Simonson, ASM MicrobeLibrary
There are thousands of species of bacteria, but all of them are basically one of three different shapes. Some are rod- or stick-shaped and called bacilli (buh-sill-eye).
Others are shaped like little balls and called cocci (cox-eye).
Others still are helical or spiral in shape, like the Borrelia pictured at the top of this page.
Some bacterial cells exist as individuals while others cluster together to form pairs, chains, squares or other groupings.
The human body consists of millions of different cells. A bacterium consists of a single cell.
Bacterial cell illustration courtesy of the University of California Museum of Paleontology.
A bacterium’s genetic information is contained in a single DNA molecule suspended in a jelly-like substance called cytoplasm. In most cases, this and other cell parts are surrounded by a flexible membrane that is itself surrounded by a tough, rigid cell wall. A few species, such as the mycoplasmas, don’t have cell walls.
Even though bacteria have only one cell each, they come in a wide range of shapes, sizes, and colors.
Some bacteria look like little balls (Micrococcus) while others appear like tangled strings or corkscrews (Leptospira) under a microscope. Others look like medicine capsules (Salmonella) or segmented ribbons (Cyanobacteria) or sticks. Still others look like fat commas (Vibrios).
Some bacteria are stalked (Caulobacter) while others have buds (Rhodomicrobium). Some have sheaths (Sphaerotilus) while others don’t.
Bacteria like mycoplasmas that lack a hard cell wall don’t have any particular shape at all.
Just like in animals, where size ranges from the giant blue whale to the tiny gnat, bacteria vary from 1 millimeter in diameter at the largest end of the scale to 20 nanometers in length at the smallest.
The largest bacteria found so far can actually be seen without the use of a microscope (Thiomargarita namibiensis and Epulopiscium fischelsoni). The smallest known bacteria are so tiny that they were once thought to be viruses (Mycoplasmas).
Some bacteria do have natural colors. Certain species contain pigments, such as various chlorophylls, that make them naturally green, yellow, orange, or brown. Colonies of millions of bacteria may appear pink, yellowish, or white.
Many of the vividly colored images on this and other Web sites, however, have been color enhanced or stained with dyes for better viewing under a microscope.