Spirochetes are a phylogenetically distinct group of Bacteria which have a unique cell morphology and mode of motility. Spirochetes are very thin, flexible, spiral-shaped procaryotes that move by means of structures called axial filaments or endoflagella. The flagellar filaments are contained within a sheath between the cell wall peptidoglycan and an outer membrane. The filaments flex or rotate within their sheath which causes the cells to bend, flex and rotate during movement. Most spirochetes are free living (in muds and sediments), or live in associations with animals (e.g. in the oral cavity or GI tract). A few are pathogens of animals (e.g. leptospirosis in dogs, syphilis in humans and  Lyme Disease in dogs and humans).

Figure 10. Spirochetes: A. Cross section of a spirochete showing the location of endoflagella between the inner membrane and outer sheath; B. Borrelia burgdorferi, the agent of Lyme disease; C. Treponema pallidum, the spirochete that causes syphilis.

Other spiral shaped and and Curved Bacteria The main thing that unifies this group of bacteria is their spiral or vibrioid (curved) shape, although they are all classified among the Proteobacteria. Nonetheless, in certain environments, their characteristic shape can instantly inform an observer of their identity. Bacteria referred to as "spirilla" are Gram-negative aerobic heterotrophic bacteria with a helical or spiral shape. Their metabolism is usually respiratory and never fermentative. Unlike spirochetes, they have a rigid cell wall and are motile by means of ordinary polar flagella. Spirilla are inhabitants of microaerophilic aquatic environments. Most spirilla require or prefer that oxygen in their environment be present in an amount that is well below atmospheric concentration. The Rhodospirillaceae are found in the Alpha group of Proteobacteria; Spirillaceae and Oceanospirillaceae are Gamma Proteobacteria.

As inhabitants of marine and fresh waters many spirilla are endowed with some interesting properties.  Magnetospirillum contains magnetosomes and exhibits the property of magnetotaxis (movement in relationship to the magnetic field of the earth). Oceanospirillum lives in marine habitats and is able to grow at NaCl concentrations as high as 9 percent. Azospirillum is a nitrogen-fixing bacterium that enters into a mutualistic symbiosis with certain tropical grasses and grain crops. Spirilla are thought to play a significant role in recycling of organic matter, particularly in aquatic environments.

Two pathogens of humans are found among the spiral forms in the Epsilon group of Proteobacteria. Campylobacter jejuni is an important cause of bacterial diarrhea, especially in children. The bacterium is transmitted via contaminated food, usually undercooked poultry or shellfish, or untreated drinking water. Helicobacter pylori is able to colonize the gastric mucosal cells of humans, i.e., the lining of the stomach, and it has been well established as the cause of peptic ulcers.

Bacteria with a curved rod or comma shape are referred to as "vibrios". Like the spiral forms, vibrios are very common bacteria in aquatic environments. They are found among the Gamma Proteobacteria and have structural and metabolic properties that overlap with both the enterics and the pseudomonads. In Bergey's Manual (2001) Vibrionaceae is a family on the level with Enterobacteriaceae. Vibrios are facultative like enterics, but they have polar flagella, are oxidase-positive, and dissimilate sugars in the same manner as the pseudomonads. In aquatic habitats they overlap with the Pseudomonadaceae in their ecology, although Pseudomonas species favor fresh water and vibrios prefer salt water. The genus Vibrio contains an important pathogen of humans, Vibrio cholerae, the cause of Asiatic cholera. Cholera is an intestinal disease with a pathology related to diarrheal diseases caused by the enteric bacteria.

Five species of marine vibrios exhibit the property of bioluminescence, the ability to emit light of a blue-green color. These bacteria may be found as saprophytes of dead fish or as symbionts of living fish and invertebrates in marine environments. Some grow in special organs of the fish and emit light for the benefit of the fish (to attract prey, or as a mating signal) in return for a protected habitat and supply of nutrients. The reaction leading to light emission, catalyzed by the enzyme luciferase, has been found to be the same in all procaryotes, and differs from light emission by eucaryotes such as the fire fly. Luciferase diverts electrons from the normal respiratory electron transport chain and causes formation of an excited peroxide that leads to emission of light.

The small vibrioid bacterium, Bdellovibrio, is a tiny curved rod that is a parasite of other Gram-negative bacteria, including E. coli. It preys on other bacteria by entering into the periplasmic space and obtaining nutrients from the cytoplasm of its host cell while undergoing an odd type of reproductive cycle.  Bdellovibriois a member of the Delta Proteobacteria.

Myxobacteria are a group of fruiting gliding bacteria that comprise a unique order of Delta Proteobacteria. They exhibit a unique type of gliding motility. The vegetative cells move (glide) about together as a swarm, and then they aggregate together to form a multicellular fruiting body in which development and spore formation takes place. They exhibit the most complex behavioral patterns and life cycles of all known procaryotes. Myxobacteria are inhabitants of the soil. They have a eucaryotic counterpart in nature in the Myxomycetes, or slime molds, and the two types of organisms are an example of parallel or convergent evolution, having adopted similar life styles in the soil environment.

The vegetative cells of myxobacteria are typical Gram-negative rods that glide across a substrate such as a decaying leaf or piece of animal dung, or colonies of other bacteria. They obtain nutrients from the substrate as they glide across it and they secrete a slime track which other myxobacterial cells preferentially follow. If their nutrients become exhausted, the cells signal to one another to aggregate and form a swarm of myxobacteria which eventually differentiate into a multicellular fruiting body that contains myxospores, a type of dormant cell descended from a differentiated vegetative cell. In the case of Stigmatella, the myxospores are packed into secondary structures called cysts, which develop at the tips of the fruiting body (Figure 11). The bright-colored fruiting bodies of myxobacteria, containing millions of cells and spores, can often be seen with the unaided eye on dung pellets and decaying vegetation in the soil.

Figure 11. Stigmatella aurantiaca, a fruiting myxobacterium: L. Life Cycle R. Fruiting Body.