Final answer:
The Gram stain classifies bacteria into Gram-positive and Gram-negative groups, determined by the structure of their cell walls and their ability to retain certain stains, which affects their coloring under a microscope.
Step-by-step explanation:
The Gram stain procedure is a differential staining technique used to classify bacteria into two groups: Gram-positive and Gram-negative. This classification is based on the different physical and chemical properties of bacterial cell walls.
Gram-positive bacteria have thick peptidoglycan layers in their cell walls and retain the primary stain, crystal violet, making them appear purple under a microscope. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and do not retain the crystal violet after the decolorizing step; they are stained by the secondary counterstain, safranin, and appear red or pink.
Bacterial cell size, shape, motility, and arrangements are also considered when identifying bacteria using Gram staining. Distinguishing attributes like endospores formation and acid-fast characteristics may be used to further differentiate Gram-positive bacteria, as noted in Bergey's Manual and other references.