Characteristics of Gram-Negative Bacterial Cell Walls
The presence of an outer membrane and the possession of only few peptidoglycan layers in the cell wall distinguish Gram-negative bacteria from Gram-positive ones. Lipids covalently linked to proteins called lipoproteins are the molecules that bind the peptidoglycan to the outer membrane. The peptidoglycan is located in the periplasm, a space filled with fluid located between the plasma membrane and the outer membrane. A high amount of degradative enzymes and transport proteins are found in the periplasm. Unlike Gram-positive cell walls, we cannot find teichoic acids in the Gram-negative cell walls. In addition, the cell walls of Gram-negative bacteria are more prone to mechanical breakage because of the low amount of peptidoglycan. (Shagam 2006; Wheelis 2007)
The outer membrane of a Gram-negative bacterium is composed of lipopolysaccharides (LPS), phospholipids, and lipoproteins. It plays a very important role in the survival of the bacterium under environmental pressure. For example, it prevents the whole bacterium from being phagocytosed (e.g. by macrophages) because of its strong negative charge. The mechanism of evading phagocytosis involves a complex biochemical pathway which is not the scope of this article. Furthermore, the outer membrane serves as a barrier for the bacterium against the destructive effects of various antibiotics (e.g. erythromycin, penicillin, amoxicillin), digestive enzymes like lysosomal enzymes, heavy metals, detergent substances, bile salts, and several dyes. (Shagam 2006; Wheelis 2007)
It is, however, necessary that the outer membrane is not totally restrictive to the entrance of molecules because it may jeopardize the health of the bacterium if important nutrients could not enter the cytoplasm. The outer membrane is actually permeable to nutrients due to the presence of porins, proteins that form channels toward the cytoplasm. Porins allow the entry of valuable molecules like disaccharides, nucleotides, peptides, amino acids, iron, and vitamin B12 but it prevents the entry of other molecules, especially the bigger ones.
The polysaccharide components of outer membrane’s LPS serves as bacterial antigens and are very helpful in identifying species of Gram-negative bacteria in the laboratory. There are certain laboratory tests that detect antigens specific for a single species. The LPS is therefore significant in medical diagnosis of pathogenic infections. (Shagam 2006; Wheelis 2007)