Atypical Bacteria & Spirochetes
A high-yield Step 1 tour of the boards' favorite wall-less, intracellular, and spiral organisms — pairing each bug's identifying lab feature and classic vignette buzzword with the disease it causes and the correct drug. Covers the atypical-pneumonia group (Mycoplasma, Chlamydia pneumoniae/psittaci, Legionella, Coxiella) and the BLT spirochetes (Treponema, Borrelia, Leptospira).
Why These Bugs Are "Atypical"
Two boards-favorite families break the usual microbiology rules. Atypical bacteria cause pneumonia (and other syndromes) but won't Gram stain reliably — either they have no cell wall (Mycoplasma), stain too faintly to see (Legionella), or live obligately inside host cells (Chlamydiae, Coxiella). Clinically they produce "walking pneumonia": prominent constitutional symptoms and an ugly chest X-ray that is out of proportion to a near-normal lung exam. Whether the organism lacks a peptidoglycan wall (Mycoplasma), builds one without muramic acid (Chlamydiae), or simply hides inside host cells where β-lactams can't reach it (Legionella, Coxiella), β-lactams routinely fail, and the go-to agents become macrolides, doxycycline, or fluoroquinolones.
Spirochetes are long, thin, spiral organisms so slender that most are invisible on Gram stain; the boards test how you detect them (dark-field, serology, or Giemsa) as much as the diseases they cause. Remember the trio with BLT — Borrelia, Leptospira, Treponema. Master the identifying feature, the classic vignette buzzword, and the drug for each organism below.
- Atypical = poor/absent Gram staining + "walking pneumonia" with a worse X-ray than exam: think Mycoplasma, Chlamydia pneumoniae, Legionella, Coxiella (plus C. psittaci with bird exposure).
- Mycoplasma pneumoniae has no cell wall (cholesterol-containing membrane) → β-lactams useless; treat with a macrolide, doxycycline, or fluoroquinolone. Grows on Eaton agar ("fried-egg" colonies); cold agglutinins (IgM) may cause cold autoimmune hemolytic anemia. Classic in young adults, military recruits, dorms.
- Legionella pneumophila is a faint gram-negative rod best shown by silver stain, grown on charcoal yeast extract with cysteine + iron (BCYE), and diagnosed by urine antigen. Clues: hyponatremia, diarrhea, high fever/confusion, relative bradycardia; spread by water aerosols (cooling towers, hot tubs), no person-to-person. Pontiac fever = mild, self-limited flu-like form with no pneumonia.
- Chlamydiae are obligate intracellular "energy parasites" (cannot make their own ATP); cell wall lacks muramic acid. Two forms: Elementary body = infectious, Enters cell; Reticulate body = Replicates.
- C. trachomatis serovars: A–C = trachoma (chronic conjunctivitis → leading cause of preventable blindness worldwide), D–K = urethritis/PID + neonatal conjunctivitis & pneumonia, L1–L3 = lymphogranuloma venereum. Treat azithromycin or doxycycline.
- C. pneumoniae is a common cause of atypical ("walking") pneumonia in adolescents/adults; treat like Mycoplasma with a macrolide or doxycycline.
- Coxiella burnetii (Q fever) is the rickettsia-like outlier: NO rash, NO arthropod vector (human spread is by inhalation); breathes in hardy spore-like particles from livestock birth products/amniotic fluid. Causes atypical pneumonia, granulomatous hepatitis, and culture-negative endocarditis. Treat doxycycline.
- "College student, 2 weeks of dry hacking cough and malaise, looks better than his diffuse interstitial X-ray; elevated cold-agglutinin titer." → Mycoplasma pneumoniae → macrolide or doxycycline (never a β-lactam — no cell wall).
- "60-year-old smoker back from a cruise: high fever, confusion, watery diarrhea, and serum Na 128; positive urine antigen." → Legionella pneumophila → azithromycin or levofloxacin.
- "Pet-shop worker develops atypical pneumonia and headache after his parrots fell ill." → Chlamydia psittaci (psittacosis; formerly Chlamydophila) → doxycycline.
- "Farm/abattoir worker with atypical pneumonia, then months later culture-negative endocarditis; no rash, no tick bite." → Coxiella burnetii (Q fever) → doxycycline.
Spirochetes — How You Find Them Matters
The three tested spirochetes are the BLT: Borrelia, Leptospira, Treponema. Their detection method is itself a buzzword. Borrelia is the largest spirochete — big enough to see with aniline dyes (Giemsa or Wright stain) on a peripheral blood smear (classically in relapsing fever). Treponema pallidum and Leptospira are too thin to resolve on Gram stain, so they require dark-field microscopy or serology. Critically, T. pallidum cannot be cultured on artificial media, which is why syphilis is diagnosed by microscopy + antibody tests rather than culture. Keep the geography and vectors straight: Treponema is sexually transmitted, Borrelia (Lyme) rides the Ixodes tick, and Leptospira is acquired from water contaminated with animal urine.
- Treponema pallidum (syphilis) — cannot culture; visualize by dark-field. Screen with nontreponemal VDRL/RPR (titers track treatment response; false-positives occur). Confirm with treponemal FTA-ABS / TP-PA (turns positive earliest and stays positive for life).
- Syphilis stages: 1° = single painless, indurated chancre; 2° = diffuse rash on palms & soles, condylomata lata, systemic symptoms ("the great imitator"); 3° = gummas, thoracic (ascending) aortic aneurysm (endarteritis of the vasa vasorum), and neurosyphilis — tabes dorsalis and Argyll Robertson pupil (accommodates but does not react to light).
- Congenital syphilis: saddle nose, Hutchinson (notched) teeth, mulberry molars, saber shins, sensorineural deafness, snuffles.
- Penicillin G treats every stage; treatment can trigger the Jarisch–Herxheimer reaction (acute fever, chills, myalgia hours after the first dose from spirochete lysis).
- Borrelia burgdorferi (Lyme) — Ixodes tick (also carries Babesia and Anaplasma); white-footed mouse reservoir, deer sustain the tick; Northeast/upper-Midwest US. Course: erythema migrans (bull's-eye) → early disseminated bilateral Bell's palsy, AV block/carditis, meningitis → late monoarthritis of large joints (knee). Doxycycline early; ceftriaxone for cardiac/neurologic disease.
- Leptospira interrogans — question-mark-shaped; from water contaminated with animal urine (surfers, tropics/Hawaii, sewer workers). Flu-like illness with conjunctival suffusion and calf myalgias; severe Weil disease = jaundice + acute renal failure + hemorrhage. Treat penicillin G or doxycycline.
- "Sexually active man with a single painless, indurated genital ulcer with a clean base and nontender inguinal nodes." → primary syphilis (Treponema pallidum) → penicillin G.
- "Weeks later: maculopapular rash on the palms and soles plus moist, wart-like perineal lesions (condylomata lata)." → secondary syphilis → penicillin G (watch for Jarisch–Herxheimer).
- "Hiker in Connecticut with an expanding red ring with central clearing on the thigh, then bilateral facial droop and a prolonged PR interval." → Lyme disease (Borrelia burgdorferi) → doxycycline early; ceftriaxone for carditis/neuro.
- "Triathlete who swam in a tropical lake: fever, severe calf myalgias, red conjunctivae without discharge, then jaundice and acute kidney injury." → Leptospira interrogans (Weil disease) → penicillin G or doxycycline.
- BLT — the spirochetes: Borrelia, Leptospira, Treponema. Only Borrelia is Big enough to see on light microscopy (Giemsa/Wright); the others need dark-field.
- "FAKE a Key Lyme pie" — Lyme disease features: Facial nerve (Bell's) palsy, Arthritis, Kardiac block (AV block), Erythema migrans.
- Chlamydia life cycle: Elementary body Enters and is Enfectious; Reticulate body Replicates.
- VDRL false-positives: Viruses (mono, hepatitis) · Drugs / pregnancy · Rheumatic fever / Rheumatoid arthritis · Lupus / Leprosy.
- Mycoplasma one-liner: walking pneumonia + cold agglutinins + no cell wall → no β-lactams.
Master Comparison
| Organism | Key identifying feature | Disease / buzzword | Treatment |
|---|---|---|---|
| Mycoplasma pneumoniae | No cell wall (sterol membrane), not on Gram stain; Eaton agar "fried-egg"; cold agglutinins (IgM) | Walking pneumonia, young adults/military; erythema multiforme | Macrolide or doxycycline (NOT β-lactams) |
| Legionella pneumophila | Faint GNR, silver stain; BCYE agar (cysteine + iron); urine antigen | Legionnaires' pneumonia + diarrhea, hyponatremia, confusion; water aerosols | Macrolide (azithromycin) or fluoroquinolone |
| Chlamydia trachomatis | Obligate intracellular, no muramic acid; EB (infectious)/RB (replicates); inclusions | Urethritis/PID, neonatal conjunctivitis & pneumonia, trachoma (A–C), LGV (L1–L3) | Azithromycin or doxycycline |
| Chlamydia pneumoniae / C. psittaci | Obligate intracellular | Atypical pneumonia; C. psittaci after bird/parrot exposure (psittacosis) | Doxycycline (or macrolide) |
| Coxiella burnetii | Obligate intracellular; no rash, no vector; inhaled hardy spore-like form | Q fever: pneumonia, granulomatous hepatitis, culture-negative endocarditis; livestock | Doxycycline |
| Treponema pallidum | Can't culture; dark-field; VDRL/RPR (screen), FTA-ABS/TP-PA (confirm) | Syphilis: painless chancre → palms/soles rash + condylomata lata → gummas/tabes/aortitis | Penicillin G (watch Jarisch–Herxheimer) |
| Borrelia burgdorferi | Largest spirochete, visible on Giemsa/Wright; Ixodes tick, mouse reservoir | Lyme: erythema migrans → Bell's palsy/AV block → monoarthritis | Doxycycline (early); ceftriaxone (cardiac/neuro) |
| Leptospira interrogans | Question-mark shape; dark-field; water + animal urine | Leptospirosis: conjunctival suffusion, calf myalgia; Weil disease (jaundice + renal failure); surfers/tropics | Penicillin G or doxycycline |
Practice Microbiology now
Board-style questions, spaced-repetition flashcards, and a Socratic AI tutor — free to start.