In vitro susceptibility testing with tobramycin.

Tobramycin (nebramycin factor 6) is an aminoglycoside antibiotic active in vitro against many gram-negative species and Staphylococcus aureus. The susceptibility of 191 recently isolated pathogenic bacteria to tobramycin was measured by both a routine broth dilution procedure and the FDA standardized disc technique using a 10-mug disc. Twenty-five isolates each of Escherichia, Enterobacter, Klebsiella, indole-negative Proteus, Pseudomonas, Serratia, and S. aureus, and 16 isolates of group D enterococci were tested. The greatest activity was seen with S. aureus and Pseudomonas species; nearly all isolates of both were inhibited by 0.20 mug or less per ml. Tobramycin was slightly less active against Klebsiella and Enterobacter and moderately active against Escherichia and Proteus, with most isolates of these genera being inhibited by 1.56 mug/ml. Neither Serratia nor the enterococci were particularly susceptible. Correlations between zones of inhibition around the 10-mug disc and minimal inhibitory concentrations were determined, and a zone diameter of 16 mm was recommended as the critical point for prediction of susceptibility.