Comparison of Ticarcillin and Carbenicillin Activity Against Random and Select Populations of Pseudomonas aeruginosa

Abstract

The in vitro activities of ticarcillin (T) and carbenicillin (C) against 253 strains of Pseudomonas aeruginosa (108 randomly selected clinical isolates and 145 isolates determined to be C resistant by 10 referring hospitals) were evaluated using both broth microdilution and disk diffusion techniques. T activity was usually twofold or greater than C activity for individual strains in both sample populations. The minimal inhibitory concentration (MIC) values of T and C were ≤0.4 to >400 (median, 25) and 0.8 to >400 (median, 50) μg/ml, respectively, for the random sample and 1.6 to >400 (median, 50) and 12.5 to >400 (median, 200) μg/ml, respectively, for the select sample. For predicting susceptibility (MIC, ≤50 μg of T and ≤100 μg of C per ml), 75-μg T and 100-μg C disks were equivalent in accuracy for the random sample (T/T = 92.6%, T/C = 92.0%, C/T = 91.7%, C/C = 89.8%). Regression-line analyses of zones of inhibition versus MIC values for the four combinations showed coefficients of correlation to be −0.81, −0.76, −0.77, and −0.75, respectively. With the sample selected as being resistant to C, T was better than C for predicting T susceptibility ( P < 0.05), and T was equivalent to C for predicting C susceptibility (T/T = 84.3%, T/C = 81.0%, C/T = 73.6%, C/C = 80.4%). Coefficients of correlation were −0.70, −0.74, −0.55, and −0.61, respectively. T and C disks were, therefore, equivalent for determining T and C susceptibilities, but the T disk was more reliable for determining T susceptibility in a population skewed with C-resistant strains.