Which of the following is true regarding spirochetes?

• A. Motility is via flagella.
• B. Spirochetes are gram positive.
• C. They are visualized best using dark field or phase optics.
• D. All human spirochete diseases are generally transmitted human to human.

Answer: (C)

Spirochetes are tiny, spiral-shaped bacteria whose movement comes from endoflagella (axial filaments) located between the outer membrane and the body, not from external flagella. Because they are so slender and stain poorly with routine Gram methods, they’re difficult to visualize with standard bright-field microscopy. Dark-field illumination or phase-contrast optics provide the contrast needed to see their corkscrew morphology and motility in clinical specimens, making these techniques the best tools for detecting spirochetes. Also, transmission differences exist among spirochetes—some are transmitted by vectors or environmental routes rather than directly from person to person—so not all human spirochete infections are purely human-to-human. This combination of thin, elusive cells and the distinctive visualization method is why seeing spirochetes best with dark-field or phase-contrast microscopy is the correct statement.