Penicillin's discovery was serendipitous

Innovation is often serendipitous, and penicillin falls under this category.

Fleming already known for this study of staphylococci, was trying to check his facts against how this type of bacteria grows in a natural condition. He left a couple of Petri dishes by the window and went off to a holiday. When he came back from his holiday, out of habit, he checked his Petri dishes and he discovered that there's a strange pattern occurring, where the fungus and bacteria are not mixed.

The growth of the bacteria and fungus that triggered this discovery was greatly affected by the change of heat and cold water in London back in 1928 (see references).

At first, penicillin was declared as impractical as the application of it was topical. It took 12 years before Florey (and team of researchers) realised that penicillin works best when it's injected to our body.

References

How Innovation Works (p. 67).

But early experiments with penicillin as a topical antiseptic applied to infected wounds were disappointing.