Although French student Ernest Duchesne noted the growth of penicillin in 1896, its true potential was not realised until Scottish scientist Alexander Fleming rediscovered it in 1928. He noticed that colonies of the bacterium Staphylococcus aureus failed to grow in areas of a culture that had been contaminated by the green mold Pénicillium notatum.
However, its ability to kill infectious bacteria wasn’t demonstrated until Dr Howard Florey’s research proved it at Oxford University over a decade later.
Penicillin, a group of antibiotics, works by inhibiting the bacterial enzymes responsible for cell wall synthesis, while activating additional enzymes to break down the protective wall of the unwanted micro-organism. Consequently, penicillin is effective in combating micro-organisms that produce cell walls, as most bacteria constantly need to remodel their peptidoglycan cell walls as they grow and divide.
This involves the pencilling chemical structure binding to the enzyme that links the peptidoglycan molecules in the cell wall, which inhibits the formation of cell wall cross-links and causes the cell to burst (known as cytolysis) and cell death due to osmotis or water pressure.
(Read good article Did Penicillin, Rather Than The Pill, Usher In Age Of Love?)