Cation-adjusted Mueller-Hinton broth (CAMHB; Becton Dickinson and Company, Sparks, MD) was used as the bacterial growth medium for experiments with MRSA.Brain heart infusion (BHI) broth (Becton Dickinson and Company, Sparks, MD) was used as the bacterial growth medium for h VISA and VISA isolates (note that internal experiments found no difference in growth characteristics between media for any of the isolates [data not shown]).Experiments were performed at 10 CFU/ml for 72 h against 2 of the isolates (MRSA STA 336 and h VISA STA 449).We explored the higher inoculum and longer study duration in an effort to observe reduced antibacterial effects for the vancomycin monotherapy regimen.
One shortcoming of previous studies is that only single drug concentrations of the combination were explored for the duration of the experiments.
aureus [h VISA] isolate [MIC, 2.0 μg/ml], and a vancomycin-intermediate S.
aureus [VISA] isolate [MIC, 8.0 μg/ml]) were evaluated in an in vitro pharmacodynamic model with a starting inoculum of 10(6) or 10(8) CFU/ml. Time-kill curves were constructed, and the area under the bacterial killing and regrowth curve (AUBC) was calculated.
All isolates were presumed to have a cefazolin MIC of ≥32 μg/ml due to their MRSA phenotype. Cefazolin (lot 101A015; expiration date, June 2012; Apotex Corp., Weston, FL) was obtained from the Department of Pharmacy at Hartford Hospital.
Vancomycin analytical powder was acquired from Sigma-Aldrich, Inc. Vancomycin and cefazolin were administered as a bolus into the model every 8 h (q8h) to simulate the peak and trough free drug concentrations achieved in the sera of healthy volunteers after intravenous doses of 1 g q8h of vancomycin infused over 1 h and 1 g q8h of cefazolin infused over 2 to 3 min (12, 13).