The three types of horizontal gene transfer in bacteria are transformation, transduction and conjugation.

Transformation

Tranformation was first demonstrated by Frederick Griffith in 1928.  Griffin (knowing nothing about DNA) concluded that some "transforming principle" was passed from the dead smooth (pathogenic) strain to the live rough (nonpathogenic) strain.
 
 


In 1945, Avery, McCarty and McCloud demonstrated that this "transforming principle" was DNA.

Transformation only occurs naturally in bacteria that are competent, meaning there are alterations of the cell envelope that enable the cell to absorb naked DNA in the surrounding environment.  Genera that have naturally competent members include Streptococcus, Staphylococcus, Haemophilus and Pseudomonas.  Some bacteria, such as Escherichia coli, can be made competent under laboratory conditions and used in biotechnology.
 
 

Transduction

Transduction is viral-mediated horizontal transfer of genetic material.


In generalized transduction, random host-cell DNA is carried by the defective (tranducing) phage.  In specialized transduction, specific genes and gene sequences are carried.  Some bacteria, when infected by a specialized transducing phage, become pathogenic, since the genes for virulence (ability to cause disease) are carried by the phage.  This process is called lysogenic conversion, since the infected bacteria are not forced to enter into the lytic viral replication cycle.  Examples of bacteria that become pathogens in this way include Clostridium botulinum, Staphylococcus aureus, Legionella pneumophila, Escherichia coli O157:H7 and Yersinia pestis.
 
 
 
 

Conjugation


Conjugation involving the transfer of an entire plasmid is the most common form.  The F+ plasmid undergoes rolling circle replication, meaning that it is replicated as a linear single nucleoside rather than a complete circular strand of DNA as occurs in replication of the chromosome.  After passing through the pilus into the F- recipient cell, the complementary nucleoside is synthesized and the plasmid is linked into circular form by ligase.
 
 


Hfr conjugation occurs when a plasmid from the F+ donor, previously recombined with the chromosome to produce a new Hfr+ cell, is partially copied along with chromosomal genes by rolling circle replication and passed to the recipient cell.  The DNA fragment is completed and recombined with the recipient chromosome, but since all of the plasmid DNA was not transferred, the recipient remains an F- cell and cannot participate in further conjugation with other cells.
 
 
 


This genetic map has been produced for the circular Hfr genome of E. coli and divided into individual operons (outside labels) and the number of minutes (inside labels) it takes for each to be transferred during Hfr conjugation.