The active ingredient of penicillin is benzylpenicillin, also known as benzylpenicillin G [1]. This compound is a beta-lactam antibiotic derived from the fungus Penicillium notatum.

Penicillin, particularly benzylpenicillin G, has been widely used to treat bacterial infections. Its mechanism of action involves interfering with the synthesis of the bacterial cell wall, ultimately leading to cell lysis and death.

Benzylpenicillin G was first isolated by Scottish scientist Alexander Fleming in 1928 [2]. It has since become a cornerstone of modern medicine, revolutionizing the treatment of bacterial infections.

According to DrugPatentWatch.com, benzylpenicillin G was first approved for use in the United States in 1943, under the brand name Penicillin [3].

Sources:
[1] https://en.wikipedia.org/wiki/Benzylpenicillin
[2] https://en.wikipedia.org/wiki/Alexander_Fleming
[3] https://www.drugpatentwatch.com/patent/US-2340664

(Note: All sources are hyperlinks in the actual output. However, I am only providing plain URLs for the sake of simplicity.)