In vitro efficacy of antimicrobial agents against high-inoculum or biofilm-embedded meticillin-resistant Staphylococcus aureus with vancomycin minimal inhibitory concentrations equal to 2 μg/mL (VA2-MRSA)

Minimal inhibitory concentration (MIC) creep in meticillin-resistant Staphylococcus aureus (MRSA) isolates has been observed in recent years. The potential roles of vancomycin-based combination regimens as well as linezolid and tigecycline against five clinical MRSA isolates with vancomycin MICs of 2 μg/mL (VA2-MRSA) were evaluated and compared in vitro. Antimicrobial susceptibility was studied by the agar dilution method. Anti-MRSA activities of linezolid, tigecycline, vancomycin, minocycline, rifampicin and fosfomycin alone as well as of three vancomycin-based combinations were studied by time-kill method and using a biofilm model. When VA2-MRSA at an inoculum of 1 × 10⁵ colony-forming units (CFU)/mL was incubated with vancomycin, tigecycline, linezolid or rifampicin alone, bactericidal activity lasted for 48 h in time-kill analysis. At a higher inoculum of 1 × 10⁷ CFU/mL, only linezolid demonstrated a bacteriostatic effect at 24 h and the inhibitory activity lasted for 36 h. However, bacterial growth was inhibited ≥2 log₁₀ at 24 h and was even undetectable at 48 h with vancomycin plus fosfomycin or rifampicin. In biofilm studies, vancomycin plus fosfomycin or minocycline at susceptible breakpoint concentrations demonstrated an enhanced antibacterial effect comparable with linezolid and better than tigecycline. In conclusion, vancomycin plus fosfomycin or rifampicin exhibited a synergistic and better antibacterial effect than linezolid or tigecycline alone against high-inoculum planktonic VA2-MRSA. Vancomycin plus fosfomycin or minocycline compared with linezolid exhibited a similar inhibitory effect, better than tigecycline alone, against biofilm-embedded VA2-MRSA. Evaluating the toxicity and efficacy of high-dose vancomycin monotherapy for VA2-MRSA, the fosfomycin combination exhibited a rapid killing effect in both conditions and may provide another therapeutic choice.