Abstract
The survival of a bacterial pathogen within a host depends upon its ability to outmaneuver the host immune response. Thus, mutant pathogens provide a useful tool for dissecting host-pathogen relationships, as the strategies the microbe has evolved to counteract immunity reveal a host's immune mechanisms. In this study, we examined the pathogen Francisella novicida and identified new bacterial virulence factors that interact with different parts of the Drosophila melanogaster innate immune system. We performed a genome-wide screen to identify F. novicida genes required for growth and survival within the fly and identified a set of 149 negatively selected mutants. Among these, we identified a class of genes including the transcription factor oxyR, and the DNA repair proteins uvrB, recB, and ruvC that help F. novicida resist oxidative stress. We determined that these bacterial genes are virulence factors that allow F. novicida to counteract the fly melanization immune response. We then performed a second in vivo screen to identify an additional subset of bacterial genes that interact specifically with the imd signaling pathway. Most of these mutants have decreased resistance to the antimicrobial peptide polymyxin B. Characterization of a mutation in the putative transglutaminase FTN_0869 produced a curious result that could not easily be explained using known Drosophila immune responses. By using an unbiased genetic screen, these studies provide a new view of the Drosophila immune response from the perspective of a pathogen. We show that two branches of the fly's immunity are important for fighting F. novicida infections in a model host: melanization and an imd-regulated immune response, and identify bacterial genes that specifically counteract these host responses. Our work suggests that there may be more to learn about the fly immune system, as not all of the phenotypes we observe can be readily explained by its interactions with known immune responses.
Highlights
The outcome of any infection, whether it be clearance of the infecting pathogen, establishment of a persistent infection, or even death of the host is determined by contributions from both the host and the microbe [1]
We examine the host-pathogen interactions of Francisella novicida with an insect host, Drosophila melanogaster, and identify aspects of fly immunity that are most important for fighting F. novicida infection as well as the bacterial virulence factors that interact with each of these specific immune responses
Previous work using the Live Vaccine Strain (LVS) of F. tularensis demonstrated that F. tularensis grows to high bacterial levels within flies and causes a lethal infection [9]
Summary
The outcome of any infection, whether it be clearance of the infecting pathogen, establishment of a persistent infection, or even death of the host is determined by contributions from both the host and the microbe [1]. In addition to genes that allow the bacteria to invade host cells and obtain nutrients from its host, a subset of the virulence factors expressed by the microbe must address the need of the bacteria to counteract the host immune response. Exploring this complex interplay between host and pathogen can help us to understand bacterial pathogenesis and define the contributions of the host immune system to bacterial virulence. The humoral AMP response has been studied extensively and shown to be regulated by two pattern recognition pathways, Toll and imd which have been well-characterized and described, but the regulatory mechanisms of the melanization and Author Summary
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
