Bacterial Genetic Exchange


Exchange of genetic material between bacterial species is mediated by the basic processes of conjugation, transduction and transformation. This exchange is fundamental for bacterial evolution and adaptation in a dynamic environment.

Keywords: conjugation; genetic exchange; integrons; pathogenicity islands; transduction; transformation

Figure 1.

Conjugation. Conjugation is the transfer of a plasmid or other self‐transmissible DNA element and sometimes chromosomal DNA from a donor cell to a recipient cell via direct contact, usually mediated by a conjugation or sex pilus. Cell‐to‐cell contact is required and the process is resistant to DNAase. In this example the F plasmid (F+) is being transferred from a donor cell to a recipient cell. The donor is resistant to the antibiotic tetracycline (tetr) which is encoded by the F plasmid. The F plasmid DNA is transferred to the recipient cell as a single‐stranded (ssDNA) bound by ssDNA‐binding protein. The complement DNA strand is synthesized in the recipient cell and the resulting transconjugant is tetr and can act as a donor cell.

Figure 2.

Transduction. Transduction is the transfer of bacterial DNA from a donor cell into a recipient bacterium via a virus particle. Transduction requires phages and is resistant to DNAase. (a) Generalized transduction. The bacterial chromosome is degraded as a result of an initial round of phage infection. Portions of the degraded chromosome encoding genes (A, B, C, etc.) are mistakenly packaged into phage particles during phage assembly. These particles can now act as generalized transducing phages which are able to deliver their portion of the bacterial chromosome to another bacterium during a second round of phage infection. (b) Specialized transduction. After initial infection, phage DNA is integrated into the host cell genome at a specific site (lysogeny). During induction (the replicative stage of phage development) the phage genome can be excised intact or aberrantly. Intact phage genomes are packaged into phage particles resulting in mature normal phage. Aberrantly excised phage genomes contain host cell genes and are packaged into phage particles producing specialized transducing phage. The specialized transducing phage are now able to introduce the host cell DNA into another bacterium during a second round of infection.

Figure 3.

Transformation. Transformation is the transfer of free DNA, present in the extracellular environment, to a recipient bacterium. In the example free DNA binds to the recipient cell membrane in a protein‐dependent manner. The DNA is transferred across the membrane as a single strand via a DNA translocation apparatus, bound by single‐stranded DNA‐binding protein, and subsequently pairs with homologous DNA sequences on the recipient's chromosome. Homologous recombination occurs and the new DNA is assimilated into the chromosome. Newly acquired traits (genes responsible for lactose utilization in the example) encoded by the assimilated DNA are expressed. The process of transformation is sensitive to DNAase.

Figure 4.

Integrons. Integrons are site‐specific recombination elements often found within transposons or defective transposons that capture and mobilize bacterial‐derived genes, especially multiple antimicrobial resistance gene cassettes. In the example, the integrase IntI integrates a circular piece of DNA encoding kanamycin resistance (kanr). The integration occurs at a specific attachment site (attI), immediately downstream of a strong promoter (Pant). This results in two adjacent antimicrobial resistance genes encoded by the integron: kanr and sulfonamide resistance encoded by sulI.



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Further Reading

Brock TD, Madigan MT, Martinko JM and Parker J (1994) The Biology of Microorganisms, chap. 7. Englewood Cliffs, NJ: Prentice‐Hall, Inc.

Dreiseikelmann B (1994) Translocation of DNA across bacterial membranes. Microbiological Reviews 58: 293–316.

Hall RM and Collis CM (1995) Mobile gene cassettes and integrons: capture and spread of genes by site‐specific recombination. Molecular Microbiology 15: 593–600.

Matic I, Taddei F and Radman M (1996) Genetic barriers among bacteria. Trends in Microbiology 4: 69–72.

Snyder L and Champness W (1997) Molecular Genetics of Bacteria, chaps 5–7. Washington, DC: American Society for Microbiology.

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McGee, David J, Coker, Christopher, Harro, Janette M, and Mobley, Harry LT(Apr 2001) Bacterial Genetic Exchange. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001416]