Comparative in vitro activity of tobramycin, gentamicin, kanamycin, colistin, carbenicillin, and ticarcillin and clinical isolates of Pseudomonas aeruginosa: epidemiological and therapeutic implications.

The susceptibility of clinical isolates of Pseudomonas aeruginosa to six antibiotics was related to the epidemiologic knowledge yielded by serotyping. The agar dilution method for determination of minimal inhibitory concentrations (MICs) and the Habs serotyping system were used. Effectiveness of each agent ranked highest to lowest as follows: tobramycin, colistin, ticarcillin, carbenicillin, gentamicin, and kanamycin. For many isolates, the MICs of gentamicin were close to the peak level in serum. Susceptibility to carbenicillin and to ticarcillin were highly correlated, with MICs of ticarcillin half those of carbenicillin. Similary, susceptibility to gentamicin and to tobramycin were correlated. For susceptible strains, MICs of gentamicin were two to four times greater than those of tobramycin. However, strains resistant to gentamicin were clearly in one of two groups: susceptible to tobramycin or resistant to the drug. Most strains resistant to both gentamicin and tobramycin were serotype 11, whereas those resistant gentamicin and kanamycin only were nonagglutinable.