Most cells have a single chromosome, but some species may have two. Vibrio cholerae and Agrobacterium tumefaciens are examples of bacterial cells having two chromosomes.
The chromosome of a typical bacterial cell is free in the cytoplasm and located in an area called the nuclear region or nucleoid.
Since the amount of DNA necessary to provide the genetic information necessary for the life of a bacterial cell far exceeds the actual volume of the cytoplasm, the chromosome is folded into loops about 50,000 to 100,000 bp in length. These loops are maintained by RNA and proteins called bacteriohistones. The molecule is further twisted by the enzyme gyrase (topoisomerase).
Plasmids are extrachromosomal rings of DNA found in some bacterial cells. These are considerably smaller than the bacterial chromosome, ranging in size from a few thousand to several million bp.
Though plasmids are necessary for the daily life of the cell, they often contain genetic information that can enhance the microbe's ability to survive and pass it genetic structure to others. Plasmids are sometimes referred to as factors, based on the type of genetic information they contain:
Fertility (F) factors arre plasmids that carry genes necessary for a cell to transfer DNA to another compatible cell through a process called conjugation.
Resistance (R) factors carry genes that allow a cell to be resistant to antibiotics or other antimicrobial compounds.
Bacteriocidin factors carry genes for the production of toxins that kill other bacteria that might compete with the cell for nutrients and space.
Virulence plasmids carry genes that enable a nonpathogenic bacterium to become a pathogen.