Subtitles section Play video Print subtitles Microorganisms are small. So small you can't even see them unless they cluster together. Each of these dots is really millions of bacteria. But down at a microscopic scale, mold and bacteria fight vicious battles. Their resources and territory are scarce and the stakes are survival. Mold has a powerful chemical weapon in its arsenal. I'm Emily, and I'd like to show you Alexander Fleming's famous discovery that bread mold kills bacteria. Here's some fresh bread. I'll add some water because mold loves moisture. Now I need bacteria, which you can find basically anywhere, including in your mouth. This petri dish has a jello-like layer of agar: food for the bacteria in my saliva. It'll take a few days before the colonies of bacteria are big enough to see. Let's set up a place to grow the mold and bacteria in a contained way, so it doesn't spread or infect anything else. These bell jars will help make sure that nothing contaminates the experiment, and the experiment doesn't contaminate anything else. I'm back! Wow! There's a lot of bacteria! Each cell divided and divided and divided to make a colony that we can see now. Some bread mold also grew. Let's see if it can kill some bacteria. I'll take a chunk of bread mold and let it do its work. Legend has it that in Fleming's original experiment, some mold from his lunch sandwich accidentally dropped into his petri dish. Let's take a closer look, and we'll come back later and see what's changed. Under a microscope mold looks something like this yellow model, and mold attacks bacteria cell walls, represented by these cups. Mold secretes a chemical: penicillin, which damages bacteria cell walls, stamping out the competition. This means the bacteria surrounding the mold will die. We're looking for bacterial death around the mold. Do you see where the bacteria have died around the mold? This is called the ring of death. Mold and bacteria have been battling for millions of years. Mold keeps coming up with new weapons: antibiotics; and bacteria keeps coming up with new shields: resistance. Alexander Fleming, after treating wounded soldiers in World War One, was the first person to realize how useful antibiotics are for treating infection. Today we remember him for his discovery of penicillin. I think his experiment is really neat, and I hope you do, too.