Bacteria are single-celled microscopic organisms that can also live in colonies. The cells are much smaller than animal or plant cells and have no distinct cell nucleus. About hundred bacteria would fit onto a single printed dot. Bacteria are all around us: some, like those that spread cholera and salmonella, are dangerous; others, like those used in the production of yoghurt and cheese, are useful. There are also millions of positive bacteria in the human body that activity assist in living processes, such as digestion (see Digestive System). Most bacteria get their energy from the breakdown of living or non-living organic matter. Bacteria are active and play an important role in the decay and decomposition of plant and animal remains, and in returning nutrients to the soil, for example in the production of compost.
