A polyvalent phage shapes bacterial dynamics

Bacteria and the viruses that infect them (known as bacteriophages or phages) are important microbial ecosystem members. Antagonistic interactions between different bacteria or between bacteria and phages can profoundly impact population dynamics. Lytic phages are efficient killers that generally infect strains from a single genus or species. Polyvalent phages that target multiple unrelated hosts have been described, but their ecological significance is largely unknown. Here, we investigated how a polyvalent phage (PSA39) alters bacterial dynamics during co-cultures with susceptible hosts. <i>Pseudomonas aeruginosa</i> and <i>Stenotrophomonas maltophilia</i> are unrelated bacterial species that inhabit the same ecological niches, are often co-members of microbial ecosystems, cause similar infections, share mobile genetic elements, and can engage in complex interactions. In the presence of <i>P. aeruginosa</i> and <i>S. maltophilia</i>, PSA39 significantly reduces the recovery of both bacteria but has a stronger impact on <i>S. maltophilia. P. aeruginosa</i> adapts in the presence of PSA39, but <i>S. maltophilia</i> survival is impaired when the two bacteria are grown together and with phage. Furthermore, propagation in the presence of <i>S. maltophilia</i> cells results in higher viral titers. Both bacterial species evolve mutations in pili genes when exposed to PSA39. We propose that <i>P. aeruginosa</i>, <i>S. maltophilia</i>, and PSA39 can serve as a model system to study how polyvalent phages alter co-existing bacterial populations.IMPORTANCEPhages are the most abundant biological entity on the planet, but polyvalent phages that infect multiple bacterial species are poorly understood. Here, we investigated how the polyvalent phage PSA39 affects two susceptible but unrelated bacterial hosts (<i>Pseudomonas aeruginosa</i> and <i>Stenotrophomonas maltophilia</i>). During co-cultures with <i>S. maltophilia</i>, <i>P. aeruginosa</i> quickly develops resistance to this virus and has an antagonistic effect on its bacterial competitor. We find that both bacterial species evolve mutations in Type IV pili genes to resist PSA39 lysis. Our study provides novel insights into the impact that polyvalent phages can have on susceptible bacteria, such as those from natural environments or from infections.