Cell Envelope
The cell envelope may be defined as the cell membrane and cell wall plus an outer membrane if one is present. The cell wall consists of the peptidoglycan layer and attached structures. Most bacterial cell envelopes fall into two major categories (Figure 1): Gram positive and Gram negative. This is based on Gram staining characteristics that reflect major structural differences between the two groups. Other types of cell wall are found in a few bacterial species (neither Gram positive nor Gram negative).
The peptidoglycan is a single bag-shaped, highly cross-linked macromolecule that surrounds the bacterial cell membrane and provides rigidity. It is huge (billions in molecular weight; compare proteins which are thousands in molecular weight). Peptidoglycan consists of a glycan (polysaccharide) backbone consisting of N-acetyl muramic acid and N-acetyl glucosamine with peptide side chains containing D- and L- amino acids and in some instances diaminopimelic acid. The side chains are cross-linked by peptide bridges. These peptide bridges vary in structure among bacterial species. Muramic acid, D-amino acids and diaminopimelic acid are not synthesized by mammals. PG is found in all eubacteria except Chlamydia andMycoplasma.