Synergistic activity of aminoglycoside-beta-lactam combinations against Pseudomonas aeruginosa with an unusual aminoglycoside antibiogram.

The bactericidal activity of aminoglycosides alone and in combination with various beta-lactams was studied by the time-kill technique against ten Pseudomonas aeruginosa isolates with an unusual antibiogram (amikacin-resistant, gentamicin-resistant, tobramycin-susceptible [ArGrTs]). Previous studies have indicated that ArGrTs isolates are moderately resistant to all aminoglycosides and many are multiply resistant to beta-lactams. Aminoglycoside-beta-lactam combinations showed infrequent synergistic (16%) or enhanced killing (12%) against the ArGrTs isolates. Synergistic activity, when present, was more likely to occur with tobramycin and amikacin than with gentamicin, even though these differences were not statistically significant. Antibiotic resistance patterns were not predictive of synergy or enhanced killing. Systemic infections produced by ArGrTs isolates that are multiply resistant to the beta-lactams may not respond to combination therapy with an aminoglycoside and beta-lactam. Alternative treatment with polymyxin B or a quinolone may be required for these infections.