Question

Which stain is used to distinguish Gram +ve bacterium from a Gram -ve type?

• A. Eosin
• B. Crystal violet
• C. Hematoxylin
• D. Carmine

Answer 1

Answer: (B)

Crystal violet

• Eosin is an acidic dye with negatively charged chromophore and stains positively charged molecules such as certain WBCs; presence of acidic substances in large quantity with negatively charged surfaces on bacterial cells inhibit their staining with eosin. Option A is incorrect.
• Haematoxylin is positive/basic dye which stains acidic substances carrying negative charge and is used with eosin in histology; option C is incorrect.
• Carmine is a natural dye produced by insect Dactylopius coccus and is used food and cosmetics; option D is incorrect.
• Gram staining includes staining of both Gram positive and Gram negative bacterial cell purple with crystal violet followed by stabilization of dye in peptidoglycan layer with mordant—Gram’s iodine and application of alcohol to dissolve outer membrane and dye from peptidoglycan layer. Presence of thick peptidoglycan layer in Gram positive cells make them retain the dye after alcohol treatment while Gram negative bacteria become colourless. Thus, option B is the correct answer.