Bacteria can be divided into two large groups on the basis of a differential staining technique, called the Gram stain, developed by the Danish physician, Christian Gram in 1884. One large group is called Gram Positive and the other, Gram negative. These two groups that are morphologically indistinguishable yet differ in their cell wall structure and metabolic character.
In this process, a heat fixed smear of bacteria is stained with crystal violet dye for thirty seconds. After rinsing off, a dilute solution of gram iodine is added. All bacteria are stained deeply blue. The smear is treated with 95% alcohol which act as a decolorizing agent. Some bacteria are completely decolorized by alcohol, while other retain the crystal violet dye. The bacteria are finally treated with some counter stain such as safranine. Two types of results are noted: those species of bacteria that are decolorized by the alcohol, appear red in color, these are gram negative; and those retained the purple primary stain, are called gram positive.
The cell wall of gram positive bacteria
In gram positive bacteria, the cell wall is composed of several layers of peptidoglycan forming a thick, rigid structure. The cell wall has a high content of peptidoglycan(20-80% of the dry weight of cell wall).
In addition to peptidoglycan, the cell of gram positive bacteria contain teichoic acid, large molecules composed of repeating units of sugars and phosphates. These acids contribute to cell wall.
The cell wall gram negative bacteria
The structure of the gram negative bacterial cell wall is more complex than that of gram positive bacteria. In gram negative bacteria beyond the cytoplasmic membrane lies the periplasmic space, bounded on the outside by the thin layer of peptidoglycan, exterior to which they possess a second 'membrane'
The periplasmic space is an aqueous space having the consistency of a gel with a loose network of peptidoglycan layer running through it.
The peptidoglycan of gram negative cell has the same fundamental structure as that found in gram positive cells, but gram negative cells generally have only a thin layer of peptidoglycan.
The outer surface of the peptidoglycan is covered by a series of layers which provide a protective coating around the cell. The first of these layers is lipoprotein which provides an anchor for the outer membrane. The outer membrane is a lipoprotein bilayer similar to the cell membrane. This layer protect gram negative bacteria from the effect of anti-peptidoglycan chemicals such as penicillin and lysozyme that could damage the cell. As a result, infection from gram negative bacteria are often more difficult to treat. The surface of the outer membrane contains molecules of lipopolysaccharides(LPS) which has toxic properties.