Gram staining

This is The Most Important staining technique in Bacteriology. Although simple stains are useful, they do not reveal details about the bacteria other than morphology and arrangement. The Gram stain is a differential stain commonly used in the microbiology laboratory that differentiates bacteria on the basis of their cell wall structure. Most bacteria can be divided into two groups based on the composition of their cell wall:

1) Gram-positive cell walls have a thick peptidoglycan layer beyond the plasma membrane. Characteristic polymers called teichoic and lipoteichoic acids stick out above the peptidoglycan and it is because of their negative charge that the cell wall is overall negative. These acids are also very important in the body’s ability to recognise foreign bacteria. Gram-positive cell walls stain blue/purple with the Gram stain.

2) Gram-negative cell walls are more complex. They have a thin peptidoglycan layer and an outer membrane beyond the plasma membrane. The space between the plasma membrane and the outer membrane is called the periplasmic space. The outer leaflet of the outer membrane is composed largely of a molecule called lipopolysaccharide (LPS). LPS is an endotoxin that is important in triggering the body’s immune response and contributing to the overall negative charge of the cell. Spanning the outer membrane are porin proteins that enable the passage of small molecules. Lipoproteins join the outer membrane and the thin peptidoglycan layer. Gram negative cells will stain pink with the Gram stain.