In 1884

In 1884, Danish scientist Hans Christian Gram developed a method known as Gram’s staining. Gram’s staining is a laboratory staining technique used to differentiate bacterial species into two large groups: Gram-positive and Gram-negative, while also enabling them to be examined under a microscope. The retention of these stains is based on the chemical and physical properties of their cell walls. Gram-positive can be identified by the color purple and Gram-Negative by pink. Staining also aids in displaying a clear view of their morphological types, rod or coccid shaped.
Gram-positive bacteria have a thick cell wall made up of peptidoglycan, which stains purple as stated earlier. Peptidoglycan is mainly a polysaccharide composed of N-acetyl glucosamine and N-acetyl muramic acid. When dyed with crystal violet, the thick peptidoglycan layer of these organisms retains the dye, ultimately staining the cell wall purple. Lipoteichoic acid, briefly known as LTA, is another major part of the cell wall embedded in the peptidoglycan layer. It is composed of teichoic acids and acts as a regulator of autolytic wall enzymes. Lipoteichoic acid also has antigenic properties that stimulate specific immune responses when it is released from the cell wall after cell death.
Gram-negative bacteria, unlike Gram-positive, have a thinner layer of peptidoglycan. Because of its thin layer, the crystal violet dye rinses out during decolorization with the alcohol rinse. However, it still retains the counter stain, Safranin, causing it to appear pink. They also have an additional outer membrane containing lipids. This outer layer is separated from the cell wall by periplasmic space.
The cell wall of Gram-negative bacteria is very harmful and enables pathogenic bacteria to cause disease. The virulence of Gram-negative bacteria is associated with specific components of the cell wall. The most crucial component is lipopolysaccharide, also referred to as LPS. In humans, LPS elicits an innate immune response characterized by the activation of the immune system and cytokine production. Because of cytokine production, inflammation occurs which can be incredibly toxic.