Abstract
Horizontal gene transfer (HGT) is a main mechanism of bacterial evolution endowing bacteria with new genetic traits. The transfer of mobile genetic elements such as plasmids (conjugation) requires the close proximity of cells. HGT between genetically distinct bacteria largely depends on cell movement in water films, which are typically discontinuous in natural systems like soil. Using laboratory microcosms, a bacterial reporter system and flow cytometry, we here investigated if and to which degree mycelial networks facilitate contact of and HGT between spatially separated bacteria. Our study shows that the network structures of mycelia promote bacterial HGT by providing continuous liquid films in which bacterial migration and contacts are favoured. This finding was confirmed by individual-based simulations, revealing that the tendency of migrating bacteria to concentrate in the liquid film around hyphae is a key factor for improved HGT along mycelial networks. Given their ubiquity, we propose that hyphae can act as focal point for HGT and genetic adaptation in soil.
Highlights
We propose two driving mechanisms for the increased contact probabilities in the hyphosphere: Reduction of the space available for bacterial dispersal and, secondly, reduction of the degree of freedom of bacterial mobility
Such reasoning becomes clear by comparing a scenario in which bacterial cells populate a cube of water with a scenario in which cells only populate a water film surrounding hyphae forming a network within a cube of the same volume
Presuming bacterial mobility within the liquid film around hyphae resembles a two- rather than a three-dimensional movement, the calculated increase of contact probability of two cells amounts to a factor of approximately 100 ⋅ t / s; i.e. that the contact probability in the lower-dimensional domain increases over time (t) and surpasses the probability in the higher-dimensional domain by two orders of magnitude already after a very short time (~1 s)
Summary
HGT frequencies in systems with or without transport networks were quantified on (semi-) solid ABC-media[21] containing varying amounts of agar to influence bacterial mobility. The aforementioned plasmid donor strain and Pseudomonas putida KT2440::yfp as potential recipient strain were inoculated at a separation distance of 7 mm on a continuous agar surface and the systems incubated for three days.
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