The location of the peptidoglycan polymer in the cell walls of bacteria allows for general identification, differentiating Gram+ from Gram- bacterial cells.
What Is Peptidoglycan?
Peptidoglycan is a huge polymer of interlocking chains of identical monomers, the backbone composed of two derivatives of glucose: N-acetylglucosamine (NAG) and N-acetlymuramic acid (NAM). The NAG NAM strands are connected by interpeptide bridges. This rigid 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, rather than the 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 – 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 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 Killed 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 these drugs.
Additional Microbiology Resources
- SPO’s Virtual Microbiology Classroom
- Todar’s Textbook of Bacteriology
- Kenyon College’s MicrobeWiki
Photo Credit
Gram stained cerebrospinal fluid showing gram-positive anthrax bacilli (purple rods) and white blood cells. Photo public domain US government.
