What is the Gram stain?

• A. A staining technique used to distinguish bacteria based on their cell walls
• B. A method to count bacteria
• C. A test to determine antibiotic susceptibility
• D. A DNA sequencing technique

Answer: (A)

Gram staining is a differential staining technique that uses crystal violet, iodine, alcohol, and a counterstain to separate bacteria by their cell wall structure. The thick peptidoglycan layer in some bacteria traps the dye and remains purple, marking them as Gram-positive, while bacteria with a thinner peptidoglycan layer plus an outer membrane do not retain the dye after decolorization and take up the counterstain, appearing pink or red as Gram-negative. This quick visual categorization helps clinicians identify the type of bacteria present and guide initial treatment decisions. It isn’t used for counting bacteria, assessing antibiotic susceptibility, or sequencing DNA.