While at St. Mary’s his most important discovery was proving that
an enzyme found in tears called lysozyme had a natural antibacterial
effect. During further research, Fleming accidentally discovered
penicillin which was a common mould found growing on stale bread.
Although that wasn't surprising, what it was doing was! The staph
bacteria in the contaminated dishes had been killed around this mould he
Fleming is said to have tested his new creation on animals without
harming them as he was an animal lover. His discovery in 1929 was
publicised in magazines, but little interest was shown until World War
II when it saved many lives. Penicillin is still used today to treat all
kinds of bacterial infections throughout the world.