Treponema

• TRANSMISSION ROUTE: Direct contact

Definition

Treponema are gram-negative, motile, spirochete with many nutritional requirements. They can be classified as anaerobes or microaerophiles. All are commensals or parasites in humans and other animals.

Natural Habitat and Features

The name Treponema was derived from the Greek words trepein and nema, meaning “turning thread.” Like other spirochetes, treponemes have two or more periplasmic flagella, each attached to opposite ends of the protoplasmic cylinder and unattached at the other end. They extend about two-thirds of the way along the cylinder. The membrane cell wall of the cylinder is rigid, while the complex outer sheath is flexible. When the flagella rotate in the space between the sheath and the cylinder, the entire organism rotates in the opposite direction, allowing for motility.

Because the diameters of these bacteria are usually less than 0.3 micrometers (m), they are difficult to visualize with a Gram stain. However, most can be visualized using dark-field or phase-contrast microscopy. They grow best at pH 7.2 to 7.4 and at temperatures between 86° and 99° Fahrenheit (30° and 37° Celsius). Treponemes have very small genomes, with approximately 1.14 million base pairs and fewer than 1,100 genes. Because of their small genomes, they have limited metabolism and depend on their hosts for many necessary compounds, including fatty acids and most amino acids that treponemes cannot make. They are difficult to grow in culture because of their extensive nutritional requirements. Some, like pallidum, have never been successfully grown in culture; others can be co-cultured only in the presence of other cultured cells; and some, whose complex nutritional requirements have been determined, can be grown in normal culture.

Although immunity does develop after some treponemal infections, that immunity is strain-specific; no Treponema vaccine has been developed.

The taxonomy of the genus has changed in the twenty-first century. The four main human pathogens, pallidum, carateum, endemicum, and pertenue, have all been reclassified as subspecies of pallidum, although the original designations, especially carateum, still appear in the literature. These pathogens were combined because, morphologically and genetically, almost no difference exists among these organisms. All are flattened spirochetes that cause a three-phase infection in humans. Stage one shows a small sore or chancre at the site of infection. It usually appears days or weeks after initial contact; during this stage, the spirochetes are multiplying. Stage two occurs a few weeks to several months later; during this stage, the bacteria disseminate, and lesions appear on various body parts. Stage three occurs after the bacteria have become fully disseminated, which may take many years and is the most serious; many lesions present both internally and externally.

The treatment of choice is penicillin, and a single dose is usually enough to wipe out the organisms in stages one and two. Other antibiotics, such as tetracycline, chloramphenicol, erythromycin, and azithromycin, have been used, but none seem as effective as penicillin in treating human primary and secondary infections. The tertiary stage requires prolonged, and sometimes more diverse, antibiotic therapy.

Pallidum naturally infects only humans, and humans serve as the only living reservoir for the bacteria. Pallidum, a thin spirochete with a diameter of less than 1.5 m, is so sensitive to environmental stress that it rarely survives more than a few seconds away from its human host. Pallidum pallidum causes syphilis, the most severe of the treponemal infections. Infections are usually contracted through sexual contact with an infected partner. Congenital syphilis can occur when the bacterium crosses the placental barrier or infects the fetus as it passes through the birth canal of an infected woman. The tertiary stage of syphilis occurs in about one-half of all untreated cases. Lesions can occur in the central nervous system and the circulatory system, leading to paralysis and death.

Pallidum pertenue causes yaws and pallidum carateum causes pinta. Both are transmitted through skin-to-skin contact and are most common in children in tropical and subtropical countries. Yaws is found in tropical regions worldwide and, in its tertiary stage, can lead to disfiguring lesions on the bones. Pinta is more common in Central America and South America and, even in its tertiary stages, involves only the skin.

Pallidum endemicum is the cause of bejel. This disease is most common in the Mediterranean region and Saharan Africa. The bacterium is usually transmitted through mouth-to-mouth contact but is sturdy enough to remain viable on eating utensils and can be transmitted by sharing utensils with an infected person. The primary and secondary stages are usually found in the mouth, while the disseminated tertiary stage leads to bone lesions.

Denticola and vincentii are two of the many treponemes that cause periodontal disease. Other Treponema spp. seem to be commensal in human oral and genital tissue without being pathogenic unless the host is debilitated. Some Treponema spp. are pathogenic in other animals (for example, hyodysenteriae in pigs and paraluiscuniculi in rabbits). Bryantii and saccharophilum, found in the rumen of cows, are important in the complete breakdown of cellulose, although neither is cellulolytic itself. In termite guts, both primitia and azotonutricium are needed for the utilization of wood as food. Azotonutricium is the only known Treponema spp. that can fix atmospheric nitrogen.

Bibliography

Antal, George M., Sheila A. Lukehart, and Andre Z. Meheus. "The Endemic Treponematoses." Microbes and Infection, vol. 4, 2002, pp. 83-94.

Johnson, Shannon, et al. "Syphilis: Symptoms, Diagnosis, Treatment, and Prevention." Healthline, 6 May 2022, www.healthline.com/health/std/syphilis. Accessed 29 Oct. 2024.

Krieg, Noel R., et al., editors. Bergey’s Manual of Systematic Bacteriology. 2d ed., Springer, 2010.

Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed., Pearson/Prentice Hall, 2010.

Radolf, Justin D. "Treponema - Medical Microbiology." NCBI, www.ncbi.nlm.nih.gov/books/NBK7716/. Accessed 29 Oct. 2024.

Stoltey, J.E., and Cohen, S.E. "Syphilis Transmission: A Review of the Current Evidence." Sexual Health, vol. 12, no. 2, Apr. 2015, pp. 103-109, doi:10.1071/SH14174. Accessed 29 Oct. 2024.