What are the two main staining categories used to classify bacteria?

• A. Gram-positive and Gram-negative
• B. Gram-Positive and Gram-Color
• C. Positive Gram and Negative Gram
• D. Gram-Blue and Gram-Red

Answer: (A)

Gram staining classifies bacteria based on cell wall structure and how they interact with the dye, which leads to two distinct appearances under the microscope. Bacteria with a thick peptidoglycan layer retain the crystal violet-iodine complex and stay purple; these are Gram-positive. Bacteria with a thinner peptidoglycan layer and an outer membrane do not retain the dye after decolorization and take up the counterstain, appearing pink or red; these are Gram-negative. The two main categories, therefore, are Gram-positive and Gram-negative. The other options mix up colors or use nonstandard terms, which don’t reflect the structural basis of the classification.