1-3.2.1 Types of Plasmids

The plasmids are divided into 6 major classes as described below depending on the phenotype:

i)  Resistance or R plasmids carry genes which give resistance to the bacteria from one or more chemical agents, such as antibacterial agents. R plasmids are very important in clinical microbiology as they can have profound consequences in the treatment of bacterial infections. Eg: RP4 plasmid, which is commonly found in Pseudomonas and in many other bacteria.

ii)  Fertility or F plasmids are conjugative plasmid found in F⁺ bacterium with higher frequency of conjugation. F plasmid carries transfer gene (tra) and has the ability to form Conjugation Bridge (F pilus) with F⁻ bacterium. Eg: F plasmid of E. coli .

iii)  Col plasmids have genes that code for colicins, proteins that kill other bacteria. Eg: ColE1 of E.coli

iv)  Degradative plasmids allow the host bacterium to metabolize unusual molecules such as toluene and salicylic acid. Eg TOL of Pseudomonas putida.

v)  Virulence plasmids confer pathogenicity on the host bacterium. Eg: Ti plasmids of Agrobacterium tumefaciens, which induce crown gall disease on dicotyledonous plants.

vi)  Cryptic Plasmids do not have any apparent effect on the phenotype of the cell harboring them. They just code for enzymes required for their replication and maintenance in the host cell.

Based on the origin or source of plasmids, they have been divided into two major classes: such as natural and artificial.

i)  Natural plasmids: They occur naturally in prokaryotes or eukaryotes. Example: ColE1.

ii) Artificial plasmids: They are constructed in-vitro by re-combining selected segments of two or more other plasmids (natural or artificial). Example: pBR322.

1-3.3 Natural Plasmids

Few examples of naturally occurring plasmids and their characteristics are listed in table below

Plasmid	Size (kb)	Origin	Host range	Antibiotic resistance	Additional marker genes showing insertional inactivation
RSF1010	8.6	E.coli (strain K-12)	Broad host range	Streptomycin and sulfonamides.	None
ColE1	6.6	E.coli	Narrow host range	None	Immunity to colicin E1
R100	94.2	E.coli	E.coli K-12, Shigella flexneri 2b	Streptomycin, chloramphenycol, tetracycline	Mercuric (ion) reductase, putative ethidium bromide (EtBr) resistant protein.