Gram staining interpretation: After proper Gram staining, what colour do Gram-negative bacteria typically appear with the counterstain?

Introduction / Context:
Correctly interpreting Gram reactions is fundamental in bacteriology. Gram-negative organisms possess a distinct cell envelope that influences dye retention during the decolorization step and determines the final observed colour.

Given Data / Assumptions:

• Standard reagents: crystal violet, iodine, alcohol/acetone, safranin.
• Procedure and timing are appropriate.

Concept / Approach:
Gram-negative bacteria have an outer membrane rich in lipopolysaccharide and a thin peptidoglycan layer. Decolorizer solubilizes the outer membrane and removes the crystal violet–iodine complex. These cells then readily take up the counterstain (safranin or fuchsine), appearing pink to red under light microscopy.

Step-by-Step Solution:
Primary stain + mordant → initial purple colour in all cells. Decolorization → Gram-negative lose the complex. Counterstain → Gram-negative become pink; Gram-positive remain purple.

Verification / Alternative check:
Use of reference strains (E. coli vs S. aureus) confirms correct colour outcomes and technique reliability.

Why Other Options Are Wrong:

• Purple: Gram-positive outcome.
• Green / Brown: Not produced by the standard Gram protocol.
• Colourless: Suggests over-decolorization or technique error.

Common Pitfalls:
Old cultures of Gram-negative bacteria can show variable staining; ensure fresh cultures and precise timing.

Final Answer:
Pink (counterstain such as safranin) is the typical colour for Gram-negative bacteria.