Plasmid refers to small, mobile piece of DNA found in most bacterial species and in some species of eukaryotes.
Plasmids are separate from the bacterial chromosome but still multiply during cell growth.
Their size ranges from 3 percent to 20 percent of the size of the chromosome. There is usually only one copy of a single plasmid per cell, but occasionally several are found. Some plasmids carry 'fertility genes' that enable them to move from one bacterium to another and transfer genetic information between strains. Plasmid genes determine a wide variety of bacterial properties including resistance to antibiotics and the ability to produce toxins.