Plasmids have been classified into five main types on the basis of their phenotypic function encoded by the genes they contain. These five plasmid types are as follows:
- R plasmids: These are plasmids carrying genes that encodes for resistance to antibiotics. Plasmids may confer on their host resistance to a single or multiple antibiotics encoded by resistance genes against antibiotics. These antibiotics are ampicillin, streptomycin, sulfomiamide, chloramphenicol, and tetracycline. These are very important in clinical microbiology. Their spread through natural populations has profound consequences in the treatment of bacterial infections.
- Col plasmids: They code for colicins, proteins that kill other bacteria. The Co1 proteins of E. coli are encoded by plasmids, such as ColEl.
- F plasmids: Plasmids containing the F or fertility system required for conjugation. A most common example is the F plasmid of E. coli.
- Degradative or catabolic plasmids: These plasmids allow host bacterium to metabolize unusual molecules usually recalcitrant (xylenes or naphthalenes), or various pesticides. They are generally found in pseudomonads and related organisms.
- Virulence plasmids: Plasmids that are able to confer pathogenicity on a host bacterium by the production of toxins or other virulence factors. For example, Ti plasmids of Agrobacterium tumefaciens.