Fundamentals of Microbiology

3.4.3.4 Plasmids

Some bacterial cells contain one or more small circular macromolecules of DNA that store additional specialized information. These are called plasmids (extra chromosomal DNA). Plasmids contain only 1 – 5% as much DNA as in the bacterial chromosome (roughly about 20 genes) which supplement the essential genetic information contained in the bacterial chromosome. However, the genetic information contained in plasmids can be important, in establishing characters such as resistance to antibiotics and tolerance to heavy metals. Thus the gene products of plasmids may permit the survival of bacteria under conditions that are normally unfavourable for growth and survival.

Plasmids can be transferred from one bacterial cell to another, sometimes even from one bacterial species to another.