The amount and location of peptidoglycan in the prokaryotic cell wall is what determines whether a bacterium is Gram-positive or Gram-negative.

Two Basic Types of Cells

All cells fall into one of the two major classifications. Prokaryotes are evolutionarily ancient. They were here first and for billions of years were the only form of life. Even with the more recent evolution of complex eukaryotic cells, prokaryotic cells are supremely successful.

Prokaryotes

Pro = “before”, karyon = “nucleus”

Prokaryotes were the first type of cell to evolve. These cells lack a membrane bound nucleus. Their genetic material is naked within the cytoplasm, and ribosomes are their only type of organelle. The term nucleoid refers to the region of cytoplasm where chromosomal DNA is located, usually a singular, circular chromosome.

Prokaryotes are most always single-celled, except when they exist in colonies. These ancestral cells reproduce by means of binary fission, duplicating their genetic material and then essentially splitting to form two daughter cells identical to the parent. All bacteria are prokaryotes.

Components of the Bacterial Cell Wall

Peptidoglycan Structure and Function

This rigid structure of peptidoglycan, specific only to prokaryotes, gives the cell shape and surrounds the cytoplasmic membrane. The cell wall provides prokaryotes with protection from environment.

Peptidoglycan is a huge polymer of interlocking chains of identical monomers. The backbone of the peptidoglycan molecule is composed of two derivatives of glucose: N-acetylglucosamine (NAG) and N-acetlymuramic acid (NAM). The NAG and NAM strands are connected by interpeptide bridges.

From the peptidoglycan inwards all bacterial cells are very similar. Going further out, the bacterial world divides into two major classes: Gram positive (Gram +) and Gram negative (Gram -).

Gram-positive Cells

In Gram-positive cells, peptidoglycan makes up as much as 90% of the thick, compact cell wall, which is the outermost structure of Gram + cells.

Gram-negative Cells

The cell walls of Gram - bacteria are more chemically complex, thinner and less compact. Peptidoglycan makes up only 5 – 20% of the cell wall, and is not the outermost layer, but lies between the plasma membrane and an outer membrane.

This outer membrane is similar to the plasma membrane, but is less permeable and composed of lipopolysaccharides (LPS). LPS is a harmful substance classified as an endotoxin, The space between the cell wall and the plasma membrane is called the periplasm. Periplasm controls molecular traffic entering and leaving the cell.

Peptidoglycan and Antibiotics

Penicillins and cephalosporins interfere with the linking of the interpeptides of peptidoglycan, but because of the LPS membrane, these antimicrobials can’t access the peptidoglycan of gram-negative bacteria. Gram-positive bacteria, with no membrane outside the peptidoclycan cell wall, are more susceptible to these antibiotics.

Cell walls without intact peptidoglycan cross-links are structurally weak, and disintegrate when cells divide. This is how penicillins and cephalosporins work.

Since the eukaryotic cells of humans do not have cell walls, our cells are not damaged by these drugs. Microorganisms that do not contain peptidoglycan are not susceptible to these drugs.

Additional Cell Biology Information

For more information on prokaryotes and cell biology, go to the excellent websites of Science Prof Online or Cells Alive. See the Suite 101 Prokaryote & Eukaryote article series page, or to learn more about the systematic classification of bacteria and other organisms see the article Binomial Nomenclature.

Sources

Bauman, R. (2005) Microbiology.

Park Talaro, K. (2008) Foundations in Microbiology.