What color do gram positive bacteria appear under a microscope?

Gram-positive bacteria appear purple or blue under a microscope due to the Gram staining process. This differentiation is based on the structural composition of the bacterial cell wall. Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain used in the Gram staining procedure, allowing them to appear purple or blue.

During the Gram stain process, after applying crystal violet, iodine is added, which forms a complex with the dye. When a decolorizer is added, it does not wash away the dye from the thick peptidoglycan layer of gram-positive bacteria, meaning they retain the color. In contrast, gram-negative bacteria, which have a thinner peptidoglycan layer and an outer membrane, lose the initial crystal violet stain when the decolorizer is applied, leading them to take up the counterstain (usually safranin) and appear pink.

Understanding this distinction is crucial in microbiology for classifying bacteria and determining appropriate treatment options, as gram-positive and gram-negative bacteria respond differently to antibiotics.