Aims

Methicillin-resistant Staphylococcus aureus (MRSA) can cause wound infections via a ‘Trojan Horse’ mechanism, in which neutrophils engulf intestinal MRSA and travel to the wound, releasing MRSA after apoptosis. The possible role of intestinal MRSA in prosthetic joint infection (PJI) is unknown.

Aim. Staphylococcus aureus is the first causative agent of bone and joints infections (BJI). It causes difficult-to-treat infections because of its ability to form biofilms, and to be internalized and persist inside osteoblastic cells. Recently, phage therapy has emerged as a promising therapy to improve the management of chronic BJI. In the present study, we evaluated the efficacy of an assembly of three bacteriophages previously used in a clinical case report (Ferry, 2018) against S. aureus in in vitro models of biofilm and intracellular osteoblast infection. Methods. Using HG001 S. aureus, the bactericidal activities of the assembly of the three bacteriophages (Pherecydes Pharma) used alone or in association with vancomycin or rifampicin were compared by quantifying the number of viable bacteria in mature biofilms and infected osteoblasts after 24h of exposure. Results. The activity of bacteriophages against biofilm-embedded S. aureus was dose-dependent. Synergistic effects were observed when bacteriophages were combined to antibiotics at the lowest concentrations, with no significant bactericidal activity in monotherapy. In the human osteoblast infection model, we were able to show that phage penetration into osteoblasts was only possible when the cells were infected, suggesting a S. aureus dependent Trojan horse mechanism. The intracellular inoculum in osteoblasts treated with bacteriophages or vancomycin was significantly higher than in cells treated with lysostaphin, used as control condition of rapid killing of bacteria released in the extracellular media after death of infected cells and absence of intracellular activity. These results suggest that bacteriophages are probably both i) inactive in the intracellular compartment and ii) unable to kill all bacteria released after cell lysis into the extracellular medium fast enough to prevent them from reinfecting other osteoblasts. Conversely, the intracellular inoculum recovered from cells treated with vancomycin+bacteriophages was significantly lower than the one inside cells treated with vancomycin or bacteriophages alone, suggesting that this combination allowed a better control of released bacteria in the extracellular media. Finally, bacteriophages did not increase the activity of rifampicin in this model. Conclusion. In conclusion, we showed that the bacteriophages tested were highly active against S. aureus in mature biofilm but had no activity against bacteria internalized in osteoblasts. Additional studies using animal models of BJI and well-conducted clinical trials are needed to further evaluate phage therapy and its positioning in the management of these infections