What does Gram staining reveal about bacteria?

• A. It differentiates bacteria by cell wall thickness and composition: Gram-positive retain crystal violet (thick peptidoglycan); Gram-negative decolorize and counterstain.
• B. It colors all bacteria blue.
• C. It can identify viral infections.
• D. It uses fluorescent dyes to detect DNA.

Answer: (A)

The key idea is that Gram staining distinguishes bacteria by their cell wall structure, which affects how they take up and hold the dye during the process. Bacteria with a thick peptidoglycan layer retain the crystal violet-iodine complex through the decolorization step and appear purple under the microscope. In contrast, bacteria with a thinner peptidoglycan layer and an outer membrane are decolorized by alcohol and then pick up a counterstain, usually safranin, appearing pink or red. This Gram status helps microbiologists categorize bacteria quickly and also inform treatment decisions, since the two groups often differ in antibiotic susceptibility. The other statements don’t fit because Gram staining isn’t about coloring all bacteria blue, identifying viral infections, or using fluorescent dyes to detect DNA.