Rhodococcus rhodnii boosts host tolerance and mitigates virulence of Serratia species in Rhodnius prolixus

Insects contend with myriad microbes throughout their lives. These microbial interactions, ranging from beneficial to pathogenic, have important consequences for the insect host, and simultaneous infection with multiple microbes is the norm. We are increasingly aware of how within-host interactions between the endogenous microbiota and invading pathogens can influence host infection outcomes. In this study, we examined how symbiont-pathogen interactions influence pathogen infection in the insect Rhodnius prolixus and how virulence can vary drastically with the context of infection, including cases where the endogenous microbiota is disrupted. Leveraging our ability to rear symbiont-free, or aposymbiotic, individuals, we first challenged symbiotic and aposymbiotic R. prolixus with three Serratia strains of varying virulence (two virulent Serratia marcescens strains and one avirulent Serratia ureilytica strain). We further altered the context of infection by varying the order of symbiont and pathogen colonization, as well as the method of inoculation (i.e., ingestion vs injection). Within R. prolixus , we showed a strong mitigating effect on virulence with the presence of Rhodococcus rhodnii , a well-established R. prolixus symbiont, despite high Serratia burdens. Virulence varied not only by Serratia strain but also by host species, with R. prolixus being particularly susceptible to infection among the four insect species tested. Our findings corroborate the importance of context in determining pathogen virulence, with symbiont presence, in particular, playing a key role in the outcome of pathogen infection for R. prolixus . IMPORTANCE Throughout their lives, insects encounter many bacteria, some beneficial and some detrimental. Increasingly, we are aware of how resident bacteria in the insect gut impact the ability of pathogenic microbes to establish and cause harm. Understanding the factors that underlie the specificity of these interactions is important in furthering our ability to leverage pathogenic microbes for the control of insect pests. This study focuses on the kissing bug, Rhodnius prolixus , a medically important insect that transmits Chagas disease. We tested the impact of different conditions in determining the virulence of the pathogen Serratia marcescens . Kissing bugs suffered high levels of mortality when infected with Serratia , particularly compared to other insect species. We determined that the presence of a beneficial bacterium ( Rhodococcus rhodnii ) in the kissing bug mitigated the ill effects of infection with Serratia . Early establishment of the beneficial bacteria was key to maximizing this effect.