Alexander Fleming was working with a bacteria called staphylococcus, which looked like bunches of grapes under the microscope. He could grow colonies of these bacteria on petri dishes with a growth medium. When he returned from vacation, he found that several of these dishes had become contaminated with a fungus. The colonies near where the fungus was growing had been destroyed. Fleming then grew the mold and purposefully added it to a petri dish that contained bacteria colonies. The colonies exposed to the mold died. Fleming named the substance within the mold that killed bacteria “penicillin.”