APIC Training Certification in Infection Prevention and Control (CIC) Practice Test

The key idea is how the dye interacts with the bacterial cell wall during Gram staining. When crystal violet is applied (with the mordant iodine), it forms a crystal violet–iodine complex that binds to the cell wall. Gram-positive bacteria have a thick peptidoglycan layer that traps this complex effectively, so they retain the dye and appear dark violet, purple, or blue under the microscope. This retained color is what makes them distinguishable from Gram-negative bacteria, which have a thinner peptidoglycan layer and an outer membrane that leads to dye loss during decolorization and eventual counterstaining. So after staining with crystal violet, Gram-positive bacteria appear the dark purple hue.