Bacteria common noun bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometers in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Most bacteria have not been characterized, and only about 27 percent of the bacterial phyla have species that can be grown in the laboratory (specifically unculturable phyla, known as candidate phyla, make up 103 out of approximately 142 known phyla).