Bacteria are prokaryotes, i.e. they do not have their DNA enclosed in a nucleus (Bacteria, 2005). They are among the earliest life forms that appeared on Earth billions of years ago, and helped change the environment, creating oxygen which enabled higher forms to evolve. The Universal Tree of Life derived from sequencing ssrRNA gives three major domains of living organisms: Eucarya, Archaea and Bacteria (Todar, 2005). Although there are thousands of different bacteria, they come in three basic forms: rod shaped, called bacilli; round, ball-shaped forms called cocci); and some are spiral in shape. Bacterial cells can exist as individual cells, or may group together in chains (rods and cocci) or cocci can aggregate in clusters. They have a cell envelope consisting of a capsule, the cell wall, and a plasma membrane, and a cytoplasmic region which contains the cell genome (DNA), ribosomes, and various cell inclusions. (Todar, 2005).
Most bacteria have a rigid cell wall which protects the cell protoplast from osmotic lysis, and consists of a polymer of disaccharides cross-linked by short amino acid chains (peptides) forming a peptidoglycans called mucin. Gram-positive bacteria (those that retain the purple dye) have a cell wall with a thick layer of murein. Gram-negative bacteria, which exclude the dye, have a relatively thin cell wall composed of murein surrounded by a membranous structure called the outer membrane. Murein is unique to bacterial cell walls. The oute
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