In almost all eubacteria the cytoplasmic membrane is surrounded by a bag-shaped macromolecule: the peptidoglycan or murein sacculus. In Gram-negative bacteria it is located within the periplasm between the cytoplasmic membrane and the outer membrane, while in Gram-positive bacteria the peptidoglycan forms the outermost part of the cell. Eubacteria known not to contain peptidoglycan are Planctomyces, mycoplasmas, Chlamydiae and Orientia (Rickettsia) tsutsugamushi.

Peptidoglycan has been studied for several decades and the chemical composition has long been solved. As the term 'peptidoglycan' suggests, it consists of polysaccharides (glycan strands) cross-linked by peptide moieties. The sacculus can be isolated as a whole and viewed under the electron microscope. Its shape corresponds exactly to the form of the original cell. Unfortunately, to date it has not been possible to actually visualize the fine structure of this macromolecule, resulting in controversial discussions about the orientation of the glycan strands relative to the rod axes. The peptidoglycan sacculus has to be elongated and divided during bacterial growth. As it is a stress-bearing structure several models have been described for a safe enlargement and separation.

As the peptidoglycan sacculus is a distinct feature of bacteria it is a target for several different kinds of bacteriolytic antibiotics. To cope with these stress factors, different resistance mechanisms have evolved, some of them changing the structure of the murein sacculus.

from Ute Bertsche in Bacterial Polysaccharides

Further reading: Bacterial Polysaccharides: Current Innovations and Future Trends

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