Study objective: To investigate the effects of pooled human serum (PHS) on the killing activity of vancomycin and teicoplanin against two isolates of Staphylococcus aureus from patients treated for endocarditis.

Design: An in vitro assessment of antibiotic susceptibility and killing rates.

Setting: An urban university teaching hospital.

Patients: Pooled human serum from patients treated for endocarditis.

Interventions: Two clinical isolates of Staphylococcus aureus were obtained from patients treated for endocarditis. Media consisted of cation-supplemented Mueller-Hinton broth alone and in 1:1 dilutions with PHS, 2-hour heat-inactivated PHS (HI-PHS), ultrafiltrate (UF), and 2-hour heat-inactivated ultrafiltrate (HI-UF). Heat inactivation of PHS and UF was accomplished by treatment at 56 degrees C for 2 hours.

Measurements and main results: Killing curves with vancomycin and teicoplanin were performed using drug concentrations of 45 micrograms/ml and a starting inoculum of approximately 1 x 10(6) colony-forming units (cfu)/ml. Bactericidal rates (-log cfu/ml/hr) were calculated from the slope of the killing curves over 0-12 hours (mean 3-8 replicates).

Conclusions: The killing activity of vancomycin in PHS and HI-PHS against both isolates was significantly greater than all other media tested (p < 0.0001). Ultrafiltrate tended to reverse this enhancement effect. Addition of PHS or UF did not enhance teicoplanin's killing activity against either isolate. Further investigations in our laboratory will determine if the factor is antibiotic class or organism specific.