Gentamicin, sisomicin, tobramycin, and kanamycin were compared in parallel tests in vitro and in vivo against a variety of bacterial strains and species. A number of differences were seen in vitro, in particular: (i) the lower activity of kanamycin, (ii) the greater activity of tobramycin against Pseudomonas, (iii) the greater activity of gentamicin and sisomicin against Serratia, and (iv) the generally similar results with tobramycin, gentamicin, and sisomicin against species other than Pseudomonas and Serratia, with the ranking in order of decreasing activity being sisomicin, gentamicin, and tobramycin. Analysis of disc test results suggested that the gentamicin disc is not adequate for testing the susceptibility of all bacteria to sisomicin or tobramycin. In vivo tests did not confirm all specifics of in vitro tests; results of in vivo tests indicated that sisomicin may be the most active. It is suggested that the place of each of the antibiotics in human therapy can best be evaluated by more rigorous in vivo tests and clinical studies rather than extensive in vitro comparisons.

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Carbenicillin is capable of inactivating gentamicin in vitro. This effect is time, temperature and medium dependent. In vitro antibacterial tests demonstrate greater than additive activity in some instances and inactivation in others, particularly after prolonged incubation. Inactivation was not observed in vivo in mouse protection tests. Additive or more than additive combined activity in mouse protection tests occurred only infrequently. Intravenous