Thus the shapes are the main way in which bacteria are identified. Bacilli , for example, are rod-shaped bacteria. Cocci are round spheres or lines of spheres linked together. Spirilla are curved into spirals. A different kind of spiral bacteria is called spirochetes.
Actinomycetes look like threads going in several directions at once.
Myxobacteria come in various fancy shapes. The smallest bacteria, however, are nearly as small as viruses, and a recently discovered giant bacterium, found in fish intestines, is larger than some one-celled protists. Even disease-causing bacteria live peacefully on or within humans without causing harm much of the time. Sometimes a particular strain of a species of bacterium is dangerous, or a bacterium that is normally harmless can cause disease when a person has been weakened by some other condition. Chlamydia trachomatis Swelling of the urethra or cervix Cholera Vibrio cholerae Diarrhea , muscle cramps, dehydration Diphtheria Corynebacterium diphtheriae Membranes form in throat, fever , difficulty breathing, weakness Food poisoning Salmonella spp.