Bacteria are single-celled microbes that reproduce by splitting in two. Each individual cell is capable of carrying out all of the activities needed to metabolise and reproduce. Bacteria grow best in moist conditions and can live in a wide range of temperatures. They cannot grow at low pH (i.e. in acid conditions). In the right conditions of warmth, acidity and moisture they can multiply very fast indeed, producing millions of cells in a few hours. Some bacteria form spores which are resistant to drying and heating. When conditions become favourable again to growth, they germinate and an active vegetative cell is released.
There are more than 5,000 known species of bacteria, with new ones constantly being discovered. Each cell is bounded by a wall which maintains the shape of the bacterium and gives it protection.

Bacteria have four basic shapes: spheres, rods, spirals and commas. They can be found as single cells, in pairs, chains or clusters.
Some bacteria can move. Usually this is by the action of corkscrew-like appendages called flagella, which rotate from the base like a ship's propeller. The flagella may be distributed randomly over the whole cell surface, in groups or singly.

Some bacteria have numerous fringe-like projections called fimbriae which enable them to stick to each other. Other bacteria produce a sticky substance around the cell wall. This provides protection and helps them to stick to substrates, as well as each other.