University of Central Florida (UCF) MCB3020C General Microbiology Lab Midterm Practice Exam

The counterstain in the Gram stain procedure is pink, which is indicative of the staining process used to differentiate bacterial species. In the Gram stain, two key dyes are employed: crystal violet, which is the primary stain that colors Gram-positive bacteria a deep purple due to their thick peptidoglycan layer, and safranin, which serves as the counterstain. Safranin is responsible for imparting a pink color to Gram-negative bacteria, which have a thinner peptidoglycan layer and do not retain the crystal violet during the decolorization step. Therefore, the final color of Gram-negative bacteria after the procedure is pink due to this counterstain, making the choice of pink the correct answer.