In vitro synergism of ceftriaxone combined with aminoglycosides against Pseudomonas aeruginosa.

The antipseudomonal activities of ceftriaxone (CEF) or ceftazidime (CAZ), each combined with tobramycin (TOB) or netilmicin (NET), against 90 clinically significant Pseudomonas aeruginosa isolates were examined both by checkerboard and time-kill assays. As expected, susceptibility testing of single antibiotics by agar dilution demonstrated good activity for CAZ (89% susceptible), TOB (94%), and NET (58%), but poor activity for CEF (15%). Checkerboard studies revealed striking synergy (FIC indices < or = 0.5) for CEF, however, in combination with either TOB (72%) or NET (81%), compared with CAZ-TOB (44%) or CAZ-NET (60%) (P < 0.01, respectively). No antagonism (FIC indices > or = 4) was found in any of these combinations. The MIC90s of CEF, CAZ, or aminoglycosides in the combinations were reduced at least fourfold: CEF, from > 128 to 32 mg/liter; CAZ, from 16 to 4 mg/liter; TOB, from 4 to 0.5 mg/liter; and NET, from 32 to 4 mg/liter. With CEF and NET, the percentage of strains sensitive to < or = 8 mg/liter of both drugs alone and in combination increased from 9% to 69%. Results of the time-kill assay for CEF-NET agreed reasonably well with the checkerboard method (Spearman rank correlation coefficient, 0.40, P < or = 0.01), and generated a bactericidal outcome in 60% (24 of the 40 isolates), when tested with combinations at 1/4 MBC of either antibiotic alone. Importantly, concentrations of CEF and aminoglycoside combinations that demonstrated synergy by either checkerboard or time-kill assays were achievable in serum clinically. These data suggest a unique interaction of CEF-aminoglycoside combinations against P. aeruginosa.(ABSTRACT TRUNCATED AT 250 WORDS)