Spirochaetes

T. pallidum is a thin, delicate spiral filament (6–14 µm × 0.2 µm) with 6–12 small, sharp, regular coils. It cannot be seen under the light microscope or stained by ordinary bacterial stains; it requires dark-ground or phase contrast microscopy, or silver impregnation methods (Fontana's for films, Levaditi's for tissue sections). Pathogenic treponemes cannot be cultured on artificial media and are maintained by animal subculture (e.g. Nichol's strain in rabbit testes); cultivable treponemes (T. phagedenis Reiter's strain, T. refringens) are non-pathogenic.

Syphilis, caused by T. pallidum, is acquired primarily by sexual contact (also blood-borne, congenital, occupational). Incubation is 10–90 days. Disease progresses through primary (hard chancre, usually genital), secondary (roseolar/papular rash, mucous patches, condylomata, appearing 1–3 months after primary lesion heals), and tertiary stages (cardiovascular lesions, gummata, meningovascular disease).

Non-treponemal tests (screening; detect reagin antibody via cardiolipin antigen): VDRL, RPR, TRUST, Wassermann reaction, Kahn test. VDRL is the most widely used, performed as a slide flocculation test and can also test CSF. RPR uses a stabilized carbon antigen readable by naked eye. A key limitation is biological false positive (BFP) reactions, seen in SLE, leprosy, malaria, relapsing fever, infectious mononucleosis, hepatitis and tropical eosinophilia.

Treponemal tests (specific, confirmatory): TPI (immobilization test), TPA (agglutination test), TPIA (immune adherence test), FTA/FTA-ABS (fluorescent treponemal antibody test), TPHA (haemagglutination test), and EIA.

All are transmitted by direct body contact in communities with poor hygiene standards, with laboratory diagnosis and treatment similar to venereal syphilis.

Borrelia are large, motile, refractile spirochetes with irregular, wide, open coils (5–30 µm × 0.3–0.7 µm), readily stained by ordinary stains, Gram negative, and transmitted by haematophagous arthropods.

B. recurrentis causes epidemic (louse-borne) relapsing fever; endemic (tick-borne) forms are caused by B. duttoni, B. hermsii, B. parkeri. Diagnosis is by direct wet film/blood film examination (dark-ground, phase contrast, Giemsa or Gram with carbol fuchsin counterstain).

B. burgdorferi causes Lyme disease, transmitted by ixodid ticks, cultured on BSK medium, presenting with erythema migrans (stage 1), disseminated infection with fever/arthralgia (stage 2), and chronic arthritis/neuropathy (stage 3). Diagnosis uses microscopy, ELISA/immunofluorescence, confirmed by immunoblot.

B. vincenti, in symbiosis with Leptotricha buccalis, causes ulcerative gingivostomatitis (Vincent's angina); diagnosed by dilute carbol fuchsin-stained smears of the ulcer.

Leptospires are actively motile, delicate spirochetes with fine, tightly coiled spirals and hooked ends like umbrella handles; not visible under light microscopy but demonstrable by silver impregnation or Giemsa stain. Genus Leptospira includes L. interrogans (pathogenic) and L. biflexa (saprophytic, found in surface water).

Leptospires are obligate aerobes, cultured on EMJH (Ellinghausen-McCullough-Johnson-Harris) or Korthof's/Fletcher's media. L. interrogans causes a zoonotic disease transmitted by contact with water contaminated by carrier animal urine, ranging from mild febrile illness to severe Weil's disease (jaundice and albuminuria).

Pathogenic treponemes lack the metabolic machinery to grow on cell-free media and are instead maintained through serial passage in susceptible animals such as rabbits (testicular inoculation).

Cardiolipin antigen used in non-treponemal tests is shared between T. pallidum and mammalian tissues, so conditions like SLE, leprosy, malaria and infectious mononucleosis can trigger a reactive but non-specific result.

Through direct or indirect contact with water, soil or food contaminated by the urine of infected animals (rodents, dogs, cattle), often through cuts, abraded skin, or mucous membranes.