## Morphology and Staining of Leptospira ### Correct Characteristics **Key Point:** Leptospira are thin, tightly coiled gram-negative spirochaetes measuring 6–20 μm in length and 0.1–0.2 μm in diameter. **High-Yield:** The organisms are notoriously difficult to visualize with routine staining methods because of their small diameter and tight helical coils. ### Visualization Methods | Method | Utility | Notes | |--------|---------|-------| | Dark-field microscopy | Gold standard for live organisms | Shows characteristic hooked or bent ends ("question mark" appearance) | | Silver staining (Levaditi, Warthin-Starry) | Best for tissue sections | Demonstrates organisms in kidney, liver, brain | | Gram stain | Poor visualization | Too thin to be reliably seen; appears gram-negative if visible | | Immunofluorescence | Sensitive and specific | Used in reference laboratories | ### Structural Features **Key Point:** Leptospira possess: - Two axial filaments (endoflagella) located in the periplasmic space between the cell wall and outer membrane - These axial filaments enable the characteristic motility and spiral morphology - Hooked or bent ends at one or both poles ("question mark" or "lasso" appearance) **Clinical Pearl:** The tight coiling and small diameter make Leptospira invisible on routine Gram stain — this is a classic exam trap. Never confuse them with other spirochaetes like Treponema or Borrelia, which are also gram-negative but larger and more easily visualized. ### Why Option 4 Is Wrong Leptospira are **NOT** gram-positive cocci. They are gram-negative spirochaetes. This option combines two false statements: (1) they are not cocci (they are spiral), and (2) they do not stain well with Gram stain due to their small diameter. This is the deliberate distractor that contradicts all established microbiological knowledge. [cite:Park 26e Ch 26] [cite:Mandell, Douglas, Bennett's Principles and Practice of Infectious Diseases Ch 242]
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