Bacteria are nearly everywhere. They are in the food we eat, in the water we drink, and in the air we breathe. They are in the ground we walk on. They are even inside us. They grow best where it is warm, dark, and damp.
There are more than a thousand different kinds of bacteria. They can all be sorted into three shapes. Some are round. Others are rod-shaped. Still others are twisted, or spiral. They may have tiny "hairs" that help them to swim about in water and in other liquids.
Bacteria never die of old age. As soon as one grows to its full size, it divides into two bacteria. It may take only 20 minutes for one of these microorganisms to grow up and then divide.
Some bacteria are disease germs. Some make our teeth decay. Some spoil food.
But many kinds of bacteria are helpful. Some take nitrogen from the air and put it into soil. Green plants can then use it. Some help make vinegar and cheese. More important, some bacteria make dead plants and animals decay. Without bacteria to cause decay, the earth would soon have no room for living things.
