Gram-Positive bacteria

The Gram stain was first used in 1884 by Hans Christian Gram. The stain used during the technique is crystal violet. Crystal violet is composed of positive crystal violet ions that can penetrate the cell wall and membrane of both Gram positive and Gram-negative bacteria. A decolorization step is achieved through the addition of either alcohol or acetone.

The Gram stain is fundamental to the phenotypic characterization of bacteria. The staining procedure differentiates organisms of the domain bacteria according to cell wall structure.

Gram positive bacteria retain the crystal violet stain and a Gram positive result is indicated by a purple coloration. Gram positive bacteria are more susceptible to antibiotics due to the lack of an outer membrane. Since gram negative bacteria contain an outer membrane, they are less susceptible to antibiotics.

Gram-positive cells have a thick peptidoglycan layer and stain blue to purple whereas gram negative bacteria contain a thin peptidoglycan cell wall with no teichoic acid, allowing the cell wall to stain in pink during counter staining.

Gram positive bacteria contain a continuous cell wall called the sacculus, which is 20­80 nm thick. The cell wall contains tightly bound acidic polysaccharides:
1- Teichoic acid
2- Lipoteichoic acid

The outer thick cell wall is composed of several peptidoglycan layer known as murein. Peptidoglycans contain a glycan backbone, which is made up of both N­acetylated muramic acid and glucosamine. In gram positive bacteria, this glycan backbone is highly cross­linked with oligopeptides.

Gram-positive cocci include Staphylococcus (catalase-positive), which grows clusters, and Streptococcus (catalase-negative), which grows in chains.
Gram-Positive bacteria