5.1.1. Classical methods

The classical methods for genome modification of microorganisms are divided into two types:

• Selection of mutations arising spontaneously and induced by different physical and chemical factors of the environment. The spontaneous mutations are consequence of rearrangements in the heritable DNA molecule due to substitution of one nucleotide with another, the addition or deletion of one or more nucleotides or other types of reformations. A lot of spontaneous mutants arise due to the movement of transposable elements to new locations in the double strand DNA. Such kinds of elements are typical for plants, animals and microbes.
• Exchange of DNA between closely related organisms. This type of gene modification in microorganisms concerns introduction of new genetic information by chromosomal or plasmid DNA. This event happens when DNA from the chromosome of the donor microorganism is integrated into DNA of the recipient one. Being self replicating, plasmids transfer the DNA of the donor into recipient without integration with chromosomal DNA. Thus, plasmid DNA can be transferred to widely divergent organisms compared to the donor ones. The movement of the plasmid could be easily observed because of the marker borne by its molecule (for instance antibiotic resistance). Three different classical types of gene transfer are characteristic of bacteria. It is considered that these three mechanisms occur naturally:
  • DNA-mediated transformation (DNA is transferred as “naked” DNA)
  • Transduction – transfer of DNA is mediated by a virus
  • Conjugation – DNA is transferred during cell-to-cell contact between donor and recipient cells.