I went to London in November 2018 and visited the lab of Alexander Fleming where he discovered penicillin.
An author wrote this:

A poorly tidied up laboratory behind a miracle cure

In the summer of 1928, in a hurry to go on vacation, Scottish doctor Alexander Fleming left a pile of dirty Petri dishes in his laboratory sink. And as if that weren’t enough, the bottoms were smeared with staphylococci, the bacteria that cause boils, sore throats, and intestinal poisoning.

On his return a few weeks later, Dr. Fleming discovered an exciting thing in the mess of the sink: one of the Petri dishes was strewn with bacteria except where the mold had formed. There was nothing around as if an invisible barrier protected the area.

Taking a closer look at the phenomenon, he found that the mold, which belongs to a rare form of Penicillium Notatum, secreted a fluid that killed several chains of deadly bacteria. Alexander Fleming published his remarkable discovery — which has gone almost unnoticed.

Years later, Howard Walter Florey, an Australian pathologist, accidentally read Fleming’s article while leafing through old medical journals. Together with biochemist Ernst Boris Chain, they explored the therapeutic effects of the fluid secreted by mold. In 1941, they collected enough penicillin to administer it to the first human subject. This 43-year-old police officer contracted a fatal bacterial infection after being scratched by rose thorns in his garden.

The results were spectacular: the patient’s fever dropped, and his appetite returned. The penicillin used to treat him was quickly called a miracle drug. Unfortunately, due to a lack of supplies, the man’s infection flared up again, and he eventually died.

Dr. Fleming would share the Nobel Prize with Professors Florey and Chain for their work on this miracle cure. “I didn’t expect to revolutionize medicine by discovering the first antibiotic,” he said humbly. “But you have to believe that’s reality.”

The mold is the big white blob in the petri dish.
The spore of Penicillium notatum mold needed cool temperature to grow, and when Fleming left on his 6 week vacation
it was cool and the mold grew well, the Staphlococcas bacteria needed warm temperatures however.
During his vacation they had a heat wave which allowed the bacteria to grow.
Fleming noticed that the bacteria did not grow well by the mold.
Fleming and his staff tried to collect the secretions but it was difficult.

Fleming wrote it up, and for the next 10 years the mold was used as a lab curiosity to see which bacteria were or were not affected by the mold.

It was not until 1939 that the hunt was on in earnest to see what it did when injected into humans.

Mr. Brown (left), the curator of the current museum. He wrote a book of all the events.

You can find a booklet using this link:
https://www.acs.org/content/dam/acsorg/education/whatischemistry/landmarks/flemingpenicillin/the-discovery-and-development-of-penicillin-commemorative-booklet.pdf

Or just read it here:

For more reading:
Alexander Fleming Discovery and Development of Penicillin – Landmark – American Chemical Society (acs.org)