penicillin

Any of a group of antibiotic (bacteria killing) compounds obtained from filtrates of moulds of the genus Penicillium (especially P. notatum) or produced synthetically. Penicillin was the first antibiotic to be discovered (by Alexander Fleming); it kills a broad spectrum of bacteria, many of which cause disease in humans.

The use of the original type of penicillin is limited by the increasing resistance of pathogens and by allergic reactions in patients. Since 1941, numerous other antibiotics of the penicillin family have been discovered which are more selective against, or resistant to, specific micro-organisms.