Characteristics of Bacteria

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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ll to be released from it: they can bud out of it. Enveloped viruses are readily infectious only if the envelope is intact and agents which damage the envelope (alcohols, detergents) reduce infectivity. There are five basic structural forms of viruses in nature: naked icosahedral, e.g. poliovirus, hepatitis A virus; naked helical, e.g. tobacco mosaic virus, no human viruses known of this type; enveloped icosahedral, e.g. herpes viruses, rubella virus; enveloped helical, e.g. rabies virus, influenza virus, measles virus; and complex, e.g. poxvirus (Hunt, 2005). The principal events involved in replication of viruses are absorption, penetration, uncoating, synthesis of viral nucleic acid and protein, assembly/maturation, and release (Hunt, 2005). The first step in infection with a virus is absorption to a cell surface. This is accomplished through ionic interactions which are temperature-independent. The viral attachment protein recognizes receptors on the cell surface which can be proteins, carbohydrates or lipids: viruses will not attach to cells without specific receptors. The next step is penetration of the virus into the cell, which can be accomplished in a variety of ways depending on the nature of the virus (Hunt,

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