n.
Antibiotic derived from the Penicillium mold.
It was discovered in 1928 by Alexander Fleming ; by 1940, Howard Walter Florey , Ernst Boris Chain , and others had produced commercial quantities that proved vital to the treatment of war casualties, making penicillin the first successful antibiotic for human bacterial infections. Many natural and semisynthetic (ampicillin, amoxicillin) variants have since been produced. All work by inhibiting the enzymes responsible for bacterial cell wall synthesis (and therefore do not work against microorganisms without cell walls or with certain variant cell walls; e.g., the tuberculosis bacillus). Among the bacteria susceptible to penicillin are those causing strep throat, spinal meningitis , gas gangrene , and syphilis . Overuse has led to drug resistance in some strains. Penicillin's chief side effect is allergy , which can be life-threatening.