Many treponemes are part of the normal flora. Significant pathogens in this genus are T. pallidum pallidum, the agent of epidemic syphilis, T. p. pertenue, the agent of yaws, and T.p. endemicum the agent of endemic syphilis. Other species are agents of oral diseases.

T. pallidum pallidum, a fastidious, strict aerobe, is the agent of syphilis. It is limited to humans. Primary syphilis appears as an infective but painless chancre that appears to heal spontaneously. Untreated infection progresses to a systemic condition called secondary syphilis, which is manifest in skin rashes. The spirochetes remain in body organs and can initiate tertiary syphilis after a latency of many years. During this stage, the spirochete invades the heart, blood vessels, bone and brain, and forms Gummas.

Syphilis is difficult to diagnose because of its symptoms and its concurrent presence with other STDs. Methods of detection include direct microscopic identification from clinical specimens and serological tests for host antibody(such as the TPI test). Treatment with penicillin G is still effective. Control depends on identification of all sexual contacts and use of barrier contraceptives.

Subspecies of T.pallidum are agents of bejel, yaws, and pinta, diseases endemic to specific tropical regions. Their symptoms mimic the stages of syphilis, but all are transmitted non-sexually.

Leptospira interrogans is a hooked spirochete that causes leptospirosis. Leptospirosis is a tropical zoonosis transmitted through direct contact with the urine of infected animals. It can be diagnosed by serological testing and treated with drugs early in infection.

Spirochetes in the genus Borrelia are larger and more loosely coiled than other genera. Pathogens in this group are agents of relapsing fever and Lyme disease. Both are transmitted by arthropods.

The two agents of relapsing fever are B.hermsii and B.recurrentis. B.hermsii is the agent of tick-borne relapsing fever. It is transmitted by soft ticks to humans from rodents. B.recurrentis is transmitted among humans by body lice. Relapsing fever is known by the recurrence of symptoms. Borrelia sequestered in organs continue to multiply changing their surface antigens simultaneously. The fever responds to drug therapy. It can be prevented by control of rodent populations, prevention of tick bites, and good personal hygiene.

Borrelia burgdorferi is the agent of Lyme disease, a slowly progressive systemic infection that can be difficult to diagnose. It is transmitted to humans by hard ticks. The natural hosts are deer and rodents. The most prominent symptom is a bull's-eye rash radiating outward form the bite site. Tetracycline and amoxicillin are effective if administered early.

Other Curviform Gram-Negative Bacteria of Medical Importance
The genera Vibrio, Campylobacter, and Spirillum are curved or short spiral-shaped bacteria that are actively motile. Vibrio cholerae is the agent of epidemic, or Asiatic, cholera. It is spread through contaminated water. Its virulence factor is an enterotoxin that causes profuse diarrhea and dehydration. Diagnosis is by microscopic identification of the bacteria in stool specimens or serological tests. Therapy includes prompt replacement of water and electrolytes followed by antibiotics. Preventive measures focus on effective water purification and waste water treatment.

V.parahaemolyticus, the agent of seafood gastroenteritis, is most often acquired by eating raw shellfish that have been exposed to infected waste water.

Campylobacter jejunum is the agent of campylobacter enteritis. This vibrio is transmitted to humans through contaminated water and animal products. Its enterotoxin, CJT, stimulates a diarrhea similar to cholera. Treatment in severe cases is similar to cholera.

Spirillum minus is the agent of rat bite fever, a self-limiting systemic infection. It is transmitted to humans by rodent bites.

Medically Important Bacteria of Unique Morphology and Biology
The rickettsia and chlamydiae are gram-negative obligate intracellular parasites that lack key metabolic enzymes. The mycoplasmas and L forms are parasites that lack a cell wall.

The rickettsia are extremely small, pleomorphic rods or coccobacilli that cannot synthesize their own AMP. They have a complex life-style that cycles between arthropod vectors and vertebrate hosts. Rickettsia are usually transmitted by tick bite or tick feces, but one species, Coxiella burnetti, is transmitted more often by food and air.

Rickettsial diseases are classified into six groups based on the vector and specific clinical characteristics. R. prowazekii is the agent of epidemic typhus, a systemic infection transmitted by the feces of body lice. If it invades the vascular endothelium, it can cause necrosis and hypotension. This disease responds well to antibiotic therapy and recovery usually provides immunity, but a mild recurrence, Brill-Zinssner disease, sometimes occurs years later. Epidemic typhus is associated with overcrowded living conditions.

R. typhi is the agent of endemic, or murine, typhus, which is less virulent than louse-borne typhus. It is transmitted by the feces of infected rat fleas and enters that host when bites are scratched.

R. tsutsugamushi is the agent of scrub typhus, a disease endemic to Asia. It is transmitted by chigger bites. Scrub typhus responds to drug therapy, but may be fatal if untreated.

R. rickettsii is the agent of Rocky Mountain spotted fever, a potentially fatal disease endemic to North America. It is transmitted by the bites of wood ticks, dog ticks, and the Lone Star Tick. RMSF produces a infection that, if untreated, can cause tissue necrosis, and cardiovascular and clotting disorders. This disease usually responds to tetracycline if given promptly.

Rochalimeae quintana is the agent of trench fever, a systemic infection lasting 5-6 days. It is transmitted to humans by body lice. Untreated hosts may have recurring infections.

Coxiella burnettii, the cause of Q fever, is the only rickettsial disease transmitted to humans primarily by infected milk, meat, and airborne contaminants. It infects a broad range of vertebrates and arthropods. It also forms resistant spores. Prevention includes pasteurization of milk.

Bartonella henselae is the agent of cat-scratch disease, a systemic infection that travels from the initial site along the lymph vessels. This disease responds to antibiotic therapy if not resolved spontaneously.

The chlamydias are small, gram-negative, pleomorphic, intracellular parasites which have no catabolic pathways. They exist in two forms: the elementary body, which is the form transmitted between human hosts through direct contact and body secretions and the reticulate body, which is the infective form.

Chlamydia trachomatis is the agent of several STDs: NGU (nongonococcal urethritis), pelvic inflammatory disease, and lymphogranuloma venereum. It also causes ocular trachoma, a serious eye infection.

Chlamydia psittaci is the agent of ornithosis, an influenza-like disease that has serious systemic complications if untreated. Antibiotic therapy is usually successful.

Chlamydia pneumoniae is the agent of respiratory infections in young adults and asthmatics.

Mollicutes and Other Cell-Wall-Deficient Bacteria
Mycoplasmas are tiny pleomorphic bacteria that lack a cell wall. Although most species are parasitic, mycoplasmas can be cultured on complex artificial media. They are also membrane parasites because they bind tightly to epithelial linings of the respiratory and urogenital tracts.

M. pneumoniae is the agent of primary atypical pneumonia (walking pneumonia).

M. hominis and Ureoplasma urealyticum are agents of sexually transmitted infections of the reproductive tract and kidneys, and more recently, fetal infections.

M. incognitus is a recently identified pathogen which causes systemic infection by suppressing the immune system.

L forms are wall-deficient variants of walled bacteria such as group a Streptococcus, Proteus, Mycobacterium, and Corynebacterium that are occasionally involved in diseases.

Bacteria in Dental Disease
The oral cavity is an ecosystem that provides a broad spectrum of niches for a wide variety of microorganisms. A high sugar diet and poor oral hygiene enhance the growth of bacteria, which cause oral diseases. Streptococcus, Lactobacillus, and Actinomyces are the main agents of dental caries. Invasion of the gums or gingiva by a mixture of spirochetes and gram-negative bacteria causes many gum diseases. Treponema vincentii is the causative agent of ANUG, or trench mouth.