In vitro comparison of aminoglycoside activities and their synergistic action with piperacillin

The antimicrobial activity of six aminoglycosides against 927 clinical isolates was compared. Gentamicin and netilmicin were most active against Enterobacteriaceae, followed by tobramycin greater than dibekacin greater than amikacin greater than kanamycin. Tobramycin was most active against Pseudomonas aeruginosa, both mucoid and non-mucoid strains, followed by dibekacin greater than gentamicin greater than netilmicin greater than amikacin greater than kanamycin. Piperacillin-aminoglycoside combinations acted (partially) synergistically against 33-40% of the Enterobacteriaceae, 34-58% of non-mucoid strains of P. aeruginosa and 63-84% of the mucoid strains of P. aeruginosa (p less than 0.01). Piperacillin + gentamicin was the most synergistic combination against Enterobacteriaceae and non-mucoid strains of P. aeruginosa (p less than 0.05). The action against Staphylococcus aureus and Streptococcus faecalis was mostly indifferent. Antagonism was not observed. In treating gram-negative infections where tissue concentrations are less important, every aminoglycoside combination may be effective; for Pseudomonas lung infections in cystic fibrosis, piperacillin + gentamicin or tobramycin will remain the drugs of first choice.