Portage Learning Microbiology Practice Exam

During a Gram stain procedure, Gram-positive bacteria typically stain violet. This distinctive color is due to the thick peptidoglycan layer present in the cell wall of Gram-positive bacteria. When subjected to the staining process, the primary stain, crystal violet, firmly binds to this thick layer, and even after the decolorization step with alcohol or acetone, the crystal violet remains trapped, resulting in the characteristic violet appearance.

In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer and an outer membrane. This structural difference allows the decolorizer to easily wash out the crystal violet stain, leading to a pink or red coloration after being counterstained with safranin. Mycobacteria have a unique cell wall composition that includes mycolic acids, making them resistant to the Gram stain, while Archaea generally have different cell wall structures that do not fit neatly into the Gram classification.