Which type of bacteria retains the primary stain during gram staining?

The classification of bacteria into Gram-positive and Gram-negative is a fundamental concept in microbiology, which hinges on the structure of their cell walls. Gram-positive bacteria are characterized by a thick peptidoglycan layer in their cell wall. During the Gram staining process, the primary stain, crystal violet, penetrates this thick layer effectively and gets retained even after the subsequent steps involving decolorization and counterstaining.

When the Gram staining procedure is applied, Gram-positive bacteria remain violet in color because the crystal violet is not washed out due to the integrity and density of their peptidoglycan layer. In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer surrounded by an outer membrane that impedes the retention of the primary stain; they lose the crystal violet during the decolorization phase and take up the counterstain, appearing pink.

Other types of bacteria mentioned, like acid-fast bacteria, require special staining techniques due to their unique mycolic acid-rich cell walls, and non-staining bacteria might not retain any stain at all due to structural factors. Thus, Gram-positive bacteria are distinctive in their ability to retain the primary stain during Gram staining, making them crucial for identification and classification in microbiological studies.