Canadian Society for Medical Laboratory Science (CSMLS) Practice Exam

Which reagent is used as a dye trapping agent mordant in the Gram stain?

1. Lugol's iodine

2. Gram's iodine

3. Acetone alcohol

4. Crystal violet

The correct answer is: Gram's iodine

In the context of the Gram stain procedure, Gram's iodine serves as a crucial mordant. A mordant is a substance that helps to fix a dye onto a material, enhancing the binding of the dye to the cell wall of bacteria. After the application of crystal violet, which is the primary stain, Gram's iodine is used to form a complex with the dye, resulting in a more substantial interaction with the peptidoglycan layer of the bacterial cell walls. This strengthening is particularly important for distinguishing between Gram-positive and Gram-negative bacteria. In Gram-positive bacteria, the thick peptidoglycan layer retains the crystal violet-iodine complex, whereas Gram-negative bacteria, which have a thinner peptidoglycan layer, will not hold onto the dye as effectively after decolorization. Thus, Gram's iodine plays a pivotal role in the overall effectiveness of the Gram staining process, contributing to the differentiation of bacterial types based on their cell wall properties.