Final answer:
The Gram stain is a differential stain used to classify bacteria based on their cell wall composition, differentiating between Gram-positive bacteria with thick peptidoglycan layers and Gram-negative bacteria with thinner walls.
Step-by-step explanation:
Gram stain is a type of differential stain that categorizes bacteria based on their cell wall composition. This staining technique distinguishes between Gram-positive and Gram-negative bacteria. Gram-positive bacteria, which have a thick peptidoglycan layer, retain the primary stain (crystal violet) and appear purple.
In contrast, Gram-negative bacteria have a thinner peptidoglycan wall and do not retain the crystal violet, but take up the counterstain (safranin) and appear red/pink. The result of a Gram stain allows scientists to infer not only the classification of bacteria but also suggests information about the bacteria's cell wall structure and antibiotic susceptibility.