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Penicillin was discovered by Alexander Fleming in 1928 but did not become widely available until the 1940s. The penicillin’s are closely related compounds comprising a β-lactam ring, a five-membered thiozolidine ring, and a side chain. The ring structures are essential for antibacterial activity, and the side chain determines the spectrum and pharmacological properties. Most penicillins in current use are semi-synthetic derivatives of 6-aminopenicillic acid. They inhibit bacterial cell wall synthesis and are thus bactericidal.
