plasmid /plaz′mid/ [Gk, plasma, something formed] , in a bacterium, a small, circular molecule of DNA that is separate from the bacterial chromosome. Plasmids often carry genes that affect the ability of bacteria to respond to environmental challenges. For example, a bacterium containing the R (resistance) plasmid is able to resist many antibacterial drugs that act in different ways. Plasmids may be passed from one bacterium to another and are replicated in later generations of any bacterium carrying them. Molecular geneticists often use plasmids to insert specific genes into the chromosomes of bacteria and other organisms.
