Which type of bacteria retains crystal violet stain during the Gram staining process?

During the Gram staining process, crystal violet stain is retained by Gram-positive bacteria due to their thick peptidoglycan layer in the cell wall. This layer is capable of trapping the crystal violet-iodine complex formed during the staining procedure. As a result, when the Gram-positive bacteria are washed with alcohol or acetone, they do not lose the stain and subsequently appear purple under a microscope.

In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer and possess an outer membrane. When subjected to the decolorization step in the Gram stain protocol, the outer membrane is disrupted, and the crystal violet stain is washed away, leaving them colorless until they are counterstained, usually with safranin, which gives them a pink appearance.

Acid-fast bacteria, such as those in the Mycobacterium genus, have a waxy cell wall that can also resist decolorization but they are typically stained using a different method called the Ziehl-Neelsen stain, rather than the standard Gram stain.

Spirochaete bacteria, which are helical in shape, can also be challenging to classify using Gram staining as they often do not retain the dye well due to their unique cell wall structure and are generally identified