Bacteria are small organisms that can be found in all natural environments. Bacteria have been around for about three-quarters of Earth's history. While some bacteria, usually called germs, cause sickness, other types of bacteria are harmless or even helpful to humans.

Habitat

Bacteria are found in more environments than any other living thing. They exist in oceans, deserts, hot springs, and snow. They have been found at the bottom of lakes and rivers, deep underground in mines, and many miles high in the atmosphere. They can even live at temperatures too high or too low for other forms of life to endure.

Physical features

Bacteria are extremely tiny organisms that are made of a single cell. Most bacteria can be seen only with a microscope. Millions of bacteria could fit on the head of a pin.

Bacteria do not have most of the internal structures found in the cells of other organisms. They are much simpler and smaller than all other cells of living things.

Life cycle

Most bacteria reproduce by dividing down the middle to form two separate cells. After the two cells become mature, they each divide again to form a total of four cells. Some types of bacteria divide in this way as often as every 15 minutes. Through this process, billions of bacteria may be formed from a single bacterium in only 24 hours.

How bacteria live

Bacteria take in food and dispose of waste through their cell wall. Some kinds of bacteria need complex food to live, and others need only simple minerals.

How bacteria cause disease

Bacteria that cause diseases can get into the human body through the nose and mouth, as well as through cuts in the skin. Once inside, bacteria reproduce so fast that they can cause illness, which is called an infection. The infection may be brought about by the bacteria themselves or by their poisonous waste products, which are called toxins. Toxins are often more threatening than bacteria alone. Luckily, the human body produces defenses against both of these threats. These defenses are called antibodies.

Fighting disease

Bacteria that cause disease and the toxins they create can be used by medical scientists. For example, bacteria can be killed and then injected into the human body. This is called a vaccine. The body senses the dead bacteria and produces the same antibodies that it would make to protect itself from live bacteria. A vaccine protects people for a year or sometimes much longer, but only against the kind of bacteria from which the vaccine was made.