Viruses of soil bacteria (phages) evolve to improve their ability to infect the bacterial hosts that surround them. This is shown in a new study by Dutch researcher Michiel Vos, published in the journal Science. Phages appear to be better able to infect bacteria from the same small soil sample than bacteria from just a few centimetres away. Evolution can therefore restructure ecosystems on a very small scale.
Working at the University of Oxford, Michiel Vos took 5 lots of 5 samples from an area of soil measuring 25cm by 25cm. From each sample, he isolated Stenotrophomonas bacteria and their associated phages. Phages infect bacteria, proliferate, burst out of the cell and then go on to infect new bacteria. More than a third of the bacteria were found to be sensitive to infection by phages from the same area of soil. Phages can therefore markedly control populations of soil bacteria.
Vos went on to investigate whether the phages were better at infecting their surrounding bacteria than bacteria from a few centimetres (further) away. This turned out to be the case: phages were better at infecting bacteria from the same soil sample than those from other soil samples. In the language of evolutionary ecologists, the phages are 'locally adapted'. Whether the bacteria in turn adapted to the phages (co-evolution) was not investigated in the experiment, but this is quite likely.
This study demonstrates the importance of interactions between different types of soil microbes in structuring of biodiversity. While microorganisms in the soil are tremendously abundant and diverse, and are key to ecosystem functioning, relatively little is known about them -- certainly compared with our knowledge of animals and plants.
The research undertaken by Vos and his colleagues shows that even at a scale of just a few centimetres, populations of microorganisms are not distributed at random. Instead, phages are found alongside the bacteria that they can best infect, by the process of natural selection. This is especially remarkable because tiny and numerous microorganisms could be expected to be easily dispersed, erasing such spatial structure.
The article on this research was published in Science on 14 August 2009. The research was funded by a Rubicon grant from NWO, which Michiel Vos received in 2006.
Materials provided by NWO (Netherlands Organization for Scientific Research). Note: Content may be edited for style and length.
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