Microbes feed on the bright portions of your screen. The more abundant and luminous the pixels, the more the culture will feed and grow. If they can't find enough food they will soon slow down and eventually die. Feed them clip art if you like.
Credit goes to A. K. Dewdney's article in Scientific American, May 1989; Jon Hess (Vividus) and Jon Hartshorne (Berkeley Systems) for programming advice.
