Vertical gene transfer occurs between parents and offspring, and horizontal gene transfer is the transfer that may occur between organisms otherwise. It is in bacteria that horizontal gene transfer has been studied most extensively, particularly in the last decade. Conjugation is a type of sexual reproduction exhibited by some bacteria, the process involving the exchange of genetic material by means of a tube or bridge, the transfer of DNA occurring either in one direction or in both directions. Transduction involves the transfer of genetic material from one bacterium to another with the intermediation of a virus. Essentially, when the virus infects one bacterium, it often carries away pieces of that bacterium's genome, and those pieces, upon the infection of a new bacterium, become incorporated into another genome.
Transformation is a common laboratory technique used in genetic engineering.
One significant aspect of these modes of horizontal bacterial gene transfer is that in principle they make possible the horizontal gene transfer of genetic material from genetically engineered bacteria to other organisms without predictable results. So if genetically engineered bacteria are to be widely used in industry, for example, it is of some importance to have a thorough understanding of the mechanisms in order to limit any harmful ecosystem transfer effects. Phytoplankton and plankton are not equivalent.