Effect of carbenicillin on Pseudomonas aeruginosa.

The activity of carbenicillin against 200 strains of Pseudomonas aeruginosa was measured by a quantitative agar dilution method. Minimal inhibitory concentrations (M.I.C.'s) for five graded inocula were measured in terms of complete inhibition (CI) and reduced growth (RG). The M.I.C. decreased progressively as inocula were reduced, median values for the 200 strains ranging from 100 to 37.5 mug. per ml. by the CI criterion, and from 75 to 25 mug. per ml. by the RG definition. Ratios of M.I.C. obtained for large and small inocula were usually small. Identical M.I.C.'s by both CI and RG criteria were most often obtained when the inoculum for the RG criterion was 1 or 2 logs higher than that for complete inhibition.Population analysis of 15 strains of Ps. aeruginosa showed that one specific drug concentration usually caused a sharp drop in proportion of viable cells, ranging from 3 to 5 logs. None of the populations were completely non-viable even at 150 mug. per ml. There was evidence that the viability of different-sized populations was reduced disproportionately by carbenicillin.Carbenicillin 300 mug. per ml. exerted appreciable bactericidal effect against nine of 15 strains of Ps. aeruginosa after a 24-hour contact period; after only six hours the bactericidal effect was very small.Quantitative sensitivity measurements for carbenicillin should include M.I.C. values for both CI and RG criteria, using a range of inocula for testing. Such M.I.C. values may well be useful in monitoring carbenicillin therapy of tissue infections.