A helicoidal, flexible organism observed in a blood smear corresponds most closely to which genus?

• A. Borrelia
• B. Leptospira
• C. Leptonema
• D. Treponema

Answer: (B)

Recognizing spirochete genera on a blood smear is what this item tests. Leptospira are slender, highly flexible spirochetes that appear as long, helically coiled filaments during the leptospiremic phase of infection. Seeing a thin, corkscrew-like organism in blood fits Leptospira best, especially in acute leptospirosis when the bacteria circulate in the bloodstream. Other spirochetes—Borrelia tend to be larger and are associated with relapsing fever or Lyme disease, while Treponema is also slender but is more often detected in lesions or via dark-field microscopy rather than routine blood smears; Leptonema is less commonly encountered clinically. The combination of slender, flexible helices in blood strongly points to Leptospira.