Which of the following stains classify bacteria into gram-positive and gram-negative?

The Gram stain is a fundamental laboratory technique used to classify bacteria based on the composition of their cell walls. In the Gram staining procedure, bacterial cells are treated with a series of dyes that react differently depending on the structure of the cell wall.

Gram-positive bacteria retain the crystal violet dye due to their thick peptidoglycan layer, appearing purple under a microscope. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that does not retain the crystal violet. Instead, they take up the counterstain, usually safranin, which makes them appear pink.

This distinction is crucial in microbiology, as it informs treatment options, understanding the bacteria's role in infection, and identification procedures. The other staining techniques mentioned do not serve the same classification purpose: acid-fast staining is primarily for identifying mycobacteria, endospore staining highlights the presence of spores, and negative staining is used for visualizing the shape and arrangement of cells without staining them directly.