Bacteria that stain purple with a Gram stain are referred to as?

• A. Neutral stain
• B. Acid-fast
• C. Gram negative
• D. Gram positive

Answer: (D)

In Gram staining, bacteria that appear purple are classified as Gram positive. This happens because their cell walls have a thick peptidoglycan layer that traps the crystal violet-iodine complex and holds onto the dye during the decolorization step. The result is a purple appearance under the microscope. Gram negative bacteria, with their thinner peptidoglycan layer and outer membrane, lose the purple dye during decolorization and take up the counterstain, appearing pink. Acid-fast staining is a different method used for certain bacteria, and a neutral stain isn’t the label used in this context. So the purple-staining bacteria are Gram positive.