Microorganisms

On your skin, in a drop of water, and in the air live creatures so small that you can never spot them with the naked eye. We call them microorganisms, or microbes for short. Among them are bacteria and certain fungi like yeast. Only under a microscope do bacteria and yeast become visible; viruses are far smaller and show up only in an electron microscope. One thing is clear: microbes are everywhere.

Bacteria and fungi

Bacteria are made of a single cell and can divide very quickly. One bacterium becomes two, two become four, and after a short time there are millions. Yeast is a fungus and also single-celled. It feeds on sugar and gives off the gas carbon dioxide as it does so. The tiny gas bubbles get caught in the dough and make it rise.

Viruses, a borderline case of life

Viruses are far smaller than bacteria and do not quite fit the picture. On their own they cannot reproduce. They always need a host, a foreign cell that they hijack and force to build new viruses. Because they have no metabolism of their own, researchers say viruses sit on the border between living and non-living.

Friend and foe

Many microbes are useful. Bacteria turn milk into yogurt, yeast bakes your bread, and in your gut a whole community of bacteria helps with digestion. Other microbes make you ill by spreading through your body. That is why hygiene helps: washing your hands removes many harmful microbes before they can enter the body.