Which organism lacks a cell wall and is best visualized by darkfield or phase microscopy?

Which organism lacks a cell wall and is best visualized by darkfield or phase microscopy?

• A. Chlamydia trachomatis
• B. Coxiella burnetii
• C. Mycoplasma hominis
• D. Rickettsia prowazekii

Answer: (C)

The main idea tested is that organisms without a cell wall don’t take up standard Gram stains, so they’re seen best with methods that visualize living, unstained cells. Mycoplasma hominis lacks a cell wall (no peptidoglycan in its membrane), which makes it poorly stainable by Gram methods and often very small and pleomorphic. Because of this, dark-field or phase-contrast microscopy can reveal these organisms as tiny, refractile forms in specimens, even though they don’t stain well with routine stains. In contrast, the other organisms listed all have cell walls and are typically identified by other approaches (Gram staining, culture in cell lines, or serology) rather than dark-field visualization.

The main idea tested is that organisms without a cell wall don’t take up standard Gram stains, so they’re seen best with methods that visualize living, unstained cells. Mycoplasma hominis lacks a cell wall (no peptidoglycan in its membrane), which makes it poorly stainable by Gram methods and often very small and pleomorphic. Because of this, dark-field or phase-contrast microscopy can reveal these organisms as tiny, refractile forms in specimens, even though they don’t stain well with routine stains. In contrast, the other organisms listed all have cell walls and are typically identified by other approaches (Gram staining, culture in cell lines, or serology) rather than dark-field visualization.