The Gram staining protocol (see Staining Microscopic Specimens ) is used to differentiate two common types of cell wall structures ( [link] ). Gram-positive cells have a cell wall consisting of many layers of peptidoglycan totaling 30–100 nm in thickness. These peptidoglycan layers are commonly embedded with teichoic acids (TAs), carbohydrate chains that extend through and beyond the peptidoglycan layer. T.J. Silhavy, D. Kahne, S. Walker. “The Bacterial Cell Envelope.” Cold Spring Harbor Perspectives in Biology 2 no. 5 (2010):a000414. TA is thought to stabilize peptidoglycan by increasing its rigidity. TA also plays a role in the ability of pathogenic gram-positive bacteria such as Streptococcus to bind to certain proteins on the surface of host cells, enhancing their ability to cause infection. In addition to peptidoglycan and TAs, bacteria of the family Mycobacteriaceae have an external layer of waxy mycolic acids in their cell wall; as described in Staining Microscopic Specimens , these bacteria are referred to as acid-fast, since acid-fast stains must be used to penetrate the mycolic acid layer for purposes of microscopy ( [link] ).
Gram-negative cells have a much thinner layer of peptidoglycan (no more than about 4 nm thick L. Gana, S. Chena, G.J. Jensena. “Molecular Organization of Gram-Negative Peptidoglycan.” Proceedings of the National Academy of Sciences of the United States of America 105 no. 48 (2008):18953–18957. ) than gram-positive cells , and the overall structure of their cell envelope is more complex. In gram-negative cells , a gel-like matrix occupies the periplasmic space between the cell wall and the plasma membrane, and there is a second lipid bilayer called the outer membrane , which is external to the peptidoglycan layer ( [link] ). This outer membrane is attached to the peptidoglycan by murein lipoprotein. The outer leaflet of the outer membrane contains the molecule lipopolysaccharide (LPS) , which functions as an endotoxin in infections involving gram-negative bacteria, contributing to symptoms such as fever, hemorrhaging, and septic shock. Each LPS molecule is composed of Lipid A, a core polysaccharide, and an O side chain that is composed of sugar-like molecules that comprise the external face of the LPS ( [link] ). The composition of the O side chain varies between different species and strains of bacteria. Parts of the O side chain called antigens can be detected using serological or immunological tests to identify specific pathogenic strains like Escherichia coli O157:H7 , a deadly strain of bacteria that causes bloody diarrhea and kidney failure.