Impact of bile salts on coevolutionary dynamics between the gut bacterium Escherichia coli and its lytic phage PP01

Although host and parasites are typically embedded in complex abiotic and biotic environments our understanding of how environmental variation impacts on host-parasite interactions, including antagonistic coevolution (AC) is poorly understood. Nonetheless, previous studies using bacteria and bacteriophages have shown that variation in just one abiotic parameter can have profound effects not only on the type of AC dynamics observed but also the time-frames over which AC interactions can persist. Here, we investigated the effect of an important component of the abiotic human gut environment, bile salts, on AC dynamics between the bacterium Escherichia coli and the lytic phage PPO1 in an in vitro model system. In the absence of bile salts E.coli and PPO1 coevolved in a manner that is consistent with a directional arms race dynamic (ARD), with bacteria and phages evolving increasing resistance and infectivity ranges through time. However, in the presence of bile salts, evidence of directional coevolution was weaker and more variable across replicate communities. These effects may be explained, in part, by the negative effect of bile salts on both host and parasite population sizes; lower population sizes for both bacteria and phages will reduce encounter rates which in turn could mitigate the benefits of generalism in both host and parasite resistance and infectivity ranges that are observed for ARDs. The negative effect of bile salts on phage population size may also be partially independent of host population size as bile salts was found to negatively impact phage viability in the absence of bacteria, as well as reducing phage adsorption efficiency. Differences in bacterial morphological diversity between treatments were also noted, with the emergence of mucoid colonies in both bile salts and non-bile salts treatments but only in the presence of phages. These data contribute to the growing body of knowledge on how environmental variation can impact on interactions between hosts and parasites. More specifically, these results are particularly relevant to our understanding of how bacteria-phage interactions may be affected by different abiotic factors relevant to the complex environment of the human gut and have clear implications for the development of phage as therapeutics to target members of the gut microbiota and/or intestinal pathogens.