Cefotaxime aminoglycoside interactions

Minimum inhibitory concentrations and minimum bactericidal concentrations were determined for cefotaxime, gentamicin, tobramycin and amikacin and for the combination of cefotaxime and each of the aminoglycosides in vitro against 200 strains of Enterobacteriaceae. 91% were susceptible to cefotaxime, 93.5% were susceptible to gentamicin, 89.5% were susceptible to amikacin and 68% were susceptible to tobramycin. There were 95 strains which could be evaluated for synergistic killing by the antimicrobial combinations. Synergism was shown against 78% of strains by cefotaxime and amikacin, against 71% by cefotaxime and tobramycin and against 64% by cefotaxime and gentamicin. In 19 of 22 instances where a bacterial strain was resistant to both cefotaxime and the aminoglycoside, synergistic killing with clinically achievable levels of both antimicrobial agents was demonstrated. There was no significant decrease in concentration of any of the three aminoglycosides after incubation with cefotaxime at 37 degrees C for 0, 4 or 24 h.