Protozoa are 1,000 times larger than bacteria. Still, most can only be seen through a microscope. They are one-celled organisms but, unlike bacteria, protozoa enclose all their genes and most internal organs in a pouch called a nucleus.

Protozoa are the hunters of the microbial world, stalking and eating other protozoa and bacteria. But they are also hunted by larger animals. In fact, protozoans and bacteria are the smallest animals in the food chain. They are eaten by everything from tiny fish to chickens to humans.