Treponema pallidum

Treponema pallidum, formerly known as Spirochaeta pallida, is a microaerophilic spirochaete bacterium with subspecies that cause the diseases syphilis, bejel (also known as endemic syphilis), and yaws. It is known to be transmitted only among humans and baboons. 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. 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 associated disease.

Subspecies
Three subspecies of T. pallidum are known:
 * Treponema pallidum pallidum, which causes syphilis
 * T. p. endemicum, which causes bejel or endemic syphilis
 * T. p. pertenue, which causes yaws

The three subspecies causing yaws, bejel, and syphilis are morphologically and serologically indistinguishable. 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. Disease transmittance in subspecies T. p. endemicum and T. p. pertenue is considered non-venereal. 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.

Physiology
Treponema pallidum is a helically shaped bacterium with high motility consisting of an outer membrane, peptidoglycan layer, inner membrane, protoplasmic cylinder, and periplasmic space. 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. It has an endoflagellum (periplasmic flagellum) consisting of four main polypeptides, a core structure, and a sheath. The flagellum is located within the periplasmic space and wraps around the protoplasmic cylinder. 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. The outer membrane's treponemal ligands' main function is attachment to host cells, with functional and antigenic relatedness between ligands. The genus Treponema has ribbons of cytoskeletal cytoplasmic filaments that run the length of the cell just underneath the cytoplasmic membrane. They are composed of the intermediate filament-like protein cytoplasmic filament protein A (CfpA). Although the filaments may be involved in chromosome structure and segregation or cell division, their precise function is unknown.

Outer 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. 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. Treponema's reputation as a "stealth pathogen" is primarily due to this unique OM structure, which serves to evade immune detection.

TP0326
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

TP0965
TP0965 is a protein that is critical for membrane fusion in T. pallidum, and is located in the periplasm. TP0965 causes endothelial barrier dysfunction, a hallmark of late-stage pathogenesis of syphilis. 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.

TP0453
TP0453 is a 287 amino acid protein associated with the inner membrane of the microbe's outer membrane. This protein lacks the extensive beta sheet structure that is characteristic of other membrane proteins, and does not traverse the outer membrane. This protein's function has been hypothesized to be involved with control of nutrient uptake.

TP0624
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." 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.

Treponema repeat family of proteins
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. 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.

Antigen variation in TprK is regulated by gene conversion. In this way, fragments of the seven variable regions (V1–V7) present in TprK and the 53 donor sites of TprD can be combined to produce new structured sequences. 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.

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.

Culture
In the past century since its initial discovery, culturing the bacteria in vitro has been difficult. Without the ability to grow and maintain the bacteria in a laboratory setting, discoveries regarding its metabolism and antimicrobial sensitivity were greatly impaired. { However, successful long-term cultivation of T. pallidum in vitro was reported in 2017. This was achieved using Sf1Ep epithelial cells from rabbits, which were a necessary condition for the continued multiplication and survival of the system. The medium TpCM-2 was used, an alteration of more simple media which previously only yielded a few weeks of culture growth. This success was the result of switching out minimal essential medium (MEM) with CMRL 1066, a complex tissue culture medium. 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. However, continuous efforts to grow T. pallidum in axenic culture have been unsuccessful, indicating that it does not satisfy Koch's postulates. 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.

Genome
The chromosomes of the T. pallidum species are small, about 1.14 Mbp. Their DNA sequences are more than 99.7% identical. About 92.9% of DNA was determined to be open reading frames, 55% of which had predicted biological functions. The genome ofT. pallidum was first sequenced in 1998. 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. 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. 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. 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. T. pallidum 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. T. pallidum is characterized by its helical, corkscrew-like shape. To avoid antibodies attacking it, the cell has few proteins exposed on the outer membrane sheath. Its chromosome is about 1000 kilobase pairs and is circular with a 52.8% G + C average. Sequencing has revealed a bundle of 12 proteins and some putative hemolysins are potential virulence factors of T. pallidum. These virulence factors are thought to contribute to the bacterium's ability to evade the immune system and cause disease.

Clinical significance
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.

Syphilis
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. 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. 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.

The incubation period for a T. p. pallidum infection is usually around 21 days, but can range from 10 to 90 days.

Laboratory identification
Treponema pallidum was first microscopically identified in syphilitic chancres by Fritz Schaudinn and Erich Hoffmann at the Charité in Berlin in 1905. 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.

Treatment
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. 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. Many of the syphilis treatment therapies only lead to bacteriostatic results, unless larger concentrations of penicillin are used for bactericidal effects. 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.

Vaccine
No vaccine for syphilis is available as of 2024. 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, and because T. pallidum outer membrane proteins have not been unambiguously identified. In contrast, some of the known antigens are intracellular, and antibodies are ineffective against them to clear the infection. 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.