Plasmids are closed, circular pieces of DNA that are able to self--replicate and are carried by many bacteria. They provide unique functions for bacteria by allowing them to sexually replicate and to pass on genetic material between each other. Plasmids are also responsible for the genetic factors that give resistance to antibiotics, and provide the enzymes needed to break down poorly metabolised food resources. The author has provided an updated treatment of the structure, function and application of plasmids suitable for undergraduates and medical students. Employing an original teaching perspective----examining plasmids as living organisms with either a symbiotic or parasitic mode of survival----this text provides an important framework for understanding the structure and function of plasmids in an evolutionary context.* The most up to date text on plasmids* An innovative teaching perspective makes for easy student understanding* Contains crucial chapters on the importance of plasmids for clinical and biological research