The long-term objective of our research is to understand molecular basis by which bacteria interact with animal hosts in both mutualistic (beneficial for both the bacterium and the host) and pathogenic associations (harmful to the host).  We investigate these processes in the tripartite system that includes Steinernema parasitic nematodes and their mutually beneficial Xenorhabdus bacterial symbionts. Together, this symbiotic pair infect, kill, and reproduce inside insect hosts. Our approaches use genetic, “omic”, biochemical, and molecular biological techniques to investigate the mechanisms by which hosts recognize and respond to bacteria. For example, we have active lines of research on the regulation and function of genes required by Xenorhabdus bacteria to associate mutualistically with the nematode host, and to independently kill the insect host. Also, we examine insect immune responses to infection and nematode cellular structures involved in bacterial symbiont recruitment and transmission between generations.