Singular 'bacterium.' Microscopic single-cell (unicellular) life form that exists practically everywhere in the Earth's environment, and is simpler than the cells of animals, fungi, and plants. Of about three million species of bacteria believed to exist, only about 4,000 are known and are divided into five general groups according to their shape: bacilli (rod shaped), cocci (ball shaped), filamentous (thread like), spirilla or spirochetes (spiral shaped), and vibrio (comma shaped). Some are free-living, harmless or beneficial (even essential) to other life forms, others are parasitic and produce toxins that cause disease or death.
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