Genotypic and phenotypic analysis for the typing and characterisation of veterinary and human isolates of 'Staphylococcus aureus'

Jamrozy, Dorota M. (2013) Genotypic and phenotypic analysis for the typing and characterisation of veterinary and human isolates of 'Staphylococcus aureus'. (PhD thesis), Kingston University, .


Staphylococcus aureus is a member of the skin commensal flora in humans and other mammals, as well as a recognised pathogen. Treatment of S. aureus infections have been complicated by the emergence of antimicrobial-resistant isolates, most notably the methicillin-resistant S. aureus (MRSA). This organism is a significant hospital-acquired pathogen, with community-acquired infections presently on the rise. Recently, MRSA has emerged in livestock, with isolates commonly identified as clonal complex 398 (CC398). The sudden emergence and dissemination of MRSA CC398 poses questions concerning the mechanisms mediating the success of this lineage as a livestock coloniser. The hypothesis addressed in this work is that MRSA CC398 has become livestock-associated due to host specific or non-host related adaptations. The work aims to investigate virulence as well as antimicrobial resistance genotypic and phenotypic features of MRSA CC398 isolates through a comparative study using a panel of isolates from different S. aureus lineages. The analysis of virulence genotypes revealed that the CC398 lineage had the lowest content of virulence genes, with all isolates lacking accessory virulence determinants and carrying mostly core-variable genes such as adhesin and staphylococcal exotoxin-like protein genes. Keratinocyte adhesion assays demonstrated inter-lineage variation in adhesion to porcine skin cells and comparatively poor binding by CC398 isolates. MRSA CC398 isolates also demonstrated a limited capacity for biofilm formation. The antimicrobial susceptibility analysis together with investigation of antimicrobial resistance genotypes found that MRSA CC398 isolates were resistant to a number of non-ß-lactam agents. This was mediated by diverse genetic resistance determinants and was also observed for MRSA isolates belonging to other CCs. A number of resistance genes were confirmed as plasmid-borne and the sequence analysis revealed carriage of novel resistance gene clusters and resistance determinants. Biological fitness analysis revealed a competitive advantage of CC398 strains over MRSA isolates of other lineages. In conclusion, MRSA CC398 isolates revealed lack of any significant virulence features. Instead the lineage demonstrated broad resistance properties, accompanied by superior biological fitness.

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