Abstract
Phages exert strong selective pressure on bacterial hosts, yet the role of abiotic factors in resistance evolution is often overlooked. Abiotic effects can shape both demographic factors, such as encounter rates, and trade-offs between resistance and fitness. We used the salt-marsh bacterium Prodigiosinella confusarubida (a.k.a. Serratia sp. ATCC 39006) to examine how salt concentration shapes resistance evolution after exposure to a virulent myovirus phage (LC53), which infects via an outer membrane protein (OmpW). We find that resistance only emerges under low-salt conditions via mutations in the regulatory region upstream of ompW or within ompW. This effect was independent of phage type or receptor utilization, as similar resistance patterns appeared when using a flagella-tropic siphovirus (JS26). These findings suggest that indirect effects influenced by salt, such as bacterial and phage population sizes and encounter rates, play a crucial role. Overall, these results may help explain patterns of bacterial and phage genomic diversity in natural microbial communities, with consequences for predicting phage resistance evolution in applied contexts, such as phage therapy and the food industry.