DAT High Yield Biology Practice Test

The distinguishing feature of Gram-positive bacteria in Gram staining is the thick peptidoglycan layer present in their cell walls. This thick layer retains the crystal violet stain used in the Gram staining process, causing gram-positive bacteria to appear purple under a microscope after the staining procedure.

In contrast, gram-negative bacteria have a much thinner peptidoglycan layer and possess an additional outer membrane. During the staining process, they do not retain the crystal violet stain due to this thinner layer and subsequently take up the counterstain (usually safranin), appearing pink.

The presence of endotoxins is associated with gram-negative bacteria, not gram-positive bacteria. Moreover, a color change occurs in both types, but it is specifically the retention of the crystal violet that is characteristic of gram-positive bacteria. Therefore, option A accurately captures the key distinguishing feature that enables the differentiation of gram-positive bacteria from gram-negative bacteria in the Gram stain procedure.