General Bacteriology Practice Test 2026 - Free Bacteriology Practice Questions and Study Guide

During the Gram stain procedure, bacteria are categorized based on the characteristics of their cell walls, which affects their staining behavior. Gram positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet stain used in the procedure. When the Gram stain protocol is followed, these bacteria will appear purple under the microscope because the crystal violet is not washed away during the decolorization step.

In contrast, Gram negative bacteria have a thinner peptidoglycan layer and an outer membrane, which doesn't retain the crystal violet after decolorization. Instead, they take up the counterstain (usually safranin) and appear pink. Mycoplasma lack a cell wall entirely, making them unable to be classified using the Gram staining method, as they do not take up the dye in a predictable manner. Thus, the only type of bacteria that typically stains purple during a Gram stain is Gram positive bacteria.