Bacteria

Bacteria are tiny one-celled micro-organisms, 0.001 mm in diameter, which need a light microscope with very high magnification to see them. They are best seen with an electron microscope.

Bacteria have a different way of growing to fungi. They multiply by cell division, one cell dividing to make 2 cells and so on. Sometimes the cells move apart and they live as separate individuals or they can stay together to form a colony ( a few cubic mm in volume) as they do when cultured in the laboratory on an agar plate. Fungal cells are different. They divide and form branching networks of filaments (mycelia) in the soil and can explore the soil environment much more effectively than bacteria.

Bacteria form resistant spores when the climate turns harsh. They can survive droughts in a spore stage and when favourable conditions return, they can germinate and grow again.

The main food of bacteria is organic matter in leaf litter, dung, soil and carrion. They (along with fungi) are the main decomposer groups in soil because of the huge number of digestive enzymes they can secrete onto organic matter to break it down.

Some soil bacteria can fix atmospheric nitrogen, ultimately making it available to plants. Some of these live free in the soil e.g. Azotobacter ,while others fix nitrogen while living in nodules on the roots of legumes e.g. Rhizobia. Here, these bacteria gain food in the form of sugars from the plants and the plant benefits through increased nitrogen fixed by the bacteria. This "win-win" relationship between bacteria and plant is called "symbiosis" - or a relationship where both organisms gain a mutual benefit from living together.

Diseases from soil bacteria:
Not all soil bacteria are "goodies" as soil-borne diseases can attack plants and animals. Anthrax and tetanus, which infect domestic animals and humans, lurk in soil as does the bacterial blight of tomatoes.