The location of the peptidoglycan polymer in the cell walls allows for bacterial identification, differentiating Gram+ from Gram- cells.
* What Is Peptidoglycan?*
Peptidoglycan is a huge polymer composed of interlocking chains of identical monomers, the backbone composed of two derivatives of glucose: N-acetylglucosamine (NAG) and N-acetlymuramic acid (NAM). The NAG and NAM strands are connected by interpeptide bridges. The rigid peptidoglycan polymer gives the cell its shape and provides protection from the external environment.
From the peptidoglycan inwards all bacteria are very similar. Going further out, the bacterial world divides into two major classes: Gram-positive and Gram-negative cells.
* Gram-positive Bacteria *
In Gram-positive cells, peptidoglycan makes up as much as 90% of the thick, compact cell wall, and is the outermost layer of the cell. The location and thickness of peptidoglycan is the important factor that results in Gram+ cells staining differently than Gram- cells.
* Gram-negative Bacteria *
The cell walls of Gram-negative bacteria are more chemically complex, thinner and less compact, with peptidoglycan comprising only 5 to 20% of the cell wall. In Gram-negative cells, peptidoglycan is not the outermost layer, but is located between the plasma membrane and an outer lipopolysaccharide (LPS) membrane.
Lipopolysaccharide Membrane: The outer membrane of Gram negative bacteria is similar to the plasma membrane, but less permeable and composed of lipopolysaccharides (LPS), a harmful substance classified as an endotoxin.
Periplasmic Space: The space between the cell wall and the plasma membrane is called the periplasm. Periplasm controls molecular traffic entering and leaving the cell.
* Why Are These Differences in Bacterial Cell Wall Structure Important? *
Cell walls without enough peptidoglycan interpeptide cross-links are structurally weak, and disintegrate when cells divide. This is how penicillins and cephalosporins work. Penicillins and cephalosporins interfere with the linking of interpeptides in the peptidoglycan.
* Antibiotic Access to Peptidoglycan *
Although Gram-negative bacteria have fewer interpeptide bridges than do Gram-positive bacteria, Gram negative cells have an outer membrane made of LPS. Because of the outer LPS layer of Gram-negative cells, antibiotics can't easily get to the peptidoglycan. Gram-positive bacteria, which have peptidoglycan as the outermost layer, are more susceptible to these antibiotics.
* Why Our Cells Aren't Damaged By Antibiotics *
Microorganisms that do not contain peptidoglycan are not susceptible to these drugs. And since the eukaryotic cells of humans do not even have cell walls, let along peptidoglycan, our cells are not damaged by antibiotics.
* Source *
Bauman, R. (2004) Microbiology. Pearson Benjamin Cummings