In Vitro Susceptibility Testing with Tobramycin

Abstract

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-μg 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 μg 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 μg/ml. Neither Serratia nor the enterococci were particularly susceptible. Correlations between zones of inhibition around the 10-μg disc and minimal inhibitory concentrations were determined, and a zone diameter of 16 mm was recommended as the critical point for prediction of susceptibility.