Mutations in filamentous bacteriophages spark eco-evolutionary feedbacks in Pseudomonas aeruginosa

Microbial populations strongly shape their environment, which can re-route adaptation toward organism-generated fitness optima. However, the conditions that promote these eco-evolutionary feedbacks are unclear. Here, we used experimental evolution to test whether high population density, by strengthening environmental construction, drives eco-evolutionary feedbacks in the bacterial pathogen Pseudomonas aeruginosa MPAO1. Unexpectedly, we found that endpoint populations had higher performance than the ancestral strain in organism-modified media across nearly all evolutionary lines regardless of population density. This was caused by the emergence of hyperactive filamentous bacterio(phage) mutants during experimental passaging, which inhibited the ancestral strain but not endpoint populations in modified media. Hyperactive phages emerged from one of two avirulent prophages in MPAO1s genome during experimental passaging. Hyperactive phages drove the evolution of phage resistance in bacterial populations via mutations in the type IV pilus (TIVP), the phages surface receptor. TIVP mutations pleiotropically reduced motility and decreased susceptibility to a TIVP-targeting virulent phage, both of which are important traits for P. aeruginosa infection and treatment. Overall, this work suggests that filamentous phage evolution can act as a driver of eco-evolutionary feedbacks in bacterial populations, causing phenotypic and genetic changes that would not be anticipated from adaptation to the extrinsic environment alone.