Synergistic bactericidal activity of rat serum with vancomycin against enterococci

Animal models of infectious diseases may not predict clinical efficacy when species-related factors come into play. Recently, unexpected bactericidal activity of vancomycin alone against enterococci was observed in a rat model of endocarditis. A factor or factors in rat serum, but not rabbit or human serum, enhanced in vitro killing by vancomycin in four of five clinical isolates of enterococci. Bactericidal activity was maintained on dilution of rat serum to 5.0% and after exposure of serum to 56 degrees C for 30 min. Activity was lost by heating at 60 degrees C for 2 h, ultrafiltration, or absorption with bentonite or heat-killed bacteria. Rat serum appears to contain a factor or factors that contribute bactericidal activity to vancomycin, a drug normally bacteriostatic for these enterococci. The mechanism by which this factor enhances killing of enterococci by vancomycin is unknown.