Supplementary material from Filamentous phages reduce bacterial growth in low salinities

Being non-lytic, filamentous phages can replicate at high frequencies and often carry virulence factors, which are important in the evolution and emergence of novel pathogens. However, their net-effect on bacterial fitness remains unknown. To understand the ecology and evolution between filamentous phages and their hosts, it is important to understand (i) fitness effects of a filamentous phages on their hosts and (ii) how these effects depend on the environment. To determine how the net-effect on bacterial fitness by filamentous phages changes across environments, we constructed phage–bacteria infection networks at ambient 15 practical salinity units (PSU) and stressful salinities (11 and 7 PSU) using the marine bacterium, Vibrio alginolyticus and its derived filamentous phages as model system. We observed no significant difference in network structure at 15 and 11 PSU. However, at 7 PSU phages significantly reduced bacterial growth changing network structure. This pattern was mainly driven by a significant increase in bacterial susceptibility. Our findings suggest that filamentous phages decrease bacterial growth, an indirect measure of fitness at stress-full environmental conditions, which might impact bacterial communities, alter horizontal gene transfer events and possibly favour the emergence of novel pathogens in environmental Vibrios.