Phenotypic and genomic comparisons of community-associated Staphylococcus aureus Clone ST93

© 2015 Justin StepnellStaphylococcus aureus is an opportunistic bacterial pathogen that primarily colonises the anterior nares of 30-50% of individuals at any one time, without causing disease. S. aureus does however cause a wide range of diseases including skin and soft tissue infections, pneumonia, osteoarticular infections and bacteraemia or septicaemia, frequently resulting in hospitalisation and a high mortality rate for invasive disease. S. aureus strains have acquired resistance to many classes of antibiotics, most importantly resistance to beta-lactam antibiotics (methicillin-resistant S. aureus, MRSA). While traditionally a health care associated issue, community-acquired MRSA (CA-MRSA) clones are increasing worldwide resulting in severe, antibiotic resistant infections occurring in patients without healthcare contact. The ST93 clone of CA-MRSA is essentially unique to Australia, and has been associated with severe, invasive S. aureus infections in otherwise healthy individuals. ST93 CA-MRSA is also the most common CA-MRSA clone in Australia. The overall aims of this study were to determine the virulence characteristics of a collection of ST93 S. aureus isolates from around Australia, and to uncover the molecular determinants of virulence in ST93 S. aureus. Fifty-eight ST93 isolates were assessed for virulence using the Galleria mellonella invertebrate virulence model and by measuring expression of key virulence factors. Whole genome sequencing and genomic analysis of all isolates was used to uncover genetic differences that might account for differences in virulence characteristics. In this study ~50% of isolates (n=28) were avirulent when compared to a virulent reference ST93 isolate JKD6159 using the G. mellonella model. This study also revealed that the G. mellonella model does not respond to exotoxin expression of isolates with no significant differences in G. mellonella mortality between culture supernatant of virulent and avirulent isolates used in the G. mellonella model. The expression levels of PSMα3 and δ-toxin varied significantly amongst the isolate collection, and the concentration of δ-toxin was found not to correlate with that of PSMα3, indicating that systems other than the quorum-sensing agr system must be controlling the expression of PSMα3 in ST93 S. aureus