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)

Bacteria that lack a cell wall behave very differently in microscopy and staining. Mycoplasma species, including Mycoplasma hominis, do not have peptidoglycan in their cell envelope, so they lack a rigid wall. Because of their tiny size and pleomorphic shape, they do not stain well with standard Gram methods and are hard to see on routine smears. Unstained, living preparations viewed with darkfield or phase-contrast microscopy let you visualize these delicate, motile forms more readily, making such organisms noticeable without staining.

The other organisms listed have cell envelopes or rely on intracellular lifestyles that make darkfield or phase-contrast visualization impractical or less informative for routine identification, so they aren’t best visualized with these methods.