Penicillin, substance produced by a class of fungi that interferes with cell wall production by bacteria and was one of the first, and remains among the most useful, antibiotics. The property was noted by A. Fleming in 1928, and production of penicillin for medical use was started by E.B. Chain and H.W. Florey in 1940. Staphylococcus, streptococcus, and the bacteria causing the venereal diseases of gonorrhea and syphilis are among the bacteria sensitive to natural penicillin, while bacilli negative to Gram's stain, which cause urinary tract infection, septicemia, etc., are destroyed by semisynthetic penicillins like ampicillin, oxacillin, and methicillin.