The in vitro activity of N-formimidoyl thienamycin compared with other broad-spectrum cephalosporins and with clindamycin and metronidazole.

N-formimidoyl thienamycin is a new semisynthetic beta-lactam antibiotic still awaiting clinical trials. We have investigated the in vitro activity of N-formimidoyl thienamycin against 413 fresh clinical isolates and compared it to other new beta-lactam drugs and to clindamycin and metronidazole in the agar dilution test. All coliforms were inhibited by less than or equal to 1 mg/l and no member of the Proteus group was resistant to more than 8 mg/l of N-formimidoyl thienamycin. The MICs for Pseudomonas aeruginosa were less than or equal to 4 mg/l. Beta-haemolytic streptococci, pneumococci and staphylococci, including methicillin-resistant strains, were all inhibited by 0.125 mg/l or less. Streptococcus faecalis strains were all susceptible to less than or equal to 2 mg/l. N-formimidoyl thienamycin was highly active against the anaerobes tested. N-formimidoyl thienamycin exhibited bactericidal activity, and changes in inoculum size had little effect on the MICs. This data has shown that N-formimidoyl thienamycin has excellent antibacterial activity and an unusually broad spectrum of activity.