Phenotypic and Genotypic Investigation of Persistent Staphylococcus aureus Bacteraemia Isolates

Staphylococcus aureus and methicillin-resistant S. aureus (MRSA) remain a serious clinical threat and a leading cause of healthcare- and community-associated infections. Blood stream infections or bacteraemia caused by S. aureus are further complicated by the phenomenon of bacterial persistence and treatment failure despite confirmed in vitro susceptibility of the infecting strain to the administered antibiotics. Moreover, it is unclear how S. aureus evades the host immune system for the prolonged duration of a persistent infection. This study aimed to answer these two clinically relevant, biological questions by characterising multiple persistent and resolved MRSA bacteraemia isolates with the view to identifying persistent isolate associated phenotypic and genotypic traits, and novel mechanism(s) for persistence development. From this work, two distinctly novel S. aureus persistence mechanisms are presented. The first involved in vivo strain evolution induced by antibiotic exposure (daptomycin) during two independent incidences of persistence (PB1 and PB3), resulting in mprF gain-of-function mutations in the persisting bacterial population. This led to daptomycin non-susceptibility and the emergence of novel persistence associated virulence phenotypes, which included a nutrient deprived growth adaption, increased immune evasion, adhesion and stress response associated surface proteins, and attenuated virulence. The second mechanism presented (PB2 and PB5) did not display any defining persistence associated traits; however these bacteraemia were not treated with daptomycin, which further supports the daptomycin induced, MprF mediated persistence mechanism presented for the PB1 and PB3 cases. This study is the first of its kind to investigate multiple persistent MRSA bacteraemia inclusive of numerous temporally spaced infective isolates, in parallel with contemporaneous resolved bacteraemia isolates of the same genetic background. The subsequent findings of this study have massive implications for the current clinical regimes implemented during complex S. aureus bacteraemia, particularly antibiotic treatment choices, and potentially for other cases of bacterial infection where persistence is frequently observed.