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A bacterium (plural: bacteria) is an organism belonging to the domain bacteria (in the three domain scheme). Traditionally classified as one of the five kingdoms, bacteria are microscopic, single-celled and their cell structure is relatively simple. Since they lack the nucleus and organelles of the more complex cells called "eukaryotes", bacteria are considered to be "prokaryotes".
This category has the following 2 subcategories, out of 2 total.