in the JKD6159 genome compared to other S. aureus genome sequences, there were no novel virulence factors to explain the heightened virulence. The JKD6159 genome did contain lukSF-PV, the genes encoding for Panton- Valentine leukocidin and like other S. aureus genomes, also contained hla (encoding α-hemolysin) and psmα genes (encoding the α-type phenol soluble modulins) in the core staphylococcal genome. These exotoxins have been demonstrated to be important virulence factors in the pathogenesis of USA300. In order to better understand virulence of ST93 MRSA, JKD6159 was compared with another three ST93 MRSA strains with varying virulence phenotypes, using both comparative and functional genomics approaches. This demonstrated that ST93 virulence is primarily dependent on high-level expression of α-hemolysin and an intact agr system. However, other regulators, including a previously uncharacterised AraC/XylS family regulator (AryK) that acts as a positive regulator of exotoxin expression, also influence virulence of this clone. This project has characterised the virulence phenotype of ST93 CA-MRSA and used comparative and functional genomics to determine the key molecular determinants of virulence in this clone.