In vitro activity of ticarcillin, carbenicillin and ampicillin against some gram-negative bacilli.

alpha-Carboxy-3-thienylmethyl penicillin (ticarcillin) is a relatively new semisynthetic penicillin which is more active than carbenillin against Pseudomonas aeruginosa. Among the strains tested, those isolated from the respiratory tract showed an increased susceptibility to carbenicillin and ticarcillin. As with carbenicillin, synergistic activity against P. aeruginosa could be demonstrated with ticarcillin in combination with gentamicin. Like other penicillins, the antibacterial activity was influenced by the inoculum size. The antibacterial activity of ticarcillin showed the compound to be almost equally active with carbenicillin and ampicillin against Escherichia coli and Klebsiella aerogenes, but less active than acrbenicillin and ampicillin against indole-negative Proteus strains. Regarding the indole-positive Proteus species, at relatively low antibiotic concentrations the proportion of strains sensitive to ticarcillin was greater than to carbenicillin or ampicillin whereas at relatively high antibiotic concentrations the converse was the case. It is interesting to note that a high proportion of strains of E. coli and K. aerogenes were resistant to the three penicillins even at a concentration of 200 mug/ml, while 70% of Proteus strains were inhibited by these drugs at the same concentration. Disc susceptibility tests with ticarcillin were carried out according to BAUER-KIRBY method.