Viruse:
A microorganism smaller than a bacteria, which cannot grow or reproduce apart from a living cell.
A virus invades living cells and uses their chemical machinery to keep itself alive and to replicate itself.
Viruses may contain either DNA or RNA as their genetic material. Herpes simplex virus and the hepatitis- B virus are DNA viruses.
RNA viruses have an enzyme called reverse transcriptase that permits the usual sequence of DNA-to-RNA to be reversed so the virus can make a DNA version of itself. RNA viruses include HIV and the hepatitis C virus.
Researchers have grouped viruses together into several major families, based on their shape, behavior, and other characteristics.
These include the herpesviruses, adenoviruses, papovaviruses (papilloma viruses), hepadnaviruses, poxviruses, and parvoviruses among the DNA viruses. On the RNA virus side, major families include the picornaviruses (including the rhinoviruses), calciviruses, paramyxoviruses, orthomyxoviruses, rhabdoviruses, filoviruses, bornaviruses, and retroviruses. There are dozens of smaller virus families within these major classifications.
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Bacteria:
Bacteria are microscopic, single-celled prokaryotic organisms, without a defined nucleus, that reproduce asexually by binary fission (one cell splitting into two). They occur singly or in colonies of cells. Bacteria are classified into two main groups based on cell wall structure, which can be determined by a simple staining procedure called the Gram stain. Gram negative bacteria stain red or pink and Gram positive bacteria stain purple. The difference in color is directly related to the chemical composition and structure of their cell walls.
Although considered structurally simple, bacteria are extremely diverse from a metabolic standpoint and are found almost everywhere on Earth in vast numbers—from living in jet fuel and on the rims of volcanoes to thriving in hydrothermal vents deep on the ocean floor. There are both beneficial and pathogenic bacteria. Beneficial bacteria are involved in such diverse processes as digestion in animals,
nitrogen fixation in the roots of certain legumes, the decomposition of animal and plant remains, and sewage disposal systems.Pathogenic bacteria, on the other hand, cause severe and often fatal diseases in humans, animals, and plants.
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