Most bacteria have one of these two types of cell walls. The differential Gram stain uses two dyes to distinguish between bacteria based on cell wall structure.

Bacteria Are Prokaryotes

All bacteria (Eubacteria), and bacteria-like Archaea are prokaryotes. Prokaryotic cells are simpler than the eukaryotic cells that make up our body. One of the most striking distinctions is that prokaryotes lack a membrane bound nucleus. Their genetic material is naked within the cytoplasm. The term nucleoid refers to the region of cytoplasm where a bacterium’s 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.

Bacterial Cell Wall

Nearly all types of bacteria have a cell wall containing structural chemical components that are unique to bacterial cells. The rigid structure of the bacterial cell wall is due to securely linked peptidoglycan monomers that surround the cytoplasmic membrane, giving the cell shape and protection.

Gram-negative Cell Wall

Peptidoglycan

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 -). The cell walls of Gram - bacteria are more chemically complex, thinner and less compact, with peptidoglycan comprising only 5 – 20% of the cell wall.

LPS Membrane

In gram-negative bacteria, peptidoglycan is not the outmost layer of the cell wall. Gram- cells have an additional, external membrane, similar to the plasma membrane, but less permeable and composed of lipopolysaccharides (LPS); a harmful substance classified as an endotoxin.

The Gram Stain

Once scientists understood that infectious disease was caused by microorganisms (Germ Theory), it was imperative to find a way to view bacteria and other microbes; because in addition to being minute, most bacteria colorless.

In the 1800’s, Christian Gram, a Danish bacteriologist, developed a technique for staining bacteria that is still widely used today. The Gram stain protocol involves the application of a series of dyes that leaves some bacteria purple (Gram +) and others pink (Gram -). This differential staining not only colors the bacteria, but the specific stain reaction distinguishes between two meaningful categories of bacteria based on the differences in their cell wall structure.

After this staining procedure, Gram + cells appear purple; their externally located peptidoglycan having retained the primary stain, crystal violet. Because Gram negative cells have a membrane outside of the peptidoglycan layer, these cells do not retain the purple primary stain. At the end of the Gram staining procedure, Gram-negative cells retain the secondary stain, safanin, and appear pink.

Gram Negative Bacteria as Pathogens

The vast majority of Gram-negative bacteria are pathogens; bacteria that can cause disease. This pathogenicity is typically associated with lipopolysaccharide (LPS) endotoxins in Gram-negative cell walls.

Some of the major taxa of Gram negative bacteria include Proteobacteria, Chlamydias, Spirochetes and Bacteroides.

Additional Microbiology Information

For more information on prokaryotes and microbiology, go to the excellent websites of Science Prof Online or MicrobeWiki.

Sources

Bauman, R. (2005) Microbiology.

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