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Any small autonomously replicating piece of DNA found in the cytoplasm of bacteria. Examples of plasmids are the R factors (R = resistance), which can carry genes conferring antibiotic resistance. Some plasmids may reversibly insert themselves into the main chromoneme (bacterial chromosome), in which case they are called episomes. Some episomes are best regarded as bacterial viruses. Another example of an episome is the sex factor, or F factor. Possession of the F+ factor confers maleness upon a bacterium, as evidenced by the ability to transmit DNA through a sex pilus (conjugation tube) to a female (F-) bacterium.
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