The helicoidal, flexible organism seen in a blood smear with length of about 12 μm and semi-circular hooked ends corresponds most closely to which genus?

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

Answer: (C)

Recognizing spirochete shapes in a blood smear relies on noting a long, slender, helically coiled organism and paying attention to its end morphology. The described organism is about 12 μm in length and has semi-circular hooked ends. This combination—a long, flexible spirochete with hooked ends—is most characteristic of Leptospira. Leptospira are classic long, tight spirals whose ends often appear with a hooked or curved shape, and they can be seen in blood during acute leptospirosis, best visualized with dark-field microscopy or special stains rather than routine Gram stains.

Borrelia also are spirochetes, but they tend to vary more in length and aren’t typically described by hooked ends in this way. Treponema are much thinner and generally shorter filaments that don’t present with prominent hooked ends. Leptonema is a less common genus and not the best fit for these described features. So the genus that best matches a helicoidal, flexible organism about 12 μm long with semi-circular hooked ends is Leptospira.