In vitro and in vivo synergy between ceftriaxone and aminoglycosides against Pseudomonas aeruginosa.

The antibacterial interaction between ceftriaxone and the aminoglycosides tobramycin, gentamicin and amikacin against pseudomonas aeruginosa was investigated in vitro and in experimental systemic infections of the mouse. In vitro synergy was observed in 70% of the strains with tobramycin, 67.5% with gentamicin, and 77.5% with amikacin. Synergistic bactericidal activity was demonstrated with all three aminoglycosides in combination with ceftriaxone. In line with these in vitro results, synergism against most isolates also occurred in mice. In leukopenic mice, the addition of ceftriaxone resulted in a more than 2.5-fold reduction of the aminoglycoside dose necessary for antiinfective protection, although ceftriaxone alone was only marginally active against the pathogen used in immunocompromised animals.