Bacteria are single-cell organisms varying in size from 0.2 |xm to 2.0 |xm in diameter. They are found everywhere in the environment but can only be seen through an optical microscope. Many bacteria are harmless and useful to humans and are called non-pathogenic bacteria. Some are used in the production of food such as cheese and yoghurt. Others help to dispose of unwanted organic material such as the breakdown of sewage, rendering it harmless. Some bacteria in the human intestine help to synthesise vitamins K and B2. The harmful bacteria that cause disease are known as pathogenic or pathogens. Bacteria are the simplest of
Figure 1.6 A bacterium.
living organisms, composed of a single cell with cytoplasm surrounded by a protective cell membrane but devoid of organelles. They multiply rapidly by dividing into two, known as binary fission. This can occur every 20 minutes and a single bacterium may give rise to 16 million bacteria per day. Some bacteria have whip-like projections on the surface of the cell, called flagella: these enable the bacteria to move around. Bacteria may be aerobic, requiring oxygen to sustain life, or they may be anaerobic, able to survive without oxygen.
The aerobic variety is found invading surface tissues of the skin and mucous membranes of the respiratory tract. The anaerobic variety is found in the bowel or deep wounds. Some bacteria develop into spores; these can lie dormant for long periods of time and become active when conditions are suitable. Spores develop a hard, thick outer shell that protects the contents and makes them very difficult to destroy. They are more resistant to heat and disinfectants; higher temperatures and strong chemical disinfectants are required to kill spores.
Bacteria cause disease by producing toxins or poisons that are harmful to body cells. They grow and multiply if the conditions are right.
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