Alexander Fleming • Penicillin Discovery Plate

How an Accidental Mold Contaminant Revealed Penicillin

Watch a lawn of staphylococci meet a stray Penicillium colony. As the mold grows, clear zones of inhibition appear where bacterial growth is suppressed.

Bacteria

Penicillium mold

Follow the steps to see how Fleming recognized an antibacterial substance.

Dense staphylococcal lawn

Penicillium mold colony

Zone of inhibition (no bacterial growth)

Agar plate: mold halo blocking bacteria

State: Clean agar plate

Start with a sterile agar plate. Add a uniform lawn of bacteria, then a single mold colony. As the mold spreads, a clear halo appears where bacteria cannot grow — this is the visual clue that led Fleming to penicillin.

How to use this plate

Click “Seed plate” to coat the agar with bacteria (a confluent lawn).
Add a Penicillium colony: a blue-violet spot appears on the plate.
Press “Let them grow” to see mold expand and bacteria cleared around it.

What to notice

The mold colony secretes an antibiotic into nearby agar.
Bacterial dots fade inside the halo: this is a zone of inhibition.
The sharp edge between clear and green lawn is how we visualize antimicrobial activity on plates.

Link to Fleming and modern antibiotic tests

Fleming saw contaminant mold surrounded by bacteria-free zones on staph plates.
He inferred the mold released a substance lethal to bacteria: later called penicillin.
Modern disk diffusion assays reuse this logic: measure antibiotic potency by zone size.