Activity of cefotaxime-aminoglycoside combinations against aminoglycoside-resistant Pseudomonas

The activity of cefotaxime, either alone or combined with an aminoglycoside, was determined against 50 isolates of Pseudomonas aeruginosa, of which 50, 33, and 10 were resistant to gentamicin, tobramycin, and amikacin, respectively. Cefotaxime inhibited 34 isolates at a concentration of 16 microgram/ml and all isolates at 128 microgram/ml. The combinations of cefotaxime with gentamicin, tobramycin, and amikacin were synergistic against 30, 17, and 9 isolates, respectively, and no antagonism was observed with any combination. Synergism was obtained at clinically significant antibiotic concentrations for nine isolates with cefotaxime-gentamicin, five isolates with cefotaxime-tobramycin, and nine isolates with cefotaxime-amikacin.