The Science of Syphilis

T. pallidum has a distinctive outer membrane that is strikingly sparse in surface-exposed proteins — the molecules that the immune system would normally recognise as foreign and mount an antibody response against. This is sometimes called the "stealth pathogen" phenotype.

Most of its outer membrane proteins are lipoproteins anchored beneath the surface, facing inward rather than outward. The few proteins that are surface-exposed — the rare outer membrane proteins (ROPs) — are present in very low density. This means the immune system has remarkably few targets, and antibodies generated early in infection are directed largely at lipid antigens (the basis of the RPR/VDRL tests) rather than protein antigens that might kill the bacterium more efficiently.

The bacterium's corkscrew shape and rotational motility allow it to bore through tissue matrices and cross epithelial barriers with exceptional efficiency. Within hours of mucosal exposure, T. pallidum can disseminate haematogenously — seeding virtually every organ system, including the central nervous system.

Syphilis is defined by its staging, and each stage reflects a distinct phase of the host-pathogen relationship:

• Primary syphilis: The classic chancre (painless ulcer) is not caused by direct bacterial toxicity. It is a granulomatous immune response — macrophages and T cells accumulate at the site of inoculation, attempting to contain the infection. The chancre is teeming with spirochaetes, which is why direct contact with it is highly infectious. The chancre resolves spontaneously as local immune responses intensify, but T. pallidum has already disseminated systemically.
• Secondary syphilis: Systemic dissemination triggers a broader immune response: the classic rash (often including palms and soles), condylomata lata, mucous patches, and constitutional symptoms represent immune complex deposition and widespread inflammatory responses in multiple tissues. Spirochaete burden is highest during this stage. The immune system eventually suppresses symptoms — not by eliminating the bacteria, but by containing them.
• Latent syphilis: The spirochaetes persist at low levels, sequestered in tissues including the central nervous system, cardiovascular system, and eyes. The immune system maintains a standoff — suppressing but not eliminating the infection. This phase can last years or decades and is clinically silent, though the individual remains seropositive.
• Tertiary syphilis: In approximately 30% of untreated patients (historical data), the immune containment eventually fails. The resulting immune response to persistent treponemes — particularly T-cell-mediated delayed hypersensitivity — causes tissue-destructive granulomas called gummas, aortitis (inflammation of the aorta), and neurosyphilis. The damage in tertiary syphilis is largely immunopathological rather than a direct effect of bacterial toxins.

T. pallidum crosses the blood-brain barrier in a significant proportion of patients during early infection — not just in tertiary disease. Studies using PCR on cerebrospinal fluid have found treponemes in a substantial minority of patients with primary and secondary syphilis. Most of these early neuroinvasions resolve without sequelae when systemic treatment is given. However, when T. pallidum persists in the CNS, it can cause a spectrum from asymptomatic CSF abnormalities through to meningitis, cranial nerve palsies, stroke (meningovascular syphilis), and the classic late manifestations of general paresis and tabes dorsalis. The mechanism of neuronal damage is complex — a combination of direct spirochaetal invasion and secondary neuroinflammation driven by perivascular infiltration of macrophages and T cells.

Penicillin has been the treatment of choice for syphilis since the 1940s and, uniquely among major pathogens, T. pallidum has never acquired penicillin resistance. This is attributed to several factors. First, the bacterium cannot acquire foreign DNA through horizontal gene transfer — it lacks the machinery for transformation, conjugation, or transduction. Resistance cannot be imported. Second, T. pallidum replicates very slowly (doubling time approximately 30–33 hours, compared to 20 minutes for E. coli), which reduces the rate at which spontaneous mutations can accumulate and be selected under antibiotic pressure. Third, penicillin targets penicillin-binding proteins (PBPs) involved in cell wall synthesis, and the specific PBPs of T. pallidum have maintained their structure without the mutations that would reduce penicillin binding. This 80-year track record makes penicillin the gold standard, particularly for neurosyphilis and syphilis in pregnancy.

T. pallidum can cross the placental barrier at any stage of pregnancy, though transmission risk is highest in primary and secondary syphilis when spirochaete burden is greatest. After approximately 18 weeks of gestation, the placenta becomes more permeable, and the risk of foetal transmission rises substantially. The consequences of congenital syphilis range from stillbirth and neonatal death to a range of developmental abnormalities including saddle-nose deformity, Hutchinson's teeth, sensorineural hearing loss, and interstitial keratitis. The resurgence of syphilis globally is driving a parallel resurgence in congenital syphilis — a preventable condition that requires only antenatal serological screening and treatment.