(Grades 3-4) Besides all of the big living things in the world—humans, animals, insects—there are many tiny creatures called bacteria that you need a powerful microscope to see. They come in all kinds of shapes and colors. The more common shapes are rod (bacillus), round (coccus) and spiral.
Bacteria are unicellular organisms that do not require other cells to exist. All functions required for survival and reproduction can be carried out by this single cell. All large organisms are multicellular, this means they are made up of many cells.​