In symbiotic associations animals, including humans, interact with bacteria that can be either beneficial or harmful. Currently there is limited knowledge of how bacterial adaptation to different host environments dictates whether a particular microbe will cause benefit or harm, despite the important implications of such knowledge in preventing or promoting specific bacterial interactions. This research focuses on understanding how bacteria sense and respond to different host environments to initiate expression of either beneficial or harmful traits. The bacterium being studied expresses beneficial traits when it is associated with a small soil-dwelling roundworm (nematode), and expresses virulence traits when it is associated with insect larvae. As part of its natural life cycle, the nematode transmits (vectors) the bacterium into the insect larva. In this new environment, the bacterium expresses virulence traits, and kills the insect. The insect cadaver then serves as a nutrient source for reproduction and further dissemination of both the nematode and bacteria. This research will examine the complex molecular circuitries that coordinate and regulate the expression of beneficial and harmful traits as the bacteria move between different animal hosts. The paradigms revealed by this research will provide broad insights into such questions as: how does bacterial adaptation to new environments alter symbiotic behavior, and how do vector-borne disease-causing bacteria adapt during transmission by the vector?
The results of this research will be published and disseminated to the public through community seminars and high school outreach programs. Undergraduate, graduate, and post-doctoral students will be trained in areas of molecular biology, nematology, entomology, and bacteriology through direct scientific inquiry. Thus, this project will provide fundamental knowledge of bacterial interactions with animal hosts, and will train future scientists with the expertise to further apply this knowledge.
All organisms, including plants and animals, exist in symbiotic association with other species, particularly bacteria. Symbiotic bacteria can result in both benefit (mutualism) and harm (pathogenesis) to the animals and plants that host them. Our abilities to control harmful bacteria and promote beneficial bacteria will be facilitated by understanding the fundamental molecular rules that govern communication between host organisms and their associated bacteria. Investigating naturally occurring, experimentally tractable host-bacterial associations can offer insights into the molecular underpinnings of symbiosis. One such association is that between the bacterium Xenorhabdus nematophila and its small, soil dwelling nematode (roundworm) host, Steinernema carpocapsae. Together these two symbiotic partners infect, kill, and reproduce within insects, including those of agricultural importance. Since all three organisms (bacteria, nematodes, and insects) can be manipulated in a laboratory setting, the system serves as an excellent model to investigate both beneficial (nematode-bacteria) and pathogenic (insect-bacteria) interactions. Intellectual Merit Under this NSF award we used genetic and molecular techniques to investigate the processes by which the bacterial symbiont senses and responds to its nematode or insect host environments to initiate expression of either beneficial or harmful traits respectively. We discovered that a protein, known as Lrp, is a regulator of gene expression, controlling which traits are expressed at each stage of bacterial symbiosis with nematodes and insects. We acquired experimental data supporting our model that the symbiotic behavior of the bacterium is dictated by levels of Lrp protein in the bacterial cell. Lrp levels naturally vary within a population, and cells with high Lrp levels are mutualistic with nematodes but are not pathogenic to insects. Conversely, cells with low levels of Lrp are virulent in insects, but are not mutualistic with nematodes. We theorize that Lrp at high or low concentrations in the cell have different DNA binding characteristics, leading to distinct sets of genes being expressed. Our data indicate that random switches in regulation that can be influenced by host environment are responsible for the population variation in Lrp levels. Based on our findings we conclude that 1) mutualistic and pathogenic behaviors of the symbiotic bacterium are inversely controlled by Lrp and 2) host interaction behaviors are regulated by Lrp throughout the bacterial life cycle to ensure at each stage the appropriate expression or repression of necessary or harmful factors respectively. These conclusions are relevant in the broader context of understanding how bacteria control gene expression during transitions between different environments. For example, similar random but environmentally responsive switches controlling levels of regulatory proteins may be responsible for necessary changes in genes expressed when a plant pathogen moves from the soil to the plant environment, or when a human pathogen moves from an insect vector into the human bloodstream. Broader Impacts: The results of this research were disseminated to the public through community seminars (e.g. Wednesday Night @ the Lab, broadcast on public television, and PODCALS, an audio interview available at http://fyi.uwex.edu/news/2011/09/23/ants-in-the-cupboards/)) and K-12 outreach programs (e.g. the PEOPLE Program for URM entering high school students, and Expanding Your Horizons for middle school girls). It served as the training research for three post-doctoral fellows (two now Assistant Professors and one pursuing a career in industry) and four graduate students (one under-represented minority), including two who have graduated and secured faculty or administrative positions at research universities. The project provided hands-on independent research experiences for six undergraduates, including two under-represented minorities. Several have now graduated and are pursuing Ph.D. graduate studies. The project also provided the basis for a senior capstone undergraduate independent lab experience (~15 undergraduates), with the goal of using deep sequencing to determine insect microbial communities at different larval stages. The project yielded two Journal of Visualized Experimentation videos intended to provide guidance to scientists wishing to investigate the bacteria-nematode-insect system. Together these videos have been viewed a total of 5761 times and requested by numerous researchers, including those from other countries (e.g. Ecuador, Brazil, Japan, Thailand).
