Description: Mecillinam (amidinocillin) is a penicillin nucleus (6 APA) derivative, active in vitro against most aerobic and anaerobic Gram-negative bacilli, including E. coli and B. fraglis, but not active against Staphylococcus aureus, Enterococcus, or Pseudomonas. Mecillinam is synergistic with other beta-lactam drugs and therefore can be used in combination to treat severe Gram-negative infections. Although its in vitro efficacy is convincing, there are not enough clinical trials to support its use for intra-abdominal infections.
Intrinsic resistance in susceptible species of enterobacteria is uncommon and many ampicillin-resistant strains are susceptible. Bacteria that are resistant to both ampicillin and mecillinam are usually those producing large amounts of -lactamase, most commonly plasmid-mediated enzymes.