Competence (bacteria)

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Competence is the ability of cells to take up DNA that is freely present in the surrounding medium ; it is therefore a prerequisite for the transformation of bacteria.

Only certain types of bacteria, such as B. Bacillus subtilis , Streptococcus pneumoniae , Haemophilus influenzae or Neisseria gonorrhoeae show a natural competence. The genome of these bacteria has specific genes whose gene products are responsible for the recognition of DNA, its uptake into the cell and integration into the genome through recombination (the so-called com genes).

Natural competence was first discovered in 1928 by Frederick Griffith in " pneumococci " ( Streptococcus pneumoniae ), which took genetic information from a strain killed by heat treatment ( Griffith's experiment ) through transformation .

In 1944, Oswald Avery used the expertise in his famous experiments to detect DNA as the carrier of genetic information. John Spizizen discovered competence in B. subtilis in 1961 and described a special nutrient medium : When B. subtilis is cultivated in this medium, the growing bacterial culture shows maximum expression of the competence genes within a time window of around 30 minutes. This discovery facilitated the development of gene maps for B. subtilis and ultimately led to B. subtilis becoming a model organism for the genetics of gram-positive bacteria.

However, the competence is strictly regulated, is therefore not constitutive and can be used in certain ways, e.g. B. B. subtilis can be controlled via quorum sensing . When cultivated in Spizizen's special medium, the B. subtilis cells show a heterogeneity of their competence properties ; at the time of maximum competence, only about 30% of the cells in a culture are actually competent.

The intestinal bacterium Escherichia coli is not naturally competent, i.e. there is no complete set of competence genes in the E. coli genome . With the help of calcium chloride treatment and subsequent heat shock, however, artificial competence can be achieved through a mechanism that is not yet known in detail, albeit at a lower rate. Alternatively, DNA can be introduced into so-called electrocompetent bacteria (e.g. Agrobacterium tumefaciens ) by means of electroporation . The artificial competence of the E. coli cells produced in this way is measured in molecular biology in the form of the transformation efficiency. It is defined as the number of bacterial colonies that result from the transformation per µg of plasmid DNA. It is usually between 10 7 and 10 10 / µg. Vectors such as pUC18 or pBR322 serve as test DNA.

In nature, competence essentially serves the repair of mutated DNA segments and evolution through the uptake of foreign genes through horizontal gene transfer . In the case of pathogenic bacteria such as S. pneumoniae and N. gonorrhoeae , competence is also responsible for the transfer of antibiotic resistance genes. E. coli can utilize DNA as a C, N and P source with the help of the incomplete reservoir of the competence genes.

swell

  1. ^ Griffith F: The significance of pneumococcal types . In: Journal of Hygiene . 27, No. 2, January 1928, pp. 113-59. doi : 10.1017 / S0022172400031879 . PMID 20474956 . PMC 2167760 (free full text).
  2. ↑ Anagnostopoulos, C. & Spizizen, J. (1961): Requirements for Transformation in Bacillus subtilis. In: J. Bacteriol. Vol. 81, pp. 741-746. PMID 16561900
  3. ↑ Palchevskiy, V. & Finkel, SE (2006): Escherichia coli competence gene homologs are essential for competitive fitness and the use of DNA as a nutrient. In: J. Bacteriol. Vol. 188, pp. 3902-3910. PMID 16707682

literature

  • Maclyn McCarthy: The Transforming Principle - Discovery that Genes are made of DNA , WW Norton Company, 1985, ISBN 0-393-01951-9 (book on Avery's experiments)
  • Michaela Scherr & Dietmar Scherr (2003): The "Avery Experiment": Milestone in Molecular Biology. In: Biology in Our Time. Vol. 33, pp. 58-61. doi : 10.1002 / biuz.200390010
  • Claverys, JP et al. (2006): Induction of Competence Regulons as a General Response to Stress in Gram-Positive Bacteria. In: Annu. Rev. Microbiol. Vol. 60, pp. 451-475. PMID 16771651
  • Chen, I. & Dubnau, D. (2004): DNA uptake during bacterial transformation. In: Nat. Rev. Microbiol. Vol. 2, pp. 241-249. PMID 15083159 PDF ( Memento from April 19, 2008 in the Internet Archive )