2.3.3.3 Cell wall and periplasm
With a few exceptions, prokaiyotic cells have a cell wall that provides the physical strength to maintain their shapes. Murein is the main component of the cell wall of bacteria. The cell wall in Gram-negative bacteria is much thinner than in Gram-positive bacteria, which have a complex cell wall with other polymers and do not possess an outer membrane (Figure 2.3).
Murein (peptidoglycan) is a polymer With abackbone of B-1 ,4—linked N—acetylglucosamine and N-acetylmuramate (Figure 2.7), the lactyl group of which is cross-linked through tetrapeptides (Figure 2.8). Some Gram-positive bacteria, including Lactobacfllus acidophflus, are resistant to lysozyme since they have murein O-acetylated at the 6-OH of N-acetyhnuramate.
D-glutamate and D-alanine occupy the 2nd and 4th positions of the tetrapeptide in most cases, and L-s erine or glycine the 1 st position depending on the species. Various amino acids are found in the 3rd position including L—o1nithine, L-lysine, L,L—d_iarninopi1ne1ate, meso- diaminopirnelate and rarely L-hornoserine. The 3rd amino acid in the murein tetrapeptide is a key criterion used for bacterial classification. The degree of cross-linking differs depending on the Species and is low at the growing tip of a single bacterial cell. Murein reduced cross-linking is more susceptible to hydrolyzing enzymes Gram-positive bacteria do not have an outer membrane but have a much thicker cell wall containing teichoic acid lipoteichoic acid and lipoglycan In addition to murein.