In vitro antagonism between N-formimidoyl thienamycin and aztreonam, ticarcillin and ticarcillin/clavulanic acid.

One-hundred eighty-seven bacterial strains were tested by the two-disk-agar diffusion method for the interaction between N-formimidoyl thienamycin and aztreonam, ticarcillin and ticarcillin-clavulanic acid. An antagonism between N-formimidoyl thienamycin and the other 3 beta-lactams was noted in half of the evaluable tests, especially against Pseudomonas, Escherichia coli and Morganella.