Which step marked the beginning of the development of penicillin?

The development of penicillin began with Alexander Fleming discovering its antibacterial properties in 1928. This pivotal moment set the stage for future research and development. Fleming noticed that a mold called Penicillium notatum produced a substance that killed a variety of bacteria, which indicated a potential for treating infections. This discovery was crucial because it was the first instance of a natural substance being identified for its medicinal properties, leading to further investigations into penicillin as a viable antibiotic.

The subsequent actions, such as Florey and Chain's experiments and the USA's funding for production, built upon Fleming's initial discovery but did not initiate the development process themselves. Fleming's findings were foundational, as they provided the scientific basis that other researchers later expanded upon.