Treponema Pallidum

Subjects: Bacteriology · Systems: Microbiology · Tags: Microbiology

The story of syphillis

Treponema pallidum is a delicate spirochete - a think, corkscrew-shaped bacterium, that moves witha twisting motion thanks to periplasmic flagella (called axial filaments). This unusual motility is not just a curiosity: it allows the organism to burrow through mucous membranes and into tissues, spreading widely very early in infection. Because it is so thin, it cannot be seen with a normal Gram stain or light microscopy, which is why dark-field microscopy, or silver staining is used. It also cannot be cultured in standard media, making serologic testing the mainstay of diagnosis.

When the bacterium first enters the body, usually through sexual contact, it slips through microscopic abrasons in the mucosa or skin. Within weeks, it produces the hallmark lesion of primary syphillis: the chancre. This lesion is a firm, painless ulcer with a clean base. It may look harmless and disappears on its own within weeks, but this is deceptive. Even while the chancre is healing, the spirochete has already spread through blood and lymphatics. Patients are highly infections during this stage. Regional lymph nodes are often enlarged, but like tha chancre, they are not tender, a feature that helps distinguish this from other genital ulcers.

If untreated, the infection progresses into secondary syphillis a few weeks later. This is when dissemination becomes obvious. The rash of secondary syphillis is classically widespread and involves the palms and soles, which is unusual and therefore memorable. Mosit, wart-like lesions called condylomata lata may form in skin folds and are packed with spiorchetes, making them very contagious. Fever, malaise, sore throuat, and generalized lymphadenopathy reflect the systemic nature of the infection. Like the chancre, these lesions resovle without treatment, leading into a quiet phase: latency.

Latency can last for years. In the early latent phase (first yera), relapses of secondary disease may occur, and patients remain infectious. Later on, patients become late latent and usually show no symptoms, though the infection still smolders in their tissues. For some, this marks the end of the story. But for about a third of untreated patients, syphillis resurfaces decades later as tertiary disease, which is devastating.

Tertiary syphillis manifests in three ways. First, there are gummas, which are granulomatous, destructive lesions that can erode skin, bone, or organs. Secon, the cardiovascular form results from endartertits of the vasa vasorum. Their obliteration weakens the wals of the ascending aorta, producing aneurysms with a characteristic “tree-bark” appearance. Finally, there is neurosyphillis, which can occur early or late. In its late form, it produces tabes dorsalis - deneration of the dorsal columns leading to sensory ataxia, lightining-like pains, and Charcot joints. Another late manifestation is general paresis, with progressive dementia and psychiatric changes. The famous Argyll Robertson pupil, which accommodates but does not react to light, is a striking clue to neurosyphillis.

Syphilis can also pass from mother to child as congenital syphilis. Early manifestations in infants include “snuffles” (a bloody nasal discharge filled with spirochetes, rash, hepatosplenomegaly, and anemia. LAter, if untreated, children may develop the classic stigmata: Hutchingson teeth (small, notched incisors), saddle nose, saber shins, and sensorineural deafness.

Since T. pallidum cannot be cultured, we rely on indirect evidence for diagnisis. The first step is usually a nontreponemal test such as VDRL or RPR, which detect antibodies against cardiolipin. These tests are sensitive and cheap but not specific - they can be positive in conditions ranging from lupus to viral infecitions. Because of that, a treponemal test is used to confirm the diagnosis. Treponema tests detect antibodies agains the actual bacterium and usually remain positive for life, even after treatment. NOntreponemal test tiers, on the other hand, fall after therapy and are therefore useful for monitoring

Treatment is simple in principle: penicillin G cures syphilis at all stages. For early disease, a single intramuscular dose suffices. For late latent or tertiary syphilis, weekly injections are given for three weeks. Neurosyphilis requires intravenous therapy for longer courses. IMportantly, in pregnant women, penicillin is the only acceptable therapy. If they are allergic, they must go desensitization, because no alternative reliably prevents congenital syphilis.

Finally, one important clinical phenomenon is the Jarisch-Herxheimer reaction, a transient fever and flu like illness that occurs within hours of starting treatment. THis is not an allergy, but rather the body’s inflammatory response to the sudden release of treponemal antigens as the organisms die. , It is self-limiting but can alarm patients if not anticipated.

Disclaimer: For education only. Not medical advice; always follow your institution's guidance.