Chemically, 60 to 90% of the Gram-positive cell wall is peptidoglycan. In electron micrographs, the Gram-positive cell wall appears as a broad, dense wall 20-80 nm thick and consisting of numerous interconnecting layers of peptidoglycan (Figures 1A and 1B). The material in the bacterial cell wall which confers rigidity is peptidoglycan. The bacterial cell wall serves to give the organism its size and shape as well as to prevent osmotic lysis. Gram-positive and Gram-negative bacteria stain differently because of fundamental differences in the structure of their cell walls. The terms positive and negative have nothing to do with electrical charge, but simply designate two distinct morphological groups of bacteria. Bacteria that stain purple with the Gram staining procedure are termed Gram-positive those that stain pink are said to be Gram-negative. It is called a differential stain since it differentiates between Gram-positive and Gram-negative bacteria. The Gram stain is the most widely used staining procedure in bacteriology.
