Which characteristic is true about gram-positive bacteria when stained?

Gram-positive bacteria are characterized by their thick peptidoglycan layer in the cell wall, which retains the crystal violet stain used during the Gram staining process. When the staining procedure is performed, the crystal violet is initially absorbed by the cells and then a mordant, typically iodine, is applied to form a complex that is also retained by the thick peptidoglycan. During the washing step with alcohol or acetone, the thick layer prevents the stain from being washed out, resulting in these bacteria appearing purple under a microscope.

The other color outcomes mentioned in the options are associated with gram-negative bacteria, which have a thinner peptidoglycan layer and an outer membrane. After the decolorization step, gram-negative bacteria lose the crystal violet stain and take up the counterstain, safranin, appearing red or pink. Hence, the staining characteristics of gram-positive bacteria specifically lead them to appear purple, confirming their identity in microbiological analysis.