Detectable effects of bacterial phylogeny on virulence but not phage efficacy in an in vivo Galleria mellonella model

Abstract

Predicting the outcomes of bacteria–phage interactions is a central challenge in microbial ecology, particularly across heterogeneous host environments. Evolutionary relatedness among bacteria can explain variation in phage susceptibility in vitro, but it remains unclear whether this predictive relationship persists in complex host environments and to what extent relatedness also explains variation in other bacterial traits, such as virulence. Here, we used an in vivo infection model to quantify how bacterial phylogeny and host environmental context mediate bacterial virulence and bacteria–phage interactions. We infected 4608 Galleria mellonella larvae with 64 phylogenetically diverse Staphylococcaceae isolates, both with and without co-inoculation of the bacteriophage ISP, and recorded mortality and melanisation over 24 hours. We found that bacterial virulence varied among Staphylococcaceae strains, and that a large proportion of this variation could be explained by the evolutionary relationships between bacteria, indicating that phylogeny may be useful in predicting bacterial virulence. The addition of phage significantly improved the survival of G. mellonella larvae, with an average 15.2% increase in endpoint survival and 10.1% reduction in endpoint melanisation across Staphylococcaceae strains. Unlike previous in vitro studies, the evolutionary relationships between bacterial isolates could not explain variation in in vivo phage efficacy across Staphylococcaceae. Concurrently, we found no evidence of a correlation between in vivo and in vitro measures of phage efficacy across bacterial isolates, highlighting the importance of the host environment in shaping bacteria–phage dynamics.