Which type of bacteria are typically stained purple in a Gram stain?

Gram-positive bacteria are stained purple in a Gram stain due to their thick peptidoglycan layer in the cell wall. The Gram stain process involves the application of crystal violet dye, which penetrates the cell wall of these bacteria, followed by iodine, which helps fix the dye. After that, a decolorizer is applied, which does not remove the crystal violet from the thick peptidoglycan layer of Gram-positive bacteria, allowing them to retain the purple color. The final step involves counterstaining with safranin, which colors Gram-negative bacteria pink, but has no effect on the already purple-stained Gram-positive bacteria.

This characteristic of Gram-positive bacteria is fundamental for their classification and aids in the selection of appropriate antibiotics for treatment, as Gram-positive and Gram-negative bacteria often respond differently to antibiotic therapies due to the structural differences in their cell walls.