Gram Positive and Gram Negative Bacteria

Gram staining is a technique used to differentiate bacteria into two major groups: gram positive and gram negative. This technique was developed by Danish bacteriologist Hans Christian Gram in 1884 and is still widely used today. Gram staining is based on the ability of bacterial cell walls to retain a purple dye called crystal violet.

Gram Positive Bacteria

Cell Wall Structure:
Gram positive bacteria have a thick peptidoglycan layer in their cell wall. This peptidoglycan layer is composed of alternating sugar chains cross-linked by short peptide bridges. The presence of this thick peptidoglycan layer is responsible for the retention of the purple dye in the cell wall during the staining process.

Staining Characteristics:
Gram positive bacteria retain the crystal violet dye and appear purple under a microscope. This is due to the ability of the thick peptidoglycan layer to trap the dye. After staining, gram positive bacteria are further treated with iodine, which forms a complex with the crystal violet and enhances the retention of the dye. Finally, the bacteria are washed with alcohol or acetone, which dehydrates the peptidoglycan layer, preventing the loss of the dye. As a result, the purple color is retained.

Examples of Gram Positive Bacteria:
Some examples of gram positive bacteria include Staphylococcus aureus, Streptococcus pneumoniae, and Bacillus subtilis.

Gram Negative Bacteria

Cell Wall Structure:
Gram negative bacteria have a thinner peptidoglycan layer in their cell wall compared to gram positive bacteria. Additionally, gram negative bacteria have an outer membrane composed of lipopolysaccharides (LPS) that surrounds the peptidoglycan layer. The outer membrane acts as a barrier, making the cell wall of gram negative bacteria more complex.

Staining Characteristics:
Gram negative bacteria do not retain the crystal violet dye and appear pink or red under a microscope. This is due to the thinner peptidoglycan layer and the presence of the outer membrane. During the staining process, the crystal violet dye is initially trapped in the peptidoglycan layer, but the alcohol or acetone used for washing disrupts the outer membrane and removes the dye. As a result, the pink or red counterstain (such as safranin) is absorbed by the gram negative bacteria.

Examples of Gram Negative Bacteria:
Some examples of gram negative bacteria include Escherichia coli, Salmonella enterica, and Pseudomonas aeruginosa.

In conclusion, gram positive and gram negative bacteria can be differentiated based on the structure and staining characteristics of their cell walls. Gram positive bacteria have a thick peptidoglycan layer and retain the crystal violet dye, appearing purple under a microscope. On the other hand, gram negative bacteria have a thinner peptidoglycan layer, an outer membrane, and do not retain the crystal violet dye, appearing pink or red. Understanding these differences is important in the field of microbiology for the identification and diagnosis of bacterial infections.

Gram positive bacteria remain coloured blue or purple with gram staining even after washing with alcohol like bacillus, leuconostoc,etc . And gram negative bacteria do not retain the stain when washed with alcohol like salmonella,vibrio,xanthomonas etc.