Plasmids, their types and functions

  • Plasmids are extra-chromosomal DNA. They are circular and double stranded DNA molecules which encode traits that are not essential for bacterial viability (normal growth and reproduction).
  • They represent the discrete extra-chromosomal genetic elements both in Gram-positive and Gram-negative bacteria.
  • Plasmids are usually much smaller than the bacterial chromosomes. They range in size from less than 5 to more than 100kbp.
  • Plasmids can replicate independently of the bacterial chromosomes.
  • They can also be found integrated with the bacterial chromosomes.
  • Episomes are the plasmids that can integrate with the host chromosome.
                                  Plasmid map

Types of plasmids:

  • Plasmids can be classified on two bases:
    • Transmissibility property
    • Nature of the factor or function
  • Plasmids, depending on their transmissibility property are of three types:

1.Transmissible plasmid:

  • They can be transferred from cell to cell by the process of genetic transfer like conjugation, hence also known as conjugative plasmid.
  • They are large plasmids (mol. wt. 40-100 million) and contain more than a dozen genes (e.g. transferase gene) responsible for synthesis of the sex pilus and enzymes required for their transfer.
  • Usually 1-3 copies of such plasmid are present in the cell.

2.Non-transmissible plasmid:

  • These cannot be transferred from cell to cell during conjugation as they lack transferase gene.
  • They are small plasmids (mol. Wt. 3-20 million) and are non-conjugative.
  • They depend on their bacterial host for replication and are randomly distributed among daughter cells during division.


  • This plasmid lies either freely in circular form or gets integrated into the host chromosome

Depending on the nature of factors or function, plasmids are of the following types:

1.The F factor:

  • It contains genetic information essential for controlling the mating process of bacteria during conjugation.
  • This is also called fertility-factor or sex-factor.
  • The F plasmid consists of tra genes that regulate the  conjugative functions in bacteria.
  • These genes determine:
    • The expression of pili
    • Synthesis and transfer of DNA during mating
    • Interference with the ability of F+ bacteria to serve as recipients

2.The R factor:

  • It is also called resistance factor.
  • R factor or R plasmid occurs in two forms; large plasmids (mil. Wt. 60 million) and small plasmids (mol. Wt. 10 million).
  • The large plasmids are conjugative ‘R’ factors and help in conjugation process.
  • The small plasmids contain only ‘r’ genes and are not conjugative.
  • The resistance transfer factor (RTF) and resistant determinant (r) are the two  components of the R factor.
  • The RTF is responsible for conjugational transfer, while each r determinant carries resistance for one of the several antibiotics.
  • The R factors are responsible for spread of multiple-drug resistance among bacteria.

3.Colicinogenic (Col) factor:

  • It is also known as bacteriocinogenic factor.
  • The Col factor encodes for the production of bacteriocins (e.g. colicins, dipthericin, pyocyanin etc.), which are antibiotics-like substances that are specifically and selectively lethal to other closely related bacteria.

4.Virulence plasmid:

  • This plasmid codes for the virulence factor in some bacteria that increases it pathogenicity. E .g. exotoxin production in many bacteria.

5.Metabolic plasmid:

  • This plasmid helps in various metabolic activities in bacteria. E. g. root nodulation and N2 fixation genes of Rhizobium are present in its plasmid.

Functions of plasmids:

  • Many plasmids control medically important properties of pathogenic bacteria like:
    • Resistance to one or more antibiotics mediated by a variety of enzymes.
    • Production of toxins (exotoxins including enterotoxins like exfoliative toxin in aureus, hemolysins of C. perfringens, tetanus toxin in C.tetani etc.)
    • Synthesis of the cell surface structures required for adherence of colonization e.g. pili that mediate adherence of bacteria to epithelial cells are coded by K88,K99 found in uro-pathogenic coli.
    • Bacteriocins like colicins ( E.coli), dipthericin (C. diptheriae), pyocyanin(P. pyocyanea) etc. are encoded by plasmids.
    • Resistance to heavy metals such as mercury and silver that is mediated by the enzyme reductase is encoded by plasmid.

Plasmids, their types and functions