BACTERIA
Microbes are the oldest form of life on Earth. They are very small and some can multiply and mutate in minutes. Some types have existed for billions of years. These single-cell organisms are invisible to the eye, but they can be seen with microscopes. Microbes live in water, food, and the air you breathe. Known to scientists as prokaryotes, bacteria are the largest class of microbial creatures on Earth. During 3.5 billions years Earth has been a bacterial planet. Bacteria are very simple organisms -- single cells with no nucleus. Nevertheless, they eat, are capable of reproducing themselves, and have a membrane or wall that separates their insides from the outside world. Bacteria don't grow bigger; instead, they multiply by dividing. Each bacterium splits into two every few minutes, hours or days, depending on the species. Under ideal conditions they can increase geometrically, producing millions from a single organism in a short time.

HARMFUL OR HARMLESS
Bacteria, the most familiar of infecting agents, cause 90 percent of hospitalised infections in developing countries. Bacteria cause most of the serious, acute infections we get and are the first group of germs to produce chemical cures, which is to say, antibiotics. From bacteria we get typhoid, cholera, gangrene, diphtheria, anthrax, and most of the serious cases of dysentery, meningitis and pneumonia. But 95% of microbes are harmless. The skin, mouth, bowels and other places are filled with them. Many are necessary to survival because they help digest food or keep infectious organisms away. The pathogens are the ones that are able to make harmful chemicals or poisons. Tetanus, for instance, is caused by a bacterial poison. It's a collection of white cells in an area of infection. Disease-causing bacteria attract the white blood cell system so that white blood cells kill the invading germs by ingesting (dissolving). Pus for the most part is formed by the dead white blood cells with destroyed bacteria inside. It is the body's response to illness.

BACTERIA FEATURES
Bacteria vary in size from 0.1 to 50 micrometers of diameter. The average bacterium such as Escherichia coli measures 1 x 3 micrometers. They cannot be seen with the naked eye. There is one exception: the giant bacterium Euplopiscium fishelsoni. There are millions of different bacteria but only four distinct shapes: coccus, rod, spirillum and vibrion. It is impossible to distinguish them by just looking at their shapes. In order to study them they have to be isolated, just one kind of bacterium at a time. They are characterized by their preferences for:
1. Temperature
2. pH
3. Need for oxygen
4. Nutrients
Most of them need some water and moderate temperatures (from 20 to 40 ºC). When moving around bacteria are always looking for a substratum or attachment. This material or surface can be anything from a rock, to a tooth, a heart valve, an egg …The most common substratum is made of organic compounds that can be used as nutrients.

BACTERIAL ECOLOGY
Many bacteria are extremophiles because they live in extreme environmental conditions. Some live at temperatures above 80ºC, others even at 100ºC in boiling waters in hot springs (Yellowstone Natural Park).
Very though bacteria live around deep-sea vents, thousands of metres under the sea, in the dark and high pressure, where hot lava mixes with cold ocean water. At 3500 m, temperature reaches 400ºC,
Some bacteria live in ice, in the Poles. Others live below the Earth surface, in the deep biosphere. Trapped in the rocks they live on iron, sulphur, hydrogen or carbon dioxide.
Special bacteria live in:
1. High salt concentration, or
2. Acidic waters, or
3. Anaerobic conditions (without oxygen)
Finally some bacteria can survive in the vacuum and tolerate high exposure to radiations