Gram Positive ( Gram+ ) Bacteria

What Makes Some Bacterial Cells Stain Gram-positive?

Apr 14, 2008
Tami Port

Staphylococcus Gram+ Bacteria - Janice Haney Car , CDC PHIL#10046

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.

Prokaryotes: Eubacteria & Archaea

All bacteria (the domain Eubacteria) and bacteria-like Archaea, are prokaryotes. Prokaryotic cells are simpler than eukaryotic cells. One of the most striking distinctions is that prokaryotes lack a membrane bound nucleus. Their genetic material is naked within the cytoplasm; their DNA typically a singular, circular chromosome.

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

The Bacterial Cell Wall

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

Gram-positive Cell Wall

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 -).

In Gram-positive cells, peptidoglycan makes up as much as 90% of the thick cell wall; more than 20 layers of peptidoglycan stacked together. These layers are the outermost cell wall structure of Gram + cells, whereas in Gram negative cells, the thinner peptidoglycan component is covered by an external lipopolysaccharide (LPS) membrane.

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 microscopic, 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.