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Bacteria (singular; bacterium)
Microscopic unicellular organisms which lack rigid cell walls and a well-defined nucleus. They reproduce by binary fission, approximately every 20 minutes - so a single cell is capable of producing 16 million copies of itself in a 24 hour period. Although generally considered harmful, certain types of bacteria are essential in many food and industrial processes. "Good" bacteria are present in foods such as "live" yoghurt, and it is now thought that their introduction into the human gut helps to protect us against many diseases.