Gram Staining Order: Step-by-Step Procedure

Gram staining is a differential staining technique used to classify bacteria based on their cell wall composition. The correct sequence ensures accurate differentiation between Gram-positive and Gram-negative bacteria.

Key Gram Staining Steps

The essential steps, in order, are:

1. Heat Fixation (C): This step involves passing the slide briefly through a flame to adhere the bacterial smear to the glass surface. It prevents the sample from washing off during staining.
2. Crystal Violet - Iodine Complex (B):
   • First, Crystal Violet (primary stain) is applied to stain all bacterial cells purple.
   • Next, Iodine (mordant) is added. It reacts with Crystal Violet to form large complexes (CV-I complexes) within the bacterial cells. This complex traps more effectively in the peptidoglycan layer of Gram-positive cells.
3. Decolorizing Agent (D): An alcohol or acetone solution is applied. This solvent dissolves the lipid-rich outer membrane of Gram-negative bacteria and disrupts the thin peptidoglycan layer, washing the CV-I complex out. Gram-positive bacteria, with their thick peptidoglycan layer, retain the CV-I complex and remain purple.
4. Counterstaining (A): A contrasting stain, typically Safranin (red/pink), is applied. It stains the decolorized Gram-negative cells pink/red. Gram-positive cells remain purple due to the retained CV-I complex.

Correct Sequence

Following these steps leads to the correct order:

Heat Fixation (C) → Crystal Violet - Iodine complex formation (B) → Decolorizing Agent (D) → Counterstaining (A)

Therefore, the correct order is C, B, D, A.