Which type of staining method is used to differentiate between Gram-negative and Gram-positive bacteria?

The Gram stain is a differential staining technique that categorizes bacteria into two groups based on their cell wall composition. When performing a Gram stain, crystal violet is first applied to the bacterial sample, followed by iodine, which forms a complex with the dye. After washing with alcohol or acetone, Gram-positive bacteria retain the crystal violet dye because of their thick peptidoglycan layer, which traps the dye within its structure. This retention causes them to appear purple under a microscope.

In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, which does not hold the crystal violet during the decolorization step. Instead, they take up the counterstain (usually safranin) and appear pink or red. This distinction not only aids in identifying the bacteria but also has implications for treatment, as Gram-negative bacteria are often more resistant to antibiotics than Gram-positive bacteria due to their outer membrane. The Gram stain is fundamental in microbiology for bacterial classification, making it a critical tool for diagnosis and treatment decisions.