In vitro activity of N-formimidoyl thienamycin in comparison with cefotaxime, moxalactam, and ceftazidime.

The in vitro activity of N-formimidoyl thienamycin (N-f-thienamycin) was compared with the activities of other B-lactam antibiotics, using over 500 clinical bacterial isolates. N-f-Thienamycin inhibited 90% of the isolates of the common Enterobacteriaceae between 0.006 and 2 microgram/ml, regardless of their resistance to amoxicillin, ticarcillin, or cephalothin. It was, however, fourfold less active than moxalactam and ceftazidime and eightfold less active than cefotaxime. N-f-Thienamycin was nearly as active as ceftazidime against Pseudomonas aeruginosa (mean minimal inhibitory concentration, 3.0 microgram/ml) and eightfold more active than cefotaxime and moxalactam. In contrast to cefotaxime, moxalactam, and ceftazidime, N-f-thienamycin was highly active against enterococci (mean minimal inhibitory concentration, 1.3 microgram/ml) and staphylococci. The oxacillin-susceptible Staphylococcus aureus were inhibited between 0.03 and 0.12 microgram/ml, and the oxacillin-resistant S. aureus were inhibited between 0.12 and 2 microgram/ml. The high activity of N-f-thienamycin against both of the most important gram-positive and gram-negative organisms makes it a very promising new antibiotic.