In vitro susceptibility of gentamicin and/or tobramycin resistant gram-negative bacilli to seven aminoglycosides.

The in vitro activity of gentamicin, tobramycin, sisomicin, netilmicin, amikacin, kanamycin and streptomycin was tested simultaneously by the agar dilution method against 584 clinical isolates of gram-negative bacilli that were resistant to gentamicin and/or tobramycin. About half of the gentamicin-resistant Pseudomonas were susceptible to tobramycin but cross-resistance was virtually complete between gentamicin and tobramycin for Enterobacteriaceae. Sisomicin was much more active than gentamicin against Klebsiella, Escherichia and Citrobacter species. Only 18.9%, 27.4% and 27.9% of Klebsiella, Enterobacter and Serratia respectively were resistant to netilmicin. Amikacin was the most effective aminoglycoside with an overall resistance of 15.6%. Kanamycin was effective against 40% of Proteus and Providencia species. Surprisingly, more than half of Klebsiella and Enterobacter species and 85.3% of Serratia species were susceptible to streptomycin.