Laboratory findings of tobramycin and their relation to clinical response.

The susceptibilities of various clinical isolates to tobramycin were studied, and the applicability of these data to clinical situations was evaluated. The bactericidal nature of tobramycin was confirmed by killing curves, and the minimal inhibitory concentrations for 500 common pathogens were used to define its spectrum. Isolates inhibited by less than or equal to 5 mug/ml were considered to be sensitive to tobramycin. The clinical response of some patients was examined in relation to the minimal inhibitory concentration for the infecting organism and the serum and urine levels of tobramycin. The activity of tobramycin was compared with that of gentamicin against Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. Tobramycin was at least twice as active as gentamicin against P. aeruginosa. The effectiveness of tobramycin against S. aureus and E. coli was compared with that of other commonly used antibiotics, and its clinical role as an alternative to gentamicin in the "best guess" treatment of septicemia was considered. Lincomycin is often added to counter the ineffectiveness of tobramycin against streptococci and anaerobes such as Bacteroides species, but this combination was antagonistic against E. coli when tested in vitro by the checkerboard technique and its graphical display, the isobologram. Tobramycin was essentially similar to gentamicin in laboratory characteristics and clinical application but was more active against P. aeruginosa in general and against gentamicin-resistant strains in particular.