ABSTRACT: Although few honey bee diseases are known to be caused by bacteria, pathogens of adult worker bees may be underrecognized due to social immunity mechanisms. Specifically, infected adult bees typically abandon the hive or are removed by guards. Serratia marcescens, an opportunistic pathogen of many plants and animals, is often present at low abundance in the guts of honey bee workers and has recently been isolated from Varroa mites and from the hemolymph of dead and dying honey bees. However, the severity and prevalence of S. marcescens pathogenicity in honey bees have not been fully investigated. Here we characterized three S. marcescens strains isolated from the guts of honey bees and one previously isolated from hemolymph. In vivo tests confirmed that S. marcescens is pathogenic in workers. All strains caused mortality when a few cells were injected into the hemocoel, and the gut-isolated strains caused mortality when administered orally. In vitro assays and comparative genomics identified possible mechanisms of virulence of gut-associated strains. Expression of antimicrobial peptide and phenoloxidase genes was not elevated following infection, suggesting that these S. marcescens strains derived from honey bees can evade the immune response in their hosts. Finally, surveys from four locations in the United States indicated the presence of S. marcescens in the guts of over 60% of the worker bees evaluated. Taken together, these results suggest that S. marcescens is a widespread opportunistic pathogen of adult honey bees and that it may be highly virulent under some conditions such as perturbation of the normal gut microbiota or the presence of Varroa mites that puncture the integument, thereby enabling entry of bacterial cells.IMPORTANCE Recently, it has become apparent that multiple factors are responsible for honey bee decline, including climate change, pests and pathogens, pesticides, and loss of foraging habitat. Of the large number of pathogens known to infect honey bees, very few are bacteria. Because adult workers abandon hives when diseased, many of their pathogens may go unnoticed. Here we characterized the virulence of Serratia marcescens strains isolated from honey bee guts and hemolymph. Our results indicate that S. marcescens, an opportunistic pathogen of many plants and animals, including humans, is a virulent opportunistic pathogen of honey bees, which could contribute to bee decline. Aside from the implications for honey bee health, the discovery of pathogenic S. marcescens strains in honey bees presents an opportunity to better understand how opportunistic pathogens infect and invade hosts.
