Developed by the Danish bacteriologist Hans Christian Gram, Gram staining is a method for rendering bacteria more visible under the microscope with the aid of a special dye. Depending on differences in their cell-wall structures, bacteria can basically be divided into two groups: Gram-positive and Gram-negative bacteria.
Gram-negative bacteria have a thin cell wall with a single-layered murein. They therefore absorb no dye during Gram staining, and appear reddish under the microscope. Gram-negative bacteria are primarily rod bacteria and are endotoxic. Well-known members of this family are Legionella, the pseudomonads, and Borrelia.
Gram-positive bacteria stain dark-blue during Gram staining. The reason for this is a thick cell wall and a multi-layered murein which may account for up to 50% of the dry matter of the membrane. Gram-positive bacteria are primarily Cocci and are exotoxic. Examples are Listeria, Clostridia, Streptococci and Staphylococci.