Microbes

Microbe, or micro-organisms, can only be seen using a microscope. There are three main types of microbes: Viruses, bacteria and fungi. The most common fungus microbes are yeast.

Viruses are smaller than bacteria which are smaller than yeast.

Bacteria and yeasts are important in making some foods and drinks. Yeasts are used to make bread dough rise. The cells use oxygen, from the air found in pockets in the dough, for aerobic respiration. This process produces carbon dioxide, which makes the bread rise.

Yeast cells are also used to make beer and wne. In this case there is no air and so the yeast cells use anaerobic respiration. When yeasts cells use anaerobic respiration it is called fermentation. The ethanol is a waste product of this reaction.

The number of an organism in an area is called a population. In good conditions (warm, moist, plenty of sugar) a population of yeast cells will grow rapidly. The population stops growing if something runs out (e.g. sugar). The thing that stops the population growing is called a limiting factor.

Some microbes cause infectious diseases (diseases that can be spread from person to person). The microbes are said to infect you. The effects the microbes have on your body are known as symptoms. A doctor observes symptoms to come up with a diagnosis. Microbes can be spread by the air, water, touch, food, animals and sex.

Some ways that diseases can be stopped from spreading are:

•  making sure sewage is treated and disposed of properly
•  adding chlorine to water to kill bacteria
•  drying, freezing or refrigerating foods
•  pasteurising milk
•  using disinfectants, antiseptics and soaps
•  immunising people with vaccines.

Your body has natural defences to stop microbes getting in (e.g. skin, mucus in the trachea and nose, ciliated epithelial cells to sweep mucus away). Your body also has ways of destroying microbes. These include: • a chemical in tears that kills some bacteria • acid in the stomach that kills some bacteria • white blood cells that engulf microbes • other white blood cells that make antibodies to help destroy microbes. Babies do not have fully developed immune systems. Antibodies can p*** through the placenta and are found in breast milk. These help the baby to fight infections. For many diseases, once you have had the disease (or been immunised) you will not get it again (e.g. chickenpox). This is because the antibodies against these microbes stay in the blood. Some diseases can be cured using antibiotics. These are medicines that kill off bacteria. Some bacteria, however, are unaffected by antibiotics – they are resistant to them. Using too many antibiotics only leaves behind the resistant bacteria, which then cause diseases that are diffi cult to treat.