Phenol-Soluble Modulins Contribute to Early Sepsis Dissemination Not Late Local USA300-Osteomyelitis Severity in Rabbits

INTRODUCTION In bone and joint infections (BJIs), bacterial toxins are major virulence factors: Panton-Valentine leukocidin (PVL) expression leads to severe local damage, including bone distortion and abscesses, while α-hemolysin (Hla) production is associated with severe sepsis-related mortality. Recently, other toxins, namely phenol-soluble modulins (PSMs) expressed by community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) strain USA300 (LAC WT) were shown to have ex vivo intracellular cytotoxic activity after S. aureus invasion of osteoblasts, but their in vivo contribution in a relatively PVL-sensitive osteomyelitis model remains poorly elucidated. MATERIALS AND METHODS We compared the outcomes of experimental rabbit osteomyelitises induced with pvl+hla+psms+ LAC WT and its isogenic Δpsm derivatives (LAC Δpsmα and LAC Δpsmαβhld) using an inoculum of 3 × 108 CFUs. Mortality, hematogenous spread (blood culture, spleen and kidney), lung and bone involvements were assessed in two groups (non-survivors of severe sepsis and survivors sacrificed on day (D) 14). RESULTS Severe sepsis-related mortality tended to be lower for Δpsm derivatives (Kaplan-Meier curves, P = .06). Non-survivors' bone LAC-Δpsmα (6.9 log10 CFUs/g of bone, P = .04) or -Δpsmαβhld (6.86 log10 CFUs/g of bone, P = .014) densities were significantly higher than LAC WT (6.43 log10 CFUs/g of bone). Conversely, lung Δpsmαβhld CFUs were significantly lower than LAC WT (P = .04). LAC Δpsmα, Δpsmαβhld and WT induced similar bone damage in D14 survivors, with comparable bacterial densities (respectively: 5.89, 5.91, and 6.15 log10 CFUs/g of bone). Meanwhile, pulmonary histological scores of inflammation were significantly higher for LAC Δpsmα- and Δpsmαβhld-infected rabbits compared to LAC WT (P = .04 and .01, respectively) but with comparable lung bacterial densities. CONCLUSION Our experimental results showed that deactivating PSM peptides significantly limited bacterial dissemination from bone during the early phase of infection, but did not affect local severity of USA300 rabbit osteomyelitis.