1. Originally, an antibiotic substance obtained from cultures of the molds Penicillium notatum or P. chrysogenum; interferes with cell wall synthesis in bacteria.
2. One of a family of natural or synthetic variants of penicillic acid. They are mainly bactericidal, are especially active against gram-positive organisms, and, with the exception of hypersensitivity reactions, show a particularly low toxic action on animal tissue.
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