Bactericidal kinetics of an in vitro infection model of once-daily ceftriaxone plus amikacin against gram-positive and gram-negative bacteria

The in vitro efficacy of ceftriaxone plus amikacin combination against gram-positive and gram-negative bacteria, clinically isolated from patients affected by pneumonia in intensive care units, was compared to that of the 2 drugs used alone. The study was performed using a dynamic model in which the human kinetics of the drugs after intramuscular administration was simulated. The antibacterial activity was tested by determining the bacterial cell count (CFU/ml). Killing curves came out from plotting the log CFU/ml versus time. In the same way, ceftriaxone and amikacin concentrations were assayed by HPLC and fluorescence polarization immunoassay, respectively. The results show that ceftriaxone plus amikacin combination exert a high killing activity against all tested strains. The two antibiotics alone initially have a good killing activity but this is followed by bacterial regrowth for all tested isolates. This data supports the results of several clinical studies which have shown a good therapeutic efficacy of ceftriaxone plus amikacin combination in the treatment of severe infections caused by organisms intermediately sensitive to these drugs.