Staphylococcus epidermidis (S. epidermidis) is one of the main pathogens responsible for bone and joint infections especially those involving prosthetic materials (PJI). Although less virulent than S. aureus, S. epidermidis is involved in chronic infections notably due to its ability to form biofilm. Moreover, it is frequently multiresistant to antibiotics. In this context, the development of additional or alternative antibacterial therapies targeting the biofilm is a priority.
The aim of this study was to evaluate in vitro the activity of phage lysin exebacase (CF-301) against biofilms formed by 19 S. epidermidis clinical strains responsible for PJI. We determined the remaining viable bacteria inside the biofilm (counting after serial dilution and plating) and the biomass (bacteria and extracellular matrix, using crystal violet staining) after 24h of exposition to exebacase at different concentrations, alone (0.05; 0.5; 5; 50 and 150 mg/L) or in combination (5, 50 and 150 mg/L) with antibiotics commonly used to treat multi-resistant S. epidermidis PJI (rifampin (1 mg/L), vancomycin (10mg/L) and daptomycin (10mg/L)). In this study, synergy was defined as a significantly higher effect of the association in comparison to the sum of the effect of each molecule.
Exebacase showed a dose-dependent reduction of biomass, ranging from 11 % at 0.5 mg/L to 66 % at 150 mg/L. Exebacase showed a significant bactericidal activity at 50 and 150 mg/l, with a mean decrease of the inoculum of 0.94 and 1.7 log, respectively. In addition, synergistic effects were observed in association with i) rifampin (1 mg/L) showing a mean decrease up to 84% of the biomass and 3.5 log CFU at 150 mg/L of exebacase, ii) vancomycin (10 mg/L) showing a mean decrease up to 81% of the biomass and 2.82 log CFU at 150 mg/L of exebacase, iii) and daptomycin (10 mg/L) showing a mean decrease up to 85% of the biomass and 3.1 log CFU at 150 mg/L of exebacase.
Exebacase showed, in vitro, synergistic activity with antibiotics against S. epidermidis biofilms. It is a promising adjuvant therapy to rifampin, vancomycin and daptomycin in the context of PJI. Further studies are needed, in vitro to understand the mechanism of action on S. epidermidis biofilm and the heterogeneity of strain behaviour and in vivo to confirm the present data.