Mycelial networks from soil oomycetes promote bacterial horizontal gene transfer.

The exchange of genetic material between bacteria, known as Horizontal Gene Transfer (HGT) is an evolutionary mechanism that allows bacteria to acquire new traits hundreds of times faster than they would do by mutation. This process normally occurs in nature in three different forms known as transformation, transduction and conjugation; And the speed at which each of these processes occurs directly affects the evolution of any bacteriome.

The soil provides sufficient bacterial richness and diversity for any of these processes to occur. However, the transfer of genetic material by conjugation requires the proximity of the bacterial cells and it is believed that this process does not occur among  bacteria discontinuously distributed. However, a group of researchers from the Helmholtz Center for Environmental Research – UfZ in Germany have shown that mycelia of oomycete organisms are able to promote the conjugation of bacteria geographically separated.

In the study published in Nature, the authors demonstrated that the mycelium of the oomycete Pythium ultimatum may act as a highway that allows the movement of  donor bacteria to the proximity of recipient bacteria, and thus promotes the conjugation of species spatially separated. The researchers performed a small but potent experiment, where they placed two different bacteria with an air gap of approximately 400 μm between them in a Petri dish and then inoculated the media with P. ultimatum. The donor bacterial strain had genetic information to express two fluorescent proteins (red and green), but due to the presence of an inhibitor in its genome, it could only express a red phenotype. In contrast, the recipient bacteria did not have any information to express a colorful genotype and only the movement of genetic information between the strains would allow it to express a green phenotype.

Figure 1 shows the interesting results of the research where the presence of green bacteria (transconjugants) in the media and along the mycelia provide evidence that conjugation occurred and that hyphae can act as focal point for HGT.

screen-shot-2016-11-27-at-10-55-11-pmFigure1. Adapted from Berthold et al., 2016 . A) Combined image of red and green fluorescence channels with an overlay of the mycelial structure shown in white. Transconjugants can be seen emerging along the network structures. Scale bars represent 500 μm. Images were taken 3 days after incubation. B) Combined epifluorescence and transmission light image showing mycelium of P. ultimum grown between separate agar pieces inoculated with donor (red) or wildtype (colourless) cells, respectively. Emerging transconjugants (green) are visible along the mycelium. Outlined area is shown in detail in (C). C) Combined image of red and green fluorescence channels showing the arrangement of donor and transconjugant cells along the mycelial segment.


Source: Tom Berthold, Florian Centler, Thomas Hübschmann, Rita Remer, Martin Thullner, Hauke Harms & Lukas Y. Wick. (2016). Mycelia as a focal point for horizontal gene transfer among soil bacteria. Scientific Reports.



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