Which fixative is considered unsatisfactory for staining spirochetes?

• A. Mercurial fixatives
• B. Formalin
• C. Ethanol
• D. Acetone

Answer: (A)

Staining spirochetes relies on fixing tissue in a way that preserves their delicate appearance while not interfering with the staining chemistry, especially silver-based methods used to visualize these organisms. Mercurial fixatives contain mercury salts that bind strongly to proteins and leave heavy metal residues, which can block silver impregnation and create high background. This makes spirochetes hard to see after staining. Therefore, mercurial fixatives are considered unsatisfactory for staining spirochetes. In contrast, fixatives like formalin, ethanol, and acetone preserve morphology and are compatible with silver stains, yielding clearer visualization of spirochetes.