Final answer:
The Gram stain is a differential staining technique used to classify bacteria into gram-positive or gram-negative groups, based on their cell wall composition. Crystal violet is the primary stain that gram-positive bacteria retain, while gram-negative bacteria do not, resulting in different coloring after the process.
Step-by-step explanation:
To classify bacteria into gram-positive and gram-negative groups, a special staining technique known as the Gram stain is used. The Gram stain method involves the application of a primary stain, crystal violet, which is retained by gram-positive bacteria due to their thick peptidoglycan cell walls. Conversely, gram-negative bacteria have a thin peptidoglycan layer and do not retain the crystal violet, appearing light red/pink due to a secondary counterstain (safranin). Atypical bacteria that cannot be classified by this method include species like Mycoplasma and Chlamydia due to differences in their cell wall structures.
The Gram staining procedure comprises four steps, involving crystal violet, a mordant (iodine), a decolorizing agent (alcohol), and the counterstain (safranin). The application of Gram's iodine serves to fix the crystal violet stain on the bacterial cell wall, and the role of alcohol is to decolorize, allowing differentiation between gram-positive and gram-negative cells based on their ability to retain or lose the crystal violet during the decolorization step.