Bacterial cell is surrounded by a cell wall. It is made up of polysaccharides, lipids and protein. It contains a substance mucopeptide in which molecules of acetylglucosamine and acetylmuramic acid join each other. In addition L-alanine D-alanine., D-glutamic acid and diaminopimelic acid (DAP) are also found in cell wall. Cellulose is also present in some bacteria. In some bacteria outside the cell wall a layer made-up of slime is found. At the time of growth of bacterium slime layer becomes swollen and hard to form a capsule. The bacteria which form capsules are" known as capsulated.
Formation of capsule is significant for both bacteria and man. It protects the bacteria under unfavourable conditions and functions for storage of accumulate food substances. In addition excretory substances of bacteria are also accumulated in the capsule. On the other hand formation of capsule helps in the disease in animal and man because white blood corpuscles (W.B.C.) are not able to destroy the bacteria due to the presence of capsule. Capsule is made up of dextran, dextrin, levan and cellulose and other polysaccharides.
Inside the cell wall of bacteria the protoplasm is present and it is surrounded by protoplasmic membrane.
This membrane is made up of phospholipids-and lipoproteins, The staining of bacteria depends upon chemical and physical nature of protoplasmic membrane and protoplasm.
A scientist named Gram stained the bacteria with crystal violet and iodine solution. Then he washed them with acetone or alcohol. Gram positive bacteria become deep violet or purple but gram negative are decolorized. If these bacteria are counterstained with safranin or carbol-fuchsine it is observed that some of the bacteria retain purple colour and other take red colour. Those bacteria which retain purple colour are termed Gram-positive and those which take red colour are termed Gram-negative.
In gram-positive bacteria the cell wall is thick (25-30 mm) and its dry weight.is. , 20%-40% of total weight of the cell. It contains peptidoglycan (20%),and less quantity of protein and polysaccharide. Teichoic acid is present in the cell wall. These bacteria are destroyed by penicillin.
Cell wall in gram-negative bacteria is thin and hard (10-15 mm) and its dry weight is 10-20% of total weight of the cell. It contains phospholipid (3%), protein (15%) and lipo-polysaccharide (50%). These bacteria are not destroyed by penicillin. Gram-negative and rod shaped bacteria are pathogenic.