In vitro synergistic effect of netilmicin, a new aminoglycoside antibiotic

The combination of netilmicin (Sch 20569) with carbenicillin, cefazolin, chloramphenicol, and clindamycin was evaluated by in vitro tests against 91 clinical isolates of Pseudomonas, Klebsiella, and Escherichia coli. Combinations were considered to show synergy if there was a fourfold or greater reduction in minimal inhibitory concentration values of both antibiotics when combined. Synergy of netilmicin with carbenicillin could be demonstrated against 21 of 46 Pseudomonas isolates. Comparison of the synergy of carbenicillin with netilmicin or gentamicin showed that netilmicin was less active than gentamicin against Pseudomonas aeruginosa. However, at therapeutic concentrations of the antibiotics, significant differences were not observed. Synergy of netilmicin and cefazolin was demonstrated against only 1 of 21 Klebsiella strains tested, but partial synergy was found for 12 of 20 isolates. Synergy and antagonism were uncommon with the netilmicin-chloramphenicol and netilmicin-clindamycin combinations against E. coli and Pseudomonas.