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Summary

4.1 Viruses

A virus is a tiny parasite living, growing and reproducing inside a host cell. When viruses damage or destroy the cells they invade, they produce viral diseases. Viruses are the smallest microbes. They are smaller than bacteria, ranging in size from about 20 nm to 300 nm. They cannot be seen through a light microscope and pass through filters which retain bacteria.

Nature of viruses. Viruses were first discovered in 1892 by a Russian scientist, D. Iwanowsky, who noted infective agents that passed through a filter for ordinary bacteria. Hence they were originally called filterable viruses. First to be discovered was the tobacco mosaic virus. In 1901 Walter Reed and his associates found the virus that caused yellow fever in man. Since then, a great many viruses, all parasites on the cells of plants, lower animals or human beings, have been identified.

Viruses contain nucleic acids such as DNA or RNA and must therefore be considered as being on the border between living and non-living. Viruses are classified by their structure and nucleic acids types. Most viruses found in animal cells and those attacking bacteria (known as bacteriophages) have the nucleic acid DNA. Other animal viruses and plant viruses contain RNA. Electron microscopy and X-ray diffraction have shown viruses to be a variety of shapes such as spherical (e.g. poliomyelitis), straight rods (e.g. tobacco mosaic virus) or flexible rods (e.g. potato virus X).

Under certain circumstances of virus reproduction, they split apart and then join together again. This is much the same process that occurs when the chromosomes in the nucleus of a living cell split apart and rejoin to form new cells. The process of wild multiplication of cancer cells also has much in common with virus duplication.

Viruses are spread in a variety of ways. Some virus diseases, like chickenpox and measles, are spread by contact or by droplets in the air. Rabies virus is transmitted only through a wound - the bite of a rabid animal. Many viruses are spread by insects. There is often a reservoir of virus infection in wild or domestic animals. Virus diseases are rarely spread by water, milk or food contaminated by virus. Immune serum from people who have had one attack of the virus disease, is often used to provide a passive immunity. Prevention of virus infection, or its damaging effects, is very often achieved by vaccines, killed or attenuated, made from the original infection virus.

4.2 Bacteria

Bacteria are the smallest cellular organisms and are the most abundant. Such cells may vary in the nature of the cell wall. In some forms the glycoprotein is supplemented by large molecules of lipopolysaccharide. Cells which lack the lipopolysaccharide combine with dyes like gentian violet and are said to be gram positive. Those with the lipopolysaccharide are not stained by gentian violet and are said to be gram negative. Gram positive bacteria are more susceptible to both antibiotics and lysozyme than are gram negative ones. Bacteria may be coated with a slime capsule which is thought to interfere with phagocytosis by the white blood cells. Bacteria are generally distinguished from each other by their shape. Spherical ones are known as cocci (singular - coccus), rod-shaped as bacilli (singular - bacillus) and spiral ones as spirilla (singular - spirillum).

Cocci may stick together in chains, e.g. streptococcus, or in clusters, e.g. staphylococcus. Bacteria show considerable diversity in their metabolism. The majority are heterotrophic. They are responsible, with the Fungi, for decaying and recycling organic material in the soil. Other bacteria are autotrophic.

Bacteria reproduce by binary fission, one cell being capable of giving rise to over 4 × 10²¹ cells in 24 hours. Under certain circumstances conjugation occurs and new combinations of genetic material result. Bacteria may also produce thick-walled spores which are highly resistant, often surviving drought and extremes of temperature.

Economic importance of bacteria

It is easy to think of all bacteria as pathogens but it is important to remember that many are beneficial to humans. These benefits include:

1. The breakdown of plant and animal remains and the recycling of nitrogen, carbon and phosphorus.

2. Symbiotic relationships with other organisms. For example supplying vitamin K and some of the vitamin B complex in humans.

3. Food production, e.g. some cheeses, yoghurts, vinegar.

4. Manufacturing processes, e.g. making soap powders, tanning leather and retting flax to make linen.

5. They are easily cultured and may be used for research, particularly in genetics. They are also used for making antibiotics, amino acids and enzymes.