Archaebacteria, eubacteria and mycoplasma

Archaebacteria thrive in harsh places: very salty water and soil (halophiles), hot springs (thermoacidophiles) and marshes (methanogens).

Their cell wall is built differently from that of other bacteria, and this wall is what lets them survive extreme conditions.

Methanogens live in the gut of ruminants such as cows and buffaloes and produce methane (biogas) from the dung of these animals.

Eubacteria, the 'true bacteria', come in thousands of kinds. They have a rigid cell wall and, if they move, a flagellum.

Cyanobacteria (blue-green algae) carry chlorophyll a like green plants and are photosynthetic autotrophs. They may be unicellular, colonial or filamentous, and live in fresh water, the sea or on land; colonies usually have a gelatinous sheath, and they often bloom in polluted water.

Some cyanobacteria, such as Nostoc and Anabaena, fix atmospheric nitrogen in special cells called heterocysts.

Chemosynthetic autotrophs get energy by oxidising inorganic compounds such as ammonia, nitrites and nitrates, and use it to make ATP. They matter greatly in recycling nitrogen, phosphorus, iron and sulphur.

Heterotrophic bacteria are the most abundant in nature, and most are important decomposers. Useful ones help set curd from milk, make antibiotics and fix nitrogen in legume roots; harmful ones cause cholera, typhoid, tetanus and citrus canker.

Bacteria reproduce mainly by fission; in bad conditions they form spores; and they show a primitive sexual reproduction by passing DNA from one bacterium to another.

Mycoplasma have no cell wall at all. They are the smallest living cells known, can survive without oxygen, and many cause disease in animals and plants.