What are microbes?
Almost everyone today has a rough idea about what a microbe is. It is some tiny form of life so small you cannot see it with your naked eye. We usually link microbes with disease, although they do not cause all diseases. We may link them with the need for good hygiene, such as cleaning the skin and teeth regularly. This is a good link to make because some microbes can harm teeth and be ready to enter the body through a cut on the skin if given a chance. We may not be aware that some microbes are used to make food and most people would find it hard to believe that the ground is teeming with useful microbes, which help to make the soil fertile for plants to grow.

People have not known about microbes for very long. In the days of Ancient Greece they had no idea about microbes and used to explain how diseases occurred in another way. They thought that the health of the body was due to four fluids called humours. These humours that doctors had found in the body were blood, phlegm, black bile and yellow bile. The doctors reasoned, without trying any experiments, that if you were healthy the four humours were in balance and if you were ill, the humours were unbalanced and needed restoring. For example if a doctor thought your illness was due to having too much blood he would set leeches on your arm to draw out blood until he thought you should recover. It was into a world with such strange ideas that the discovery of microbes was made in the seventeenth century.

The first microbes were discovered by Anton van Leeuenhoek in 1673. He was a draper in Holland but had a hobby making microscopes. These microscopes were not like the ones you might have in school. A school microscope has two lenses: one near the eye and one near the object you are looking at. The two lenses are separated by a tube which can be moved closer to the object or further away to help you see the object clearly.

Leeuenhoek's microscope was about the size of a matchbox and had only one lens. It also had a device for holding objects or liquids. To use this kind of microscope you had to bring it very close to your eye. If you fold up the first finger on your right hand but leave a little gap in the middle, then hold your hand close to your eye so you can see through the gap, you will get an idea of what it was like to use one of Leeuwenhoek's microscopes. Using this instrument Leeuwenhoek saw protozoa, algae, yeast and bacteria. He shared his results with other scientists and they soon began to make more discoveries about these microbes.

There used to be another strange idea around in Leeuwenhoek's lifetime which concerned where things came from. For example, if you left a piece of meat out, maggots could soon be found on it. People reasoned that the meat turned into maggots. It was not until an Italian doctor did an experiment on meat that people saw that their idea was wrong. In 1668 Francesco Redi set up meat which was open to the air and meat which was kept under gauze. He found that meat open to the air developed maggots because flies could reach the meat and lay their eggs on it. No sooner had people accepted the results of his experiment than scientists began to find microbes appearing in all kinds of substances. Many scientists believed that surely these microbes just appeared from nowhere.

It was not until Pasteur performed an experiment on broths in the nineteenth century that people realised that microbes, like other life forms, did not just appear from nowhere. Pasteur set up broths in flasks, which were open to the air, but had S-shaped necks. Dust collected in the necks and the broths stayed clear. When the dust was added to the broths, the broths became putrefied and full of microbes. Pasteur had shown that microbes are so small they can travel though the air with dust which is why they seemed to appear from nowhere.

The discoveries we have made about microbes have sometimes come from testing strange ideas with experiments. During the twentieth century we found out a huge amount of information about microbes and we are still making many discoveries today.

How can a lens make you see microbes?
A lens has two curved surfaces. When light rays strike a lens they are bent. This makes them appear to come from a different place. If you look at a microbe with a microscope, the light rays coming from it are bent so it looks as if it is coming from something much larger. This is the large picture, called an image, that you see when you look down the microscope.

Can all kinds of microbes be seen with a microscope?
You can see bacteria, algae, fungi and protozoa with a microscope that uses light but you cannot see viruses. In the twentieth century the electron microscope was invented. This is much more powerful than a microscope that uses light. Viruses can be seen with an electron microscope.

How were viruses discovered?
They were not seen until the electron microscope was invented but scientists believed they existed long before then. In the nineteenth century a scientist was studying a disease afflicting tobacco plants. The disease made the plants grow very poorly and the leaves had blotches on them. He thought the disease was caused by a bacterium so he squeezed out the plant juices and looked at them with a microscope. He could not find any bacteria but when he added the juice to another healthy plant, the healthy plant developed the disease. At first it was thought that the juice contained a poison and the word virus, which is Latin for poison, was used to describe it. Soon it was found that the liquid containing the virus could cause disease to spread in ways that a poison could not do. This meant that scientists knew there was a disease-causing form of life in the juice even though they did not have the technology to see it. Later, in the twentieth century, viruses were seen when the electron microscope was invented.

Are all microbes germs?
No. A germ is a microbe, which causes disease. Some microbes, such as yeast, live on the juices of fruit and do not cause disease. They are not germs. There are many microbes which live in the soil, and in water, that are not germs. However, there are also microbes which are germs, that live in soil and water so both substances should be treated with care.

Do all the different kinds of microbes cause disease?
There are disease-causing bacteria, fungi and protozoa. All viruses cause disease but algae do not cause disease.