Abstract
Even when successfully surviving an infection, a host often fails to eliminate a pathogen completely and may sustain substantial pathogen burden for the remainder of its life. Using systemic bacterial infection in Drosophila melanogaster, we characterize chronic infection by three bacterial species from different genera - Providencia rettgeri, Serratia marcescens, and Enterococcus faecalis–following inoculation with a range of doses. To assess the consequences of these chronic infections, we determined the expression of antimicrobial peptide genes, survival of secondary infection, and starvation resistance after one week of infection. While higher infectious doses unsurprisingly lead to higher risk of death, they also result in higher chronic bacterial loads among the survivors for all three infections. All three chronic infections caused significantly elevated expression of antimicrobial peptide genes at one week post-infection and provided generalized protection again secondary bacterial infection. Only P. rettgeri infection significantly influenced resistance to starvation, with persistently infected flies dying more quickly under starvation conditions relative to controls. These results suggest that there is potentially a generalized mechanism of protection against secondary infection, but that other impacts on host physiology may depend on the specific pathogen. We propose that chronic infections in D. melanogaster could be a valuable tool for studying tolerance of infection, including impacts on host physiology and behavior.
Highlights
Animal physiology is profoundly impacted by constant association with microbes
To test whether the relationship between infectious dose and chronic burden is general, we determined the bacterial load in flies that survive infection with P. rettgeri, S. marcescens and E. faecalis across a 5000-fold range of initial infectious doses over two weeks post-infection
For the two combinations that showed a significant block by infection interaction (P. rettgeri-P. rettgeri: p = 0.03, S. marcescens-P. rettgeri: p = 0.003), every block individually showed that flies carrying chronic infection and secondary infection survived better than flies only carrying the secondary infection, the magnitude of this difference varied among individual blocks
Summary
Animal physiology is profoundly impacted by constant association with microbes. Commensal microbes that live on epithelial surfaces, in the gut, and in other body compartments have recently come to prominent attention. Another source of microbial association comes from pathogenic infections that remain in the host even after symptoms resolve. We and others have observed that many bacterial infections in Drosophila melanogaster are never eradicated; they are only controlled into chronic persistence [1,2,3,4,5].
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