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Coevolution between bacteria and phages in soil: ecological and genetic bases of its specificity

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How bacteria and phages evolve together

Coevolution, where closely associated species influence the evolution of each other, can be seen in the relationship between a host and its associated parasite. An EU-funded project investigated coevolution between bacteria and bacteriophage in the soil environment to gain a clearer understanding of the process.

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The evolutionary 'arms race' between parasites and their hosts can have far-reaching consequences for ecology, agriculture and medicine. This is especially true for bacteria and the bacteriophages that infect and replicate inside them. Phages are ubiquitous in nature and are employed as antibiotics, whereas bacteria play a crucial role in the function of ecosystems. The 'Coevolution between bacteria and phages in soil: Ecological and genetic bases of its specificity' (SOILCOEV) project investigated how the evolutionary relationship between bacteria and phages is maintained. Two different approaches were used to study how host and parasite evolve together in the natural environment. Scientists inoculated and studied a sterilised and simplified ecosystem (known as a microcosm). This was done using the well-understood coevolutionary system Pseudomonas fluorescens SBW25 and a virulent associated phage, both with and without the natural microbial community. The results indicated that genetic variation was not limited by a lack of evolution as the soil environment, presence of phages and the microbial community act to strengthen selection. Researchers also studied how phages interact with the rest of the natural viral and microbial community in soil to drive the evolution of P. fluorescens SBW25. After 48 days, the mutation rates of P. fluorescens were examined, revealing that interactions between viruses and the rest of the soil microbial community played a minor role in driving evolution. Further experiments between wild type and mutator bacteria in soil suggested that other factors in the soil environment could be responsible for selecting for relatively low mutation rates. These results could explain why bacteria with high mutation rates are found at relatively low frequencies in soil, but at much higher frequencies in the lab and in clinical infections. Increased resource availability led to greater bacterial resistance to the phages, but without an attendant increase in phage infectivity or change in coevolution. This suggests that the phages may be less effective in controlling bacterial densities and communities in stable environments with high levels of resources. SOILCOEV provided valuable insights into the way in which bacteria and phages evolve together in natural environments. This will give scientists a clearer understanding of the relationship between hosts and their parasites at the microbial level.

Keywords

Pseudomonas fluorescens, SBW25, parasite, host, bacteria, phage, coevolution, soil environment, microbial community

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