Minimum Inhibitory Concentrations of Carbenicillin Against Pseudomonas aeruginosa

Carbenicillin, a recently introduced semi-synthetic penicillin, has a relatively high activity against Pseudomonas aeruginosa. A marked inoculum effect on the minimum inhibitory concentration has been recorded. A number of in vitro experiments have been carried out with four Pseudomonas aeruginosa strains. No enzymatic degradation of carbenicillin has been noted not even after induction with penicillin G and carbenicillin. The strains rapidly degraded penicillin G and ampicillin. Variants with decreased sensitivity, 8 to 32 times that of the original population, could be found after a number of transfers in liquid substrates containing increasing concentrations of carbenicillin. These variants proved stable and they were unable to break down carbenicillin. By using viable counts it has been shown that variations in inocula do not influence the MIC value when this is defined as the smallest concentration of antibiotic just inhibiting an increase in viable counts after inoculation. Employing large inocula, an initial bacteriostatic effect of carbenicillin is recorded.