A Level Biology Practice Exam

Which color indicates Gram positive bacteria when stained?

1. Red

2. Blue

3. Purple

4. Yellow

The correct answer is: Purple

When stained using the Gram staining method, Gram-positive bacteria are indicated by a characteristic purple color. This is due to the presence of a thick peptidoglycan layer in their cell walls, which retains the crystal violet stain used during the procedure. During the Gram staining process, after applying crystal violet, the bacteria are treated with iodine, which helps to form a complex with the crystal violet. When the cells are washed with alcohol or acetone, Gram-positive bacteria retain this complex due to their thick peptidoglycan layer, resulting in a purple appearance. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane. When the alcohol or acetone is applied, they lose the crystal violet stain and take up a counterstain, typically safranin, leading to a pink or red appearance. This difference in color is a vital part of bacterial classification in microbiology and helps in determining appropriate treatments for bacterial infections.