What is the purpose of crystal violet in a Gram stain?

The purpose of crystal violet reagent in the Gram's stain procedure is to dye the bacteria sample. Once the sample of bacteria is on a slide, crystal violet is applied and left for 10-60 seconds, depending on the sample size, and then the excess is carefully poured off.

Gram's staining is used when a sample of bacteria needs to be differentiated into two groups: gram-positive and gram-negative. It is used to detect infections in samples of bodily fluids and can be used more quickly than culturing the bacteria, when the length of time to diagnose an illness is integral to treating the patient.