In Vitro Activity of Netilmicin, Gentamicin, and Amikacin

The in vitro activity of netilmicin (Sch 20569), a new semisynthetic derivative of gentamicin, was compared with that of gentamicin and amikacin. One hundred and ninety-two clinical isolates of Enterobacteriaceae, Pseudomonas aeruginosa , and Staphylococcus aureus were tested using both agar and broth dilution techniques. Netilmicin was comparable to gentamicin, with the following exceptions: (i) for Serratia marcescens and P. aeruginosa , gentamicin was more active than netilmicin; (ii) all strains of Escherichia coli, Klebsiella, Enterobacter, Proteus mirabilis , and Citrobacter freundii , which were resistant to gentamicin, were susceptible to netilmicin; (iii) some strains of S. marcescens , indole-positive Proteus , and Providencia , which were resistant to gentamicin, were susceptible to netilmicin. Netilmicin was more active than amikacin for all Enterobacteriaceae and S. aureus and equal to amikacin in activity against gentamicin-susceptible strains of P. aeruginosa . All strains of P. aeruginosa , resistant to gentamicin, were also resistant to netilmicin but were susceptible to amikacin. Minimal inhibitory concentrations (MICs) obtained with broth and agar showed no significant differences except for P. mirabilis , where broth MICs were twofold greater than agar MICs, and for P. aeruginosa , where agar MICs were twofold higher than broth MICs. The minimal bactericidal concentration (MBC) was either identical to or within one twofold dilution of the MIC for the strains tested. A 100-fold increase in inoculum size produced less increase in MIC and MBC with netilmicin than with gentamicin or amikacin.