Morphological, biochemical and genetic studies have shown that penicillin-binding protein 2 of Escherichia coli is the target at which mecillinam interacts to result in the production of osmotically-stable round cells. The mechanism of action appears to be identical in a wide variety of Gram-negative rod-shaped bacteria including Salmonella typhimurium, Kiebsiella aerogenea, Serratis marcescens and Pseudomonas fluorescens . In Bacillus subtilis mecillinam probably acts as a typical, but relatively ineffective β-lactam.