Sisomicin: evaluation in vitro and comparison with gentamicin and tobramycin.

Sisomicin is a new antibiotic produced by Micromonospora inyoensis. The in vitro activities of sisomicin, gentamicin, and tobramycin, three similar aminoglycosides, were determined against 228 clinical isolates representing 10 genera of common pathogens. No difference was noted in the activities of these antimicrobial agents when assayed by a standard broth dilution technique against Klebsiella, Enterobacter, Escherichia, Salmonella, Citrobacter, enterococci, or Staphylococcus aureus. Sisomicin was significantly more active than tobramycin against Serratia and indole-positive Proteus strains. Sisomicin was significantly more active than gentamicin against indole-negative Proteus strains and slightly more active against indole-positive Proteus strains. Tobramycin was more active than sisomicin or gentamicin against Pseudomonas and indole-negative Proteus strains. Gram-negative bacilli resistant to one of the three antimicrobial agents were not necessarily resistant to either of the other two. Activity of sisomicin was independent of the susceptibility or resistance of these isolates to nine other antimicrobial agents as assayed by the Bauer-Kirby technique. The presence of 50% human serum did not antagonize the in vitro activity of sisomicin against gram-negative isolates. Because sisomicin showed certain advantages over gentamicin or tobramycin in vitro, further investigation of this new antimicrobial agent is warranted.