+A bacterium is a single-celled [microbe](/wiki/Microbe) lacking a membrane-bound nucleus, classified as a [prokaryote](/wiki/Prokaryote). These ubiquitous organisms are among the most ancient forms of [life](/wiki/Life) on Earth, playing critical roles in all ecosystems.
+## See also
+- [Cell](/wiki/Cell)
+- [DNA](/wiki/DNA)
+- [Virus](/wiki/Virus)
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