Abstract
Bacterial infections are a major threat to public health. Pathogen host shifts-where a pathogen jumps from one host species to another-are important sources of emerging infectious diseases. However, compared with viruses, we know relatively little about the factors that determine whether bacteria can infect a novel host, such as how host phylogenetics constrains variation in pathogen host range and the link between host phylogeny and the infectivity and virulence of a pathogen. Here, we experimentally examined susceptibility to bacterial infections using a panel of 36 Drosophilidae species and four pathogens (Providencia rettgeri, Pseudomonas entomophila, Enterococcus faecalis, Staphylococcus aureus). The outcomes of infection differed greatly among pathogens and across host species. The host phylogeny explains a considerable amount of variation in susceptibility, with the greatest phylogenetic signal for P. rettgeri infection, explaining 94% of the variation in mortality. Positive correlations were observed between mortality and bacterial load for three out of the four pathogens. Correlations in susceptibility between the four pathogens were positive but largely non-significant, suggesting susceptibility is mostly pathogen-specific. These results suggest that susceptibility to bacterial pathogens may be predicted by the host phylogeny, but the effect may vary in magnitude between different bacteria.