What kind of stain is used to differentiate between Gram-positive and Gram-negative bacteria?

The Gram stain is a fundamental technique used to differentiate between Gram-positive and Gram-negative bacteria based on the structural differences in their cell walls. This staining method involves a series of steps that apply different reagents, which lead to distinctive color changes in the bacterial cells.

In the Gram stain procedure, crystal violet is applied first, which stains all cells purple. A mordant, usually iodine, is added to enhance the color retention. Following this, a decolorizer, often alcohol or acetone, is used, which has a significant effect on the two types of bacteria: Gram-positive bacteria retain the crystal violet stain due to their thick peptidoglycan layer, while Gram-negative bacteria lose the purple color as their thinner peptidoglycan layer cannot retain the dye after decolorization. Finally, a counterstain, such as safranin, is applied, which stains the now colorless Gram-negative bacteria pink.

This differential staining ability makes the Gram stain a crucial tool in microbiology for identifying bacterial species and informing treatment strategies, especially since the structural differences in their cell walls correspond to distinct responses to antibiotics and other treatments. Other stains, like the acid-fast stain or simple stain, do not provide this level of differentiation between bacterial types