In vitro interactions of amikacin and beta-lactam antibiotics against amikacin-resistant gram-negative bacilli.

We tested 42 strains of amikacin-resistant gram-negative bacilli with amikacin in combination with six beta-lactam antibiotics using the checkerboard and time-kill curve techniques. Synergism was demonstrated with time-killing curve in 43-68% of the strains tested. Ceftazidime plus amikacin was the most active combination by the checkerboard technique, while amikacin-cefoperazone was the most active combination by the time-killing curve technique against Pseudomonas aeruginosa. Discrepancies were found between the results of the two methods used.