Spirochaetes — Treponema, Leptospira & Borrelia
Microbiology · Bacteriology · lean revision notes
Spirochaetes — Treponema, Leptospira & Borrelia
Spirochaetes are long, thin, helically coiled, motile Gram-negative bacteria with axial filaments (endoflagella/periplasmic flagella) that produce a characteristic corkscrew/flexion-extension motility. Three medically important genera dominate NEET PG: Treponema (syphilis), Leptospira (Weil's disease), and Borrelia (Lyme disease, relapsing fever). They share the trait of being too thin to be seen on routine Gram stain and are best demonstrated by dark-field microscopy, silver stains, or immunofluorescence.
Classification & general features
| Feature | Treponema | Leptospira | Borrelia |
|---|---|---|---|
| Length / coils | 6–15 µm, regular tight coils | 6–20 µm, fine coils with hooked ends (question-mark "?" shape) | 10–30 µm, few large open irregular coils |
| Endoflagella | 3–4 per cell | 2 (one per end) | 7–20 |
| Stain | Silver (Fontana), not Gram | Silver, not Gram | Stains with Giemsa/aniline dyes (only spirochaete seen on light microscopy of blood) |
| Culture | Not cultivable in vitro (T. pallidum) | Aerobic; EMJH / Fletcher's / Korthof's media | Microaerophilic; BSK (Barbour-Stoenner-Kelly) medium |
| Key disease | Syphilis | Leptospirosis (Weil's) | Lyme disease, relapsing fever |
| Vector/source | Sexual contact (no vector) | Rodent urine (water) | Tick (Lyme/hard tick), Louse/soft tick (relapsing) |
High-yield: Borrelia is the only spirochaete large enough and stainable enough to be reliably seen on a Giemsa-stained peripheral blood smear. Treponema and Leptospira require dark-field or special methods.
Mnemonic for genera coil pattern → "Treponema Tight, Leptospira Loops with hooks, Borrelia Big/Few coils."
1. Treponema — Syphilis
Etiology & transmission
Treponema pallidum subsp. pallidum causes venereal syphilis. It is an obligate human pathogen, microaerophilic, non-cultivable on artificial media (maintained only in rabbit testes). Other treponemes cause non-venereal endemic treponematoses: Yaws (T. pallidum subsp. pertenue), Pinta (T. carateum), Bejel/endemic syphilis (subsp. endemicum).
Transmission: sexual contact, transplacental (congenital), blood transfusion. Incubation ≈ 9–90 days (average 3 weeks).
Stages & clinical features
| Stage | Timing | Lesion / features | Infectivity |
|---|---|---|---|
| Primary | 3 wk | Hard chancre (painless, indurated, clean ulcer) at inoculation site; non-tender regional lymphadenopathy | High |
| Secondary | 6 wk–6 mo | Maculopapular rash incl. palms & soles, condylomata lata, mucous patches, "snail-track" ulcers, generalized lymphadenopathy, alopecia | Highest (spirochaetes in lesions) |
| Latent | After secondary | Asymptomatic; early latent (<1 yr) infectious, late latent (>1 yr) non-infectious; serology positive | Variable |
| Tertiary | 3–30 yr | Gumma (granuloma), cardiovascular (aortitis, aneurysm of ascending aorta), neurosyphilis (tabes dorsalis, general paresis, Argyll-Robertson pupil) | Low |
High-yield: Argyll-Robertson pupil ("Prostitute's pupil" — accommodates but does not react to light) is classic for neurosyphilis (tabes dorsalis). A painless ("hard") chancre distinguishes syphilis from the painful soft chancre of chancroid (Haemophilus ducreyi).
Congenital syphilis
Transmission after the 4th month of gestation (when cytotrophoblast atrophies). Features:
- Early (<2 yr): snuffles (rhinitis, infectious), hepatosplenomegaly, maculopapular rash, osteochondritis, "saddle nose," pseudoparalysis of Parrot.
- Late stigmata: Hutchinson's teeth (notched, peg incisors), mulberry molars, interstitial keratitis, 8th nerve deafness (the Hutchinson's triad), saber shins, Clutton's joints, rhagades, frontal bossing.
High-yield: Hutchinson's triad = interstitial keratitis + 8th nerve deafness + Hutchinson's teeth. Highly tested as a triad name.
Diagnosis & investigation of choice
Direct demonstration: Dark-ground microscopy of chancre/lesion exudate (primary/secondary) shows motile spirochaetes — investigation of choice in early primary syphilis (before serology turns positive). DFA-TP (direct fluorescent antibody) is more specific.
Serology — two classes:
| Type | Tests | Antigen | Use | Notes |
|---|---|---|---|---|
| Non-treponemal (reaginic) | VDRL, RPR | Cardiolipin–lecithin–cholesterol | Screening + monitoring treatment (titres fall with cure) | Cheap; becomes negative after treatment; biological false positives |
| Treponemal (specific) | FTA-ABS, TPHA/TPPA, TP-EIA | T. pallidum antigen | Confirmatory | Remain positive for life; not for monitoring |
Stepwise flow (traditional algorithm): Clinical suspicion → VDRL/RPR screen → if reactive → confirm with FTA-ABS / TPHA → titre VDRL for follow-up.
- FTA-ABS is the earliest to become positive and the most sensitive in primary syphilis.
- VDRL on CSF is the test for neurosyphilis (specific but not very sensitive).
High-yield: Biological false-positive VDRL occurs in pregnancy, SLE/antiphospholipid syndrome, leprosy, malaria, infectious mononucleosis, viral infections. A prozone phenomenon (false-negative VDRL due to antibody excess in secondary syphilis) is corrected by diluting the serum.
High-yield: Jarisch-Herxheimer reaction — fever, chills, myalgia, hypotension within hours of starting penicillin, due to release of treponemal endotoxin-like products. Self-limiting; manage with antipyretics, NOT a penicillin allergy.
Management / drug of choice
- Drug of choice = Penicillin (no documented resistance).
- Early (primary/secondary/early latent): Benzathine penicillin G 2.4 MU IM single dose.
- Late latent/tertiary (non-neuro): 2.4 MU IM weekly × 3.
- Neurosyphilis: aqueous crystalline penicillin G IV 18–24 MU/day × 10–14 days.
- Penicillin allergy: doxycycline or ceftriaxone (NOT in pregnancy/neuro — desensitize and give penicillin instead).
2. Leptospira — Leptospirosis / Weil's disease
Etiology & epidemiology
Leptospira interrogans (pathogenic) with >200 serovars. Classic severe serovar: L. icterohaemorrhagiae (rat reservoir) → Weil's disease. Aerobic, tightly coiled with hooked ends ("?" shape), cultured on EMJH or Fletcher's medium (slow, weeks).
Transmission: contact of skin/mucosa with water/soil contaminated by rodent urine; occupational/recreational (farmers, sewage workers, fishermen, monsoon flooding, triathletes). Incubation ≈ 1–2 weeks. Endemic in India, especially post-monsoon Kerala / coastal regions.
Pathophysiology & biphasic illness
Leptospires penetrate intact mucosa/abraded skin → leptospiraemia → disseminate → vasculitis/endothelial damage. Illness is classically biphasic:
Phase 1 (Septicaemic/leptospiraemic, days 4–7): abrupt high fever, severe myalgia (especially calf), headache, conjunctival suffusion (suffusion without exudate is near-pathognomonic). Organism in blood/CSF.
Phase 2 (Immune phase): antibody appears, organism cleared from blood but excreted in urine (leptospiruria); aseptic meningitis, uveitis, and in severe cases Weil's syndrome.
Weil's disease (severe icteric leptospirosis): triad of jaundice + acute kidney injury + bleeding (haemorrhage), often with myocarditis and pulmonary haemorrhage. Jaundice is conjugated (cholestatic) with relatively preserved transaminases (helps distinguish from viral hepatitis where ALT/AST are very high).
High-yield: Conjunctival suffusion + severe calf myalgia + AKI with disproportionately high bilirubin but only mildly raised transaminases = leptospirosis. AKI is often non-oliguric with hypokalaemia (proximal tubular dysfunction).
Diagnosis & investigation of choice
- First week: organism in blood/CSF → blood culture (EMJH), dark-field microscopy (low sensitivity), PCR (best early).
- After ~1 week: serology. MAT (Microscopic Agglutination Test) is the GOLD STANDARD / reference serological test (serovar-specific, needs live antigen). A fourfold rise in paired sera is diagnostic.
- Rapid: IgM ELISA (screening, becomes positive ~5th day), lateral-flow / latex agglutination.
- Urine culture/PCR positive after the 2nd week.
High-yield: MAT = gold-standard serology for leptospirosis (frequently asked). PCR is best in the early leptospiraemic phase before antibodies appear.
Management / drug of choice
- Mild: oral doxycycline (also used for chemoprophylaxis 200 mg weekly).
- Severe (Weil's): IV penicillin G (drug of choice) or ceftriaxone; supportive dialysis for AKI.
- Jarisch-Herxheimer reaction may also occur.
3. Borrelia — Lyme disease & relapsing fever
A. Lyme disease — Borrelia burgdorferi
Vector: Ixodes (hard) tick (I. scapularis/dammini in US, I. ricinus in Europe); reservoir = deer/mice. The largest spirochaete; microaerophilic, cultured on BSK medium.
Stages:
- Early localized: Erythema chronicum migrans (ECM) — expanding "bull's-eye / target" rash at bite site (pathognomonic, days–weeks). ✔ enough for clinical diagnosis in endemic area.
- Early disseminated: multiple ECM, facial (Bell's) palsy (can be bilateral), carditis with AV block, meningitis.
- Late: Lyme arthritis (large joints, esp. knee), acrodermatitis chronica atrophicans, chronic neuroborreliosis.
Diagnosis — two-tier serology flow: ELISA screen → if positive/equivocal, confirm with Western blot (IgM/IgG). ECM in an endemic patient needs no testing — treat empirically.
High-yield: Erythema chronicum migrans (target rash) + Ixodes tick exposure = Lyme disease. Two-tier testing = ELISA then Western blot.
Treatment: Doxycycline (DOC) for early disease (also amoxicillin/cefuroxime in children/pregnancy); IV ceftriaxone for neuroborreliosis/carditis/arthritis.
B. Relapsing fever
Caused by Borrelia recurrentis and others. Recurrent febrile episodes due to antigenic variation of surface proteins (each relapse = new antigenic variant escaping antibodies).
| Type | Organism | Vector | Pattern |
|---|---|---|---|
| Epidemic (Louse-borne) | B. recurrentis | Human body louse (Pediculus) — crushed louse, not bite | Few relapses, more severe, epidemics |
| Endemic (Tick-borne) | B. duttonii, B. hermsii | Soft tick (Ornithodoros) | Multiple relapses, milder |
Diagnosis: Peripheral blood smear (Giemsa/Wright stain) during a febrile spike shows spirochaetes — Borrelia is the spirochaete visible on light microscopy of blood. Treatment: tetracycline/doxycycline or erythromycin (watch for marked Jarisch-Herxheimer reaction).
High-yield: Antigenic variation explains the relapsing nature. Louse-borne = B. recurrentis (epidemic, fewer relapses); tick-borne (Ornithodoros) = endemic, more relapses.
Key differentials & comparisons
Genital ulcer differentials (syphilis chancre):
| Disease | Organism | Ulcer | Lymph node |
|---|---|---|---|
| Syphilis | T. pallidum | Painless, hard, clean | Non-tender, rubbery |
| Chancroid | H. ducreyi | Painful, soft, ragged, dirty | Painful, suppurative bubo |
| LGV | C. trachomatis L1-3 | Small transient | "Groove sign," matted |
| Donovanosis | Klebsiella granulomatis | Beefy-red, painless | Pseudobubo |
Leptospirosis vs viral hepatitis: Lepto has conjunctival suffusion, calf myalgia, AKI, bilirubin out of proportion to mildly raised transaminases; viral hepatitis has very high ALT/AST.
Lyme carditis → AV block; differentiate from rheumatic/diphtheritic myocarditis by ECM/tick history.
Recently asked / exam angle
- VDRL is non-treponemal (cardiolipin antigen), becomes negative after treatment, used for screening & monitoring; FTA-ABS/TPHA are treponemal, stay positive for life. (Repeated single-best answer.)
- MAT = gold-standard serology for leptospirosis.
- Argyll-Robertson pupil and Hutchinson's triad named-association questions.
- Conjunctival suffusion + calf tenderness image/clinical-vignette → leptospirosis.
- Erythema chronicum migrans / Ixodes → Lyme disease.
- Borrelia = only spirochaete seen on Giemsa-stained blood film; relapsing fever via antigenic variation.
- Jarisch-Herxheimer reaction after penicillin in syphilis/leptospirosis.
- Prozone phenomenon = false-negative VDRL in secondary syphilis; correct by dilution.
- Drug of choice for syphilis = benzathine penicillin (neurosyphilis = aqueous crystalline penicillin G IV).
- Dark-ground microscopy = investigation of choice in primary syphilis chancre.
Rapid revision
- Spirochaetes have axial filaments (periplasmic flagella); too thin for Gram stain — use dark-field/silver stain.
- T. pallidum cannot be cultured in vitro (grown in rabbit testes only).
- Primary syphilis = painless hard chancre; secondary = rash on palms & soles + condylomata lata.
- Argyll-Robertson pupil = accommodation intact, light reflex lost → neurosyphilis.
- Hutchinson's triad = interstitial keratitis + 8th nerve deafness + notched teeth (congenital syphilis).
- VDRL/RPR = screen & monitor (cardiolipin); FTA-ABS/TPHA = confirm, positive for life.
- Prozone = false-negative VDRL; BFP-VDRL in SLE, pregnancy, leprosy, malaria.
- DOC for syphilis = penicillin; Jarisch-Herxheimer reaction may follow first dose.
- Leptospira has hooked "?" ends; cultured on EMJH/Fletcher's; spread via rat urine in water.
- Weil's disease (L. icterohaemorrhagiae) = jaundice + AKI + haemorrhage; MAT = gold-standard serology; clue = conjunctival suffusion + calf myalgia.
- Lyme disease = B. burgdorferi, Ixodes tick, erythema chronicum migrans (bull's-eye); two-tier ELISA → Western blot; DOC doxycycline.
- Relapsing fever via antigenic variation; louse-borne = B. recurrentis (epidemic), tick-borne = Ornithodoros (endemic); Borrelia visible on Giemsa blood smear.