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The classification of bacteria into gram-positive and gram-negative is achieved through the use of the Gram stain. This staining technique was developed by Hans Christian Gram in the 1880s and is fundamental in microbiology for identifying and differentiating bacterial species based on the structure of their cell walls.

Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain used in the procedure, resulting in a purple appearance under the microscope. In contrast, gram-negative bacteria possess a thinner peptidoglycan layer surrounded by an outer membrane. During the Gram staining process, the crystal violet is washed out, and these bacteria are counterstained with safranin, appearing pink.

While crystal violet is a component of the Gram staining process, it does not classify bacteria by itself; it serves merely as the primary stain. The broader classification of bacteria into the gram-positive or gram-negative categories relies on the entire method of Gram staining, making the Gram stain the correct and comprehensive answer.