Which type of bacteria cannot retain a crystal violet stain?

The retention of crystal violet stain is a key feature in differentiating between Gram-positive and Gram-negative bacteria. Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which allows them to retain the crystal violet stain during the Gram staining process. In contrast, Gram-negative bacteria possess a thinner peptidoglycan layer and an outer membrane that does not retain the stain after the decolorization step.

When subjected to the staining process, Gram-negative bacteria will initially take up the crystal violet, but upon decolorization with alcohol or acetone, they will lose this stain and will require a counterstain, often safranin, to be visualized. This distinct characteristic helps identify Gram-negative organisms, as they appear pink under the microscope after the staining process, while Gram-positive organisms remain purple.

In the context of the other choices, Gram-positive bacilli and staphylococci are examples of Gram-positive bacteria that will retain the stain, while acid-fast bacilli, such as Mycobacterium species, can be identified using a different staining method because they have mycolic acid in their cell walls, not primarily due to their Gram stain properties.