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.