Comparison of five aminocyclitol antibiotics in vitro against Enterobacteriaceae and Pseudomonas

The in vitro susceptibility of 163 strains of Enterobacteriaceae and 23 isolates of Pseudomonas aeruginosa to various concentrations of gentamicin, kanamycin, spectinomycin, tobramycin, and BB-K8, a new semisynthetic aminoglycoside antibiotic, was determined. Studies were performed in Mueller-Hinton agar and broth, and two different sizes of bacterial inocula were used. On a weight basis, gentamicin and tobramycin demonstrated comparable activity in vitro and were the most active of the five drugs tested against Escherichia coli, Klebsiella, Enterobacter, and Proteus species. All of these organisms were inhibited by gentamicin or tobramycin at concentrations of 5.0 mug or less/ml in agar with both inocula of bacterial cells. In addition, tobramycin was the most active drug against isolates of P. aeruginosa and gentamicin was the most active against Salmonella and Shigella species. Although kanamycin and BB-K8 demonstrated a high degree of activity against most Enterobacteriaceae, they were not the most active agents tested for any genus. Spectinomycin was the least active compound, and many isolates grew in concentrations higher than those readily attainable in serum.