A Level Biology Practice Exam 2025 - Free A Level Biology Practice Questions and Study Guide

The structure of Gram-positive bacteria significantly influences their staining properties due to the presence of a thick peptidoglycan layer in their cell wall. This thick layer retains the crystal violet stain used in the Gram staining procedure, leading to a purple coloration under the microscope. When the Gram stain is performed, the thick peptidoglycan traps the dye, preventing it from being washed out during the decolorization step with ethanol or acetone, which is crucial in differentiating Gram-positive from Gram-negative bacteria.

In contrast, the other structural attributes associated with Gram-negative bacteria, such as a thinner peptidoglycan layer combined with an outer membrane, affect their staining by allowing the crystal violet to be washed away easily. Therefore, the specific thick peptidoglycan layer of Gram-positive bacteria is pivotal for their classification and identification in microbiological studies.