Infections by filamentous phages influence bacterial fitness in various ways. While phage-encoded accessory genes, e.g., virulence genes, can be highly beneficial, the production of viral particles is energetically costly and often reduces bacterial growth. Consequently, if costs outweigh benefits, bacteria evolve resistance which can shorten phage epidemics. Abiotic conditions are known to influence the net-fitness effect for infected bacteria. Their impact on the dynamics and trajectories of host resistance evolution, however, remains yet unknown.
Combining experimental evolution, genomics, and mathematical modelling we show that phage epidemics are prolonged in sub-optimal environmental conditions, which reduce bacterial growth rate. The data set comprises populations dynamics data, including bacterial and phage density as well as evolutionary dynamics data, e.g. measured resistance of bacteria against the ancestral phage as well as the proportion of phage-infected clones in bacterial populations. Furthermore, growth data and phage production of individual clones are provided.
At reduced salinity, bacteria carrying the co-evolving phage remained twice as long in the bacterial population before being outcompeted by phage-resistant mutants. Our deterministic model suggests that the delayed presence of phage-infected clones and resistance evolution is driven by a lower bacterial growth rate at 7 PSU, which in turn reduces the encounter rate of phage and bacteria. Reduced absolute growth rates at low salinity and relaxed costs of phage carriage (i.e. reduced phage production) result in a prolonged presence of phage infected bacterial clones. These results suggest that environmental parameters, which reduce the costs of carrying a filamentous phage might increase the prevalence of filamentous phages in bacterial populations, even if they do not carry any known accessory genes.
