A 24-year-old man presents with a painless genital ulcer for the past 2 weeks. He reports that he recently has been having unprotected sex with multiple partners. Past medical history is unremarkable. On physical examination, a single ulcer is present on the dorsal shaft of the penis which is circumscribed, indurated, and partially healed. There is moderate inguinal lymphadenopathy but no buboes. Which of the following tests would confirm the most likely diagnosis in this patient?

Perform a darkfield microscopic examination of a swab from the chancre

Swab the chancre and perform a saline wet mount

Fluorescent treponemal antibody absorption (FTA-ABS) test

Venereal Disease Research Laboratory (VDRL) test

A 25-year-old professional surfer presents to the emergency room with leg pain and a headache. He recently returned from a surf competition in Hawaii and has been feeling unwell for several days. He regularly smokes marijuana and drinks 6-7 beers during the weekend. He is otherwise healthy and does not take any medications. His temperature is 102.2°F (39°C), blood pressure is 121/78 mmHg, pulse is 120/min, and respirations are 18/min saturating 99% on room air. He is sitting in a dim room as the lights bother his eyes and you notice scleral icterus on physical exam. Cardiopulmonary exam is unremarkable. Which of the following findings would most likely be seen in this patient?

Question mark-shaped bacteria on dark-field microscopy

Epithelial cells covered with gram-variable rods

Treponemes on dark-field microscopy

Granulocytes with morulae in the cytoplasm

Monocytes with morulae in the cytoplasm

A previously healthy 25-year-old male comes to his primary care physician with a painless solitary lesion on his penis that developed 4 days ago. He has not experienced anything like this before. He is currently sexually active with multiple partners and uses condoms inconsistently. His temperature is 37.0°C (98.7°F), pulse is 67/min, respirations are 17/min, and blood pressure is 110/70 mm Hg. Genitourinary examination shows a shallow, nontender, firm ulcer with a smooth base along the shaft of the penis. There is nontender inguinal adenopathy bilaterally. Which of the following is the most appropriate next step to confirm the diagnosis?

Urine polymerase chain reaction

Fluorescent treponemal antibody absorption test

Spirochetes overview for USMLE Step 2 CK: High-Yield Microbiology Lessons

Spirochetes - The Corkscrew Crew

Darkfield microscopy of Treponema pallidum spirochetes

Gram-negative cell wall structure, but too thin to be visualized by Gram stain.
Motile via periplasmic flagella (axial filaments), producing a characteristic corkscrew motion.
Key genera: Borrelia, Leptospira, and Treponema.

⭐ Spirochetes are best visualized using special techniques like darkfield microscopy or silver stain (e.g., Warthin-Starry).

Treponema pallidum - The Great Pretender

Microbiology: Thin, helical spirochete (0.1-0.2 µm wide); visualized by darkfield microscopy or direct fluorescent antibody (DFA) staining. Not readily culturable in vitro.
Transmission: Primarily sexual contact; vertical transmission (congenital syphilis).
Disease Stages:
- Primary: Painless chancre.
- Secondary: Maculopapular rash (palms & soles), condylomata lata.
- Tertiary: Gummas, aortitis (vasa vasorum destruction), neurosyphilis.
Congenital: Classic triad of interstitial keratitis, sensorineural hearing loss, and Hutchinson teeth.

⭐ A key feature of tertiary neurosyphilis is the Argyll Robertson pupil: pupils constrict with accommodation but not with light ("prostitute's pupil").

Darkfield microscopy of Treponema pallidum spirochetes

Borrelia - Tick-Borne Twisters

Organism: Large spirochetes visualized with aniline dyes (Wright, Giemsa); don't Gram stain well.
Transmission: Arthropod vectors.

B. burgdorferi (Lyme Disease)
- Vector: Ixodes ticks (deer tick).
- Reservoir: White-footed mouse.
- Phases: 1) Erythema migrans (bullseye rash), flu-like illness. 2) Facial (Bell's) palsy, AV block. 3) Migratory arthritis, encephalopathy.
- 📌 Mnemonic (BAKE a Key Lyme pie**):** Bell's palsy, Arthritis, Kardiac block, Erythema migrans.
B. recurrentis (Relapsing Fever)
- Vector: Louse.
- Pathogenesis: Recurrent fevers due to antigenic variation.

⭐ Erythema migrans is pathognomonic but absent in ~20% of Lyme cases. Bilateral Bell's palsy is highly suggestive of Lyme disease.

Ixodes Tick Life Cycle and Erythema Migrans Rash

Leptospira - Pond Scum Peril

Source: Water contaminated with animal urine (rodents, dogs); common in tropics.
- High-risk: Surfers, farmers, veterinarians.
Organism: Spirochete with characteristic hook-shaped ends (looks like a question mark).
Clinical: Biphasic illness.
- Leptospirosis: Flu-like illness, myalgias (calves), conjunctival suffusion (redness without pus).
- Weil's Disease: Severe icteric form with jaundice, azotemia, and hemorrhage.

⭐ Conjunctival suffusion is a classic, highly specific finding for leptospirosis.

Spirochete structure and movement

Treatment: Doxycycline or Penicillin G.

Spirochete Smackdown - Comparative Table

Feature	Treponema pallidum	Borrelia burgdorferi	Leptospira interrogans
Disease	Syphilis	Lyme Disease	Leptospirosis (Weil's)
Shape	Tight, regular coils	Large, irregular coils	Fine coils, hooked ends
Transmission	Sexual contact, vertical	Ixodes tick bite	Animal urine in water
Microscopy	Darkfield microscopy	Giemsa/Wright stain	Darkfield microscopy
Key Sign	Painless chancre	Erythema migrans	Conjunctival suffusion

⭐ Exam Favorite: Spirochetes possess internal endoflagella (axial filaments) located in the periplasmic space, enabling their characteristic corkscrew motility, which facilitates invasion of tissues.

High‑Yield Points - ⚡ Biggest Takeaways

Spirochetes are spiral-shaped bacteria with endoflagella (axial filaments) enabling their distinctive corkscrew motility.

They stain poorly with Gram stain; visualize using darkfield microscopy or silver stain.

The three major pathogenic genera are Treponema, Borrelia, and Leptospira.

Treponema pallidum causes syphilis; it cannot be cultured in vitro.

Borrelia burgdorferi, transmitted by Ixodes ticks, is the agent of Lyme disease.

Leptospirosis is a zoonosis from animal urine, potentially causing Weil's disease.

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