Abstract

The influence of medium composition, pH, serum, and magnesium on the action of tobramycin is compared to that on the action of gentamicin. The minimum inhibitory concentration of tobramycin, gentamicin, kanamycin, and streptomycin was determined for 472 strains including 19 species: tobramycin was found to be the most active against Ps. aeruginosa and kanamycin against Providence, but gentamicin was usually the most active against all other species. Ps. aeruginosa passaged on medium containing either gentamicin or tobramycin showed increased resistance to both. Sensitive strains of this species were shown to contain a small minority of cells resistant to these antibiotics, the presence of which is believed to account for the marked bactericidal synergy which was demonstrated between tobramycin and carbenicillin.