Ticarcillin, carbenicillin and BL-P1908. In vitro comparison of three anti-pseudomonal semisynthetic penicillins.

Ticarcillin, carbenicillin and BL-P1908, a new anti-pseudomonal semisynthetic penicillin, were subjected to several in vitro comparisons-including minimal inhibitory and lethal concentrations (MIC and MLC), effects of inoculum size, effects of buffering the media and regression analyses of MIC and disc diffusion data. Whereas ticarcillin and carbenicillin showed comparable spectra of activity, with ticarcillin being more active against Pseudomonas aeruginosa and many Enterobacteriaceae, BL-P1908 exhibited a quite different spectrum, being much more active than either or the other two drugs against P. aerrrginosa and Streptococcus faecalis, but virtually inactive against most of the Enterobacteriaceae. However, the MIC/MLC disparity against P. aeruginosa was much greater for BL-P1908 than for the other two drugs. The increase in MIC's resulting from increasing the inoculum sizes was comparable for the three drugs. The regression analyses showed good correlations between the disc diffusion and MIC data for all three drugs. Based on the regression analyses we conclude that for the 100-meg carbenicillin disc the NCCLS recommended zone size breakpoints for P. aerrrginosa should be utilized for all organisms, and that if a single disc should be selected as a representative for these two currently available antipseudomonal penicillins, the 75-meg ticarcillin disc gave slightly better correlations with the MLC data of both drugs than the 100 meg carbenicillin disc.