Rifampicin plays an important role in the treatment of staphylococcal prosthetic joint infection, as rifampicin-containing combinations have shown a high efficacy against S. aureus biofilm infections. However, the emergence of rifampin-resistant strains is a feared complication and the use of rifampicin in those cases seems unwarranted. Therefore, we evaluated the activity of bacteriophage Sb1 in combination with different antibiotics against the biofilm of four rifampicin-resistant MRSA strains as alternative therapeutic approach.
Four rifampicin-resistant MRSA strains were used in this study. The MIC for all tested antibiotics was determined by Etest. Biofilms were formed on porous glass beads for 24h and exposed to Sb1 (107 PFU/mL) for 24h followed by exposure to antibiotic for 24h. Viability of bacteria after antimicrobial treatment was detected by beads sonication and plating of the sonication fluids. The minimum biofilm eradication concentration (MBEC) was defined as the lowest concentration of antibiotic required to kill all cells resulting in the appearance of no colony after plating of the sonication fluid (detection limit <20 CFU/mL). The synergistic effects were observed when Sb1 combined with antibiotics used at least 2 log-reduction lower concentrations.
All strains were susceptible to the three antibiotics except for MRSA3, resistant to fosfomycin, according to the EUCAST breakpoints. All tested antibiotics presented high MBEC values (ranging from 64 to >1024 µg/mL) when tested alone against biofilm (Table 1). A sequential administration of Sb1 followed by vancomycin (VAN) showed no synergistic effect against any of the tested strains, whereas the combination with fosfomycin (FOF) showed synergism in 50% of the strains with improvement of the eradication activity. The combination of Sb1 with daptomycin (DAP) showed the highest synergistic effects in 100% of the strains, with a 2 or 3-log reduction in the MBEC.
Sb1 bacteriophage in combination with daptomycin seems a promising alternative for the treatment of rifampicin-resistant MRSA biofilm infections.
Table 1 (not included). Antimicrobial activities against rifampicin-resistant MRSA strains. For the table, please contact authors directly.