Treponema pallidum | |
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Scientific classification | |
Domain: | Bacteria |
Phylum: | Spirochaetota |
Class: | Spirochaetia |
Order: | Spirochaetales |
Family: | Treponemataceae |
Genus: | Treponema |
Species: | T. pallidum |
Binomial name | |
Treponema pallidum | |
Treponema pallidum, formerly known as Spirochaeta pallida, is a microaerophilic, gram-negative, spirochaete bacterium with subspecies that cause the diseases syphilis, bejel (also known as endemic syphilis), and yaws. [1] It is known to be transmitted only among humans and baboons. [2] T. pallidum can enter the host through mucosal membranes or open lesions in the skin and is primarily spread through sexual contact. [3] It is a helically coiled microorganism usually 6–15 μm long and 0.1–0.2 μm wide. T. pallidum's lack of both a tricarboxylic acid cycle and processes for oxidative phosphorylation results in minimal metabolic activity. [4] As a chemoorganoheterotroph, Treponema pallidum is an obligate parasite that acquires its glucose carbon source from its host. Glucose can be used not only as a primary carbon source but also in glycolytic mechanisms to generate ATP needed to power the bacterium given its minimal genome. [5] The treponemes have cytoplasmic and outer membranes. Using light microscopy, treponemes are visible only by using dark-field illumination. T. pallidum consists of three subspecies, T. p. pallidum, T. p. endemicum, and T. p. pertenue, each of which has a distinct related disorder. The ability of T. pallidum to avoid host immune defenses has allowed for stealth pathogenicity. [6] The unique outer membrane structure and minimal expression of surface proteins of T. pallidum has made vaccine development difficult. Treponema pallidum can be treated with high efficacy by antibiotics that inhibit bacterial cell wall synthesis such as the beta-lactam antimicrobial penicillin-G. [7]
Three subspecies of T. pallidum are known: [8]
The three subspecies causing yaws, bejel, and syphilis are morphologically and serologically indistinguishable. [9] The three subspecies can be distinguished by genetics, using restriction fragment length polymorphism (RFLP), which utilizes techniques such as PCR, restriction digest and gel electrophoresis. [10] Genes TprC, TprI, and the 5' flanking region of tpp15 can be used to differentiate between the three subspecies based on DNA fragment lengths and location of bands in gel electrophoresis. [11] [12] These bacteria were originally classified as members of separate species, but DNA hybridization analysis indicates they are members of the same species. Treponema carateum, the cause of pinta, remains a separate species because no isolate is available for DNA analysis. [13] Disease transmittance in subspecies T. p. endemicum and T. p. pertenue is considered non-venereal. [14] T. p. pallidum is the most invasive pathogenic subspecies, while T. carateum is the least invasive of the species. T. p. endemicum and T. p. pertenue are intermediately invasive. [9]
Treponema pallidum was first microscopically identified in syphilitic chancres by Fritz Schaudinn and Erich Hoffmann at the Charité in Berlin in 1905. [15] This bacterium can be detected with special stains, such as the Dieterle stain. T. pallidum is also detected by serology, including nontreponemal VDRL, rapid plasma reagin, treponemal antibody tests (FTA-ABS), T. pallidum immobilization reaction, and syphilis TPHA test. [16]
Treponema pallidum is a helically shaped bacterium with high motility consisting of an outer membrane, peptidoglycan layer, inner membrane, protoplasmic cylinder, and periplasmic space. [9] It is often described as gram-negative, but its outer membrane lacks lipopolysaccharide, which is found in the outer membrane of other gram-negative bacteria. [17] It has an endoflagellum (periplasmic flagellum) consisting of four main polypeptides, a core structure, and a sheath. [18] The flagellum is located within the periplasmic space and wraps around the protoplasmic cylinder. The peptidoglycan layer interacts with the endoflagellum which may aid in motility. [19] T. pallidum's outer membrane has the most contact with host cells and contains few transmembrane proteins, limiting antigenicity, while its cytoplasmic membrane is covered in lipoproteins. [20] [21] The outer membrane's treponemal ligands' main function is attachment to host cells, with functional and antigenic relatedness between ligands. [22] The genus Treponema has ribbons of cytoskeletal cytoplasmic filaments that run the length of the cell just underneath the cytoplasmic membrane.
The outer membrane (OM) of T. pallidum has several features that have made it historically difficult to research. These include details such as its low protein content, its fragility, and that it contains fewer gene sequences related to other gram negative outer membranes. [23] Progress has been made using genomic sequencing and advanced computational models. The treponemal outer membrane proteins are key factors for the bacterium's pathogenesis, persistence, and immune evasion strategies. The relatively low protein content prevents antigen recognition by the immune system and the proteins that do exist protrude out of the OM, enabling its interaction with the host. [23] Treponema's reputation as a "stealth pathogen" is primarily due to this unique OM structure, which serves to evade immune detection. [23]
The TP0126 protein has been linked to the outer membrane protein family (OMP). This protein will sit in the outer membrane like a porin, which is supported by circular dichroism recombinant TP0126, and will increase the virulence factor. [24] Researchers have classified the TP0126 protein in this class due to the homology between the protein and the porins of the OMPs. [25] [24] This protein is encoded by the tp0126 gene, which is conserved over all strains of T. pallidum. [25]
TP0326 is an ortholog of the β-barrel assembly machine Bam A. BamA apparatus inserts newly synthetized and exported outer membrane proteins into the outer membrane. [26]
TP0453 is a 287 amino acid protein associated with the inner membrane of the microbe's outer membrane. [27] This protein lacks the extensive beta sheet structure that is characteristic of other membrane proteins, and does not traverse the outer membrane. [28] This protein's function has been hypothesized to be involved with control of nutrient uptake. [29]
Outer Membrane Protein A (OmpA) domain-containing proteins are necessary for maintaining structural integrity in gram-negative bacteria. These domains contain peptidoglycan binding sites which creates a "structural bridge between the peptidoglycan layer and the outer memebrane." [30] The protein TP0624 found in T.pallidum has been proposed to facilitate this structural link, as well as interactions between outer membrane proteins and corresponding domains on the thin peptidoglycan layer. [30]
The TP0751 protein is a protein that is unique to T. pallidum, and it is thought to aid in attachment to the host's extra cellular membrane. [31] Since this protein aids in the attachment to the host, it sits on the surface of the cells, and in 2005, it was discovered that the TP0751 protein will attach to the laminin component in the host's extracellular matrix. [32] With that, it is thought that the TP0751 protein plays a key role in dissemination with the host. [32] [31]
TP0965 is a protein that is critical for membrane fusion in T. pallidum, and is located in the periplasm. [27] TP0965 causes endothelial barrier dysfunction, a hallmark of late-stage pathogenesis of syphilis. [33] It does this by reducing the expression of tight junction proteins, which in turn increases the expression of adhesion molecules and endothelial cell permeability, which eventually leads to disruption of the endothelial layer. [34]
The Treponema repeat family of proteins (Tpr) are proteins expressed during the infection process. Tprs are formed by a conserved N-terminal domain, an amino-terminal stretch of about 50 amino acids, a central variable region, and a conserved C-terminal domain. [26] The many different types of Tpr include TprA, TprB, TprC, TprD, and TprE, but variability of TprK is the most relevant due to the immune escape characteristics it allows. [35]
Antigen variation in TprK is regulated by gene conversion. In this way, fragments of the seven variable regions (V1–V7), by nonreciprocal recombination, present in TprK and the 53 donor sites of TprD can be combined to produce new structured sequences. [36] [37] TprK antigen variation can help T. pallidum to evade a strong host immune reaction and can also allow the reinfection of individuals. This is possible because the newly structured proteins can avoid antibody-specific recognition. [35] This is possible because the newly structured proteins can avoid antibody-specific recognition. It is also suspected that the genes that encode for the TprK protein are essential in pathogenesis during the infection of syphilis. [35]
To introduce more phenotypic diversity, T. pallidum may undergo phase variation. This process mainly happens in TprF, TprI, TprG, TprJ, and TprL, and it consists of a reversible expansion or contraction of polymeric repeats. These size variations can help the bacterium to quickly adapt to its microenvironment, dodge immune response, or even increase affinity to its host. [37]
In the past century since its initial discovery, culturing the bacteria in vitro has been difficult. [38] Without the ability to grow and maintain the bacteria in a laboratory setting, discoveries regarding its metabolism and antimicrobial sensitivity were greatly impaired. [23] However, successful long-term cultivation of T.pallidum in vitro was reported in 2017. [38] This was achieved using Sf1Ep epithelial cells from rabbits, which were a necessary condition for the continued multiplication and survival of the system. [39] The medium TpCM-2 was used, an alteration of more simple media which previously only yielded a few weeks of culture growth. [39] This success was the result of switching out minimal essential medium (MEM) with CMRL 1066, a complex tissue culture medium. [38] With development, new discoveries about T.pallidum's requirements for growth and gene expression may occur and in turn, yield research beneficial for the treatment and prevention of syphilis, outside of a host. [40] However, continuous efforts to grow T. pallidum in axenic culture have been unsuccessful, indicating that it does not satisfy Koch's postulates. [41] The challenge likely stems from the organism's strong adaptation to residing in mammalian tissue, resulting in a reduced genome and significant impairments in metabolic and biosynthetic functions. [39]
The genome of T. pallidum was first sequenced in 1998. [42] It is characterized by its helical, corkscrew-like shape. [43] T. pallidum is not obtainable in a pure culture, meaning that this sequencing played an important role in filling gaps of understanding regarding the microbes' functions. The DNA sequences of T. pallidum species are more than 99.7% identical, and PCR-based assays are effective at differentiating these species. [44] [45] About 92.9% of DNA was determined to be open reading frames, 55% of which had predicted biological functions. [20] T. pallidum was found to rely on its host for many molecules typically provided by biosynthetic pathways, and it is missing genes responsible for encoding key enzymes in oxidative phosphorylation and the tricarboxylic acid cycle. [46] The T. pallidum group and its reduced genome is likely the result of various adaptations, such that it no longer contains the ability to synthesize fatty acids, nucleic acids, and amino acids, instead relying on its mammalian hosts for these materials. [40] The recent sequencing of the genomes of several spirochetes permits a thorough analysis of the similarities and differences within this bacterial phylum and within the species. [47] [48] [49] The chromosomes of the T. pallidum species are small, about 1.14 Mbp. It has one of the smallest bacterial genomes and has limited metabolic capabilities, reflecting its adaptation through genome reduction to the rich environment of mammalian tissue. It conserves almost 99.8% of its small genome, and uses its constantly mutating protein TprK to avoid immune response from its host. [50] To avoid antibodies attacking it, the cell has few proteins exposed on the outer membrane sheath. [51] Its chromosome is about 1000 kilobase pairs and is circular with a 52.8% G + C average. [42] Sequencing has revealed a bundle of 12 proteins and some putative hemolysins are potential virulence factors of T. pallidum. [52] These virulence factors are thought to contribute to the bacterium's ability to evade the immune system and cause disease. [52]
The clinical features of syphilis, yaws, and bejel occur in multiple stages that affect the skin. The skin lesions observed in the early stage last for weeks or months. The skin lesions are highly infectious, and the spirochetes in the lesions are transmitted by direct contact. The lesions regress as the immune response develops against T. pallidum. The latent stage that results can last a lifetime in many cases. In a few cases, the disease exits latency and enters a tertiary phase, in which destructive lesions of skin, bone, and cartilage ensue. Unlike yaws and bejels, syphilis in its tertiary stage often affects the heart, eyes, and nervous system, as well. [13]
Treponema pallidum pallidum is a motile spirochete that is generally acquired by close sexual contact, entering the host via breaches in squamous or columnar epithelium. The organism can also be transmitted to a fetus by transplacental passage during the later stages of pregnancy, giving rise to congenital syphilis. [53] The helical structure of T. p. pallidum allows it to move in a corkscrew motion through mucous membranes or enter minuscule breaks in the skin. In women, the initial lesion is usually on the labia, the walls of the vagina, or the cervix; in men, it is on the shaft or glans of the penis. [9] It gains access to the host's blood and lymph systems through tissue and mucous membranes. In more severe cases, it may gain access to the host by infecting the skeletal bones and central nervous system of the body. [9]
The incubation period for a T. p. pallidum infection is usually around 21 days, but can range from 10 to 90 days. [54]
The causative agent of yaws is Treponema pallidum pertenue, which is transmissible by direct physical contact between infected people. [55] Yaws is not sexually transmitted, and occurs in tropical, humid environments of Africa, Pacific Islands, Asia and South America. [56] [57] Unlike syphilis, which displays vertical transmission, one strain of T. p. pertenue researched was not vertically transmissible in a guinea pig model, and yaws cannot be spread from mother to offspring. [58] [56] Yaws appears as skin lesions, usually papules, commonly on the lower extremities, but present in other areas such as the arms, trunk and hands. [59] Three stages of yaws disease have been documented: primary yaws which presents as inflamed sores on the lower body, secondary yaws which presents as a variety of skin abnormalities along with bone inflammation, and tertiary yaws, also referred to as latent yaws, which occurs when T. p. pertenue is serologically detected in the host but no clinical signs are displayed until relapse, which often occurs years later. [60] [59] Yaws is treated with antibiotics such as azithromycin and benzathine penicillin-G. [61]
Bejel is caused by Treponema pallidum endemicum and is a disease is that endemic in hot and dry climates. The transmission path has not been fully mapped, however infections are thought to be transmitted via direct contact with lesion secretions or fomites rather than by sexual transmission. [62] Bejel typically causes skin lesions, which first appear as small ulcers in the mouth, and secondary lesions that form in the oropharynx, or around the nipples of nursing women. [63] Bejel can be treated with benzathine penicillin-G. [64]
During the early 1940s, rabbit models in combination with the drug penicillin allowed for a long-term drug treatment. These experiments established the groundwork that modern scientists use for syphilis therapy. Penicillin can inhibit T. pallidum in 6–8 hours, though the cells still remain in lymph nodes and regenerate. Penicillin is not the only drug that can be used to inhibit T. pallidum; any β-lactam antibiotics or macrolides can be used. [65] The T. pallidum strain 14 has built-in resistance to some macrolides, including erythromycin and azithromycin. Resistance to macrolides in T. pallidum strain 14 is believed to derive from a single-point mutation that increased the organism's livability. [66] Many of the syphilis treatment therapies only lead to bacteriostatic results, unless larger concentrations of penicillin are used for bactericidal effects. [65] [66] Penicillin overall is the most recommended antibiotic by the Centers for Disease Control, as it shows the best results with prolonged use. It can inhibit and may even kill T. pallidum at low to high doses, with each increase in concentration being more effective. [66] The Guideline Development Group has recommended the development of a new treatment, a short course treatment that is administered orally and can cross the placental barriers in pregnant women. [67]
No vaccine for syphilis is available as of 2024, but doxycycline postexposure prophylaxis can be used to prevent infections. [68] [69] [70] The outer membrane of T. pallidum has too few surface proteins for an antibody to be effective. Efforts to develop a safe and effective syphilis vaccine have been hindered by uncertainty about the relative importance of humoral and cellular mechanisms to protective immunity, [71] and because T. pallidum outer membrane proteins have not been unambiguously identified. [72] [73] In contrast, some of the known antigens are intracellular, and antibodies are ineffective against them to clear the infection. [74] [75] In the last century, several prototypes have been developed, and while none of them provided protection from the infection, some prevented bacteria from disseminating to distal organs and promoted accelerated healing. [76]
Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum subspecies pallidum. The signs and symptoms depend on the stage it presents: primary, secondary, latent or tertiary. The primary stage classically presents with a single chancre though there may be multiple sores. In secondary syphilis, a diffuse rash occurs, which frequently involves the palms of the hands and soles of the feet. There may also be sores in the mouth or vagina. Latent syphilis has no symptoms and can last years. In tertiary syphilis, there are gummas, neurological problems, or heart symptoms. Syphilis has been known as "the great imitator" because it may cause symptoms similar to many other diseases.
Yaws is a tropical infection of the skin, bones, and joints caused by the spirochete bacterium Treponema pallidum pertenue. The disease begins with a round, hard swelling of the skin, 2 to 5 cm in diameter. The center may break open and form an ulcer. This initial skin lesion typically heals after 3–6 months. After weeks to years, joints and bones may become painful, fatigue may develop, and new skin lesions may appear. The skin of the palms of the hands and the soles of the feet may become thick and break open. The bones may become misshapen. After 5 years or more, large areas of skin may die, leaving scars.
The rapid plasma reagin test is a type of rapid diagnostic test that looks for non-specific antibodies in the blood of the patient that may indicate an infection by syphilis or related non-venereal treponematoses. It is one of several nontreponemal tests for syphilis. The term reagin means that this test does not look for antibodies against the bacterium itself, Treponema pallidum, but rather for antibodies against substances released by cells when they are damaged by T. pallidum. Traditionally, syphilis serologic testing has been performed using a nontreponemal test (NTT) such as the RPR or VDRL test, with positive results then confirmed using a specific treponemal test (TT) such as TPPA or FTA-ABS. This method is endorsed by the U.S. Centers for Disease Control and Prevention (CDC) and is the standard in many parts of the world. After screening for syphilis, a titer can be used to track the progress of the disease over time and its response to therapy.
Congenital syphilis is syphilis that occurs when a mother with untreated syphilis passes the infection to her baby during pregnancy or at birth. It may present in the fetus, infant, or later. Clinical features vary and differ between early onset, that is presentation before 2-years of age, and late onset, presentation after age 2-years. Infection in the unborn baby may present as poor growth, non-immune hydrops leading to premature birth or loss of the baby, or no signs. Affected newborns mostly initially have no clinical signs. They may be small and irritable. Characteristic features include a rash, fever, large liver and spleen, a runny and congested nose, and inflammation around bone or cartilage. There may be jaundice, large glands, pneumonia, meningitis, warty bumps on genitals, deafness or blindness. Untreated babies that survive the early phase may develop skeletal deformities including deformity of the nose, lower legs, forehead, collar bone, jaw, and cheek bone. There may be a perforated or high arched palate, and recurrent joint disease. Other late signs include linear perioral tears, intellectual disability, hydrocephalus, and juvenile general paresis. Seizures and cranial nerve palsies may first occur in both early and late phases. Eighth nerve palsy, interstitial keratitis and small notched teeth may appear individually or together; known as Hutchinson's triad.
Pinta is a human skin disease caused by infection with the spirochete Treponema carateum, which is morphologically and serologically indistinguishable from the bacterium that causes syphilis and bejel. The disease was previously known to be endemic to Mexico, Central America, and South America; it may have been eradicated since, with the latest case occurring in Brazil in 2020.
Treponema is a genus of spiral-shaped bacteria. The major treponeme species of human pathogens is Treponema pallidum, whose subspecies are responsible for diseases such as syphilis, bejel, and yaws. Treponema carateum is the cause of pinta. Treponema paraluiscuniculi is associated with syphilis in rabbits. Treponema succinifaciens has been found in the gut microbiome of traditional rural human populations.
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Bejel, or endemic syphilis, is a chronic skin and tissue disease caused by infection by the endemicum subspecies of the spirochete Treponema pallidum. Bejel is one of the "endemic treponematoses", a group that also includes yaws and pinta. Typically, endemic trepanematoses begin with localized lesions on the skin or mucous membranes. Pinta is limited to affecting the skin, whereas bejel and yaws are considered to be invasive because they can also cause disease in bone and other internal tissues.
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Neurosyphilis is the infection of the central nervous system in a patient with syphilis. In the era of modern antibiotics, the majority of neurosyphilis cases have been reported in HIV-infected patients. Meningitis is the most common neurological presentation in early syphilis. Tertiary syphilis symptoms are exclusively neurosyphilis, though neurosyphilis may occur at any stage of infection.
The fluorescent treponemal antibody absorption (FTA-ABS) test is a diagnostic test for syphilis. Using antibodies specific for the Treponema pallidum species, such tests would be assumed to be more specific than non-treponemal testing such as VDRL but have been shown repeatedly to be sensitive but not specific for the diagnosis of neurosyphilis in cerebrospinal fluid (CSF). In addition, FTA-ABS turns positive earlier and remains positive longer than VDRL. Other treponemes, such as T. pertenue, may also produce a positive FTA-ABS. The ABS suffix refers particularly to a processing step used to remove nonspecific antispirochetal antibodies present in normal serum.
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Lyme disease, or borreliosis, is caused by spirochetal bacteria from the genus Borrelia, which has 52 known species. Three species are the main causative agents of the disease in humans, while a number of others have been implicated as possibly pathogenic. Borrelia species in the species complex known to cause Lyme disease are collectively called Borrelia burgdorferisensu lato (s.l.), not to be confused with the single species Borrelia burgdorferi sensu stricto, a member of the complex, which is responsible for nearly all cases of Lyme disease in North America.
Condylomata lata is a cutaneous condition characterized by wart-like lesions on the genitals. They are generally symptoms of the secondary phase of syphilis, caused by the spirochete Treponema pallidum. Condylomata lata occurs in about one-third of secondary syphilis patients and is characterized by painless, mucosal, and warty erosions which are flat, velvety, moist and broad base in nature. They tend to develop in warm, moist sites of the genitals and perineum. These lesions hold a high accumulation of spirochetes and are highly infectious. Complete resolution of the lesions is spontaneous and occurs after a few days to many weeks, where it is either resolved completely or enters the tertiary phase, defined by a latent state.
Treponema denticola is a Gram-negative, obligate anaerobic, motile and highly proteolytic spirochete bacterium. It is one of four species of oral spirochetes to be reliably cultured, the others being Treponema pectinovorum, Treponema socranskii and Treponema vincentii. T. denticola dwells in a complex and diverse microbial community within the oral cavity and is highly specialized to survive in this environment. T. denticola is associated with the incidence and severity of human periodontal disease. Treponema denticola is one of three bacteria that form the Red Complex, the other two being Porphyromonas gingivalis and Tannerella forsythia. Together they form the major virulent pathogens that cause chronic periodontitis. Having elevated T. denticola levels in the mouth is considered one of the main etiological agents of periodontitis. T. denticola is related to the syphilis-causing obligate human pathogen, Treponema pallidum subsp. pallidum. It has also been isolated from women with bacterial vaginosis.
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Sheila Lukehart is an American physician who is Emeritus Professor of Medicine at the University of Washington. Her research covered immune responses and the pathogenesis of syphilis. In 2023, she was elected a Fellow of the American Society for Microbiology.