As previously mentioned, Gram-positive bacteria are characterized by their blue-violet color reaction in the Gram-staining procedure. The blue-violet color reaction is caused by crystal-violet, the primary Gram-stain dye, complexing with the iodine mordant. When the decolorizer is applied, a slow dehydration of the crystal-violet/iodine complex is observed due to the closing of pores running through the cell wall. Because the crystal-violet is still present in the cell, the counter stain is not incorporated, thus maintaining the cell's blue-violet color. If you recall, most cell walls contain peptidoglycan, a molecule made of amino acids and sugar. A distinguishing factor among Gram-positive bacteria is that roughly 90% of their cell wall is comprised of peptidoglycan and a Gram-positve bacteria can have more than 20 layers of peptidoglycan stacked together to form the cell wall. Examples of common Gram-positive cells include Staphylococcus aureusand Streptococcus cremoris, a bacterium used in dairy production.
