Over 400 recognized genera of bacteria are known to exist. Bacterial species are listed in Bergey's Manual of Systematic Bacteriology. The entire kingdom of bacteria, including cyanobacteria, is entitled Prokaryotae. Four divisions of bacteria based on their cell wall characteristics are included in the Prokaryotae kingdom. Not all bacteria are assigned to a division, but all are assigned to one of 33 “sections.”
Spirochetes have a spiral shape, a flexible cell wall, and motility mechanisms based on structures called axial filaments. Each axial filament is composed of fibrils extending toward each other between two layers of the cell wall.
Spirochetes are very slender and difficult to see under the light microscope. They are cultivated with great difficulty (some cannot be cultivated), and their classification is based on their morphology and pathogenicity. Certain species inhabit water environments, while others are parasites of arthropods (such as ticks and lice) as well as warm‐blooded animals. Spirochetes include Borrelia burgdorferi, the agent of Lyme disease, Treponema pallidum, the cause of syphilis, and Leptospira interrogans, the agent of leptospirosis.
Spirilla have a spiral shape, a rigid cell wall, and motility mechanisms based on polar flagella. The genera Spirillum, Aquaspirillum, and Azospirillum are widely dispersed among and readily isolated from numerous environments. These organisms are aerobic bacteria wound like helices. Species S. minor is a cause of rat bite fever in humans. The genus Campylobacter contains several pathogenic species, including C. jejuni, which causes campylobacteriosis, an intestinal infection accompanied by diarrhea.