Most bacteria also contain smaller places DNA that are not part of the chromosome. These extra chromosomal circles of DNA are called plasmids. Although certain plasmids can integrate into the chromosome, most plasmids remain autonomous in the cytoplasm. The genes carried on plasmids specify different traits from those on the chromosome, traits that normally are not essential to the organism.
Gene on the plasmids may encode information for such advantageous traits as an increase in metabolic options, resistance to antibiotics, or the capacity to synthesize compounds toxic to competing bacteria.