THE WORM BREEDER’S GAZETTE
VOLUME 19 NUMBER 4 | AUGUST 2013
“Bridging the Divide” abstract: A tripartite association: Steinernema
nematodes, Xenorhabdus bacteria, and insects
Heidi Goodrich-Blair 1
1
Department of Bacteriology, University of Wisconsin-Madison, Madison WI
Correspondence to: Heidi Goodrich-Blair (hgblair@bact.wisc.edu)
Although microbes influence the health and disease of all animals, we still understand little
about mechanisms underlying microbial specificity for hosts and tissues or microbial contributions to
host biology. To address these questions, my lab investigates a relatively simple tripartite association in
which nematodes of the genus Steinernema parasitize insects by virtue of their mutualistic association
with Xenorhabdus bacteria. The Steinernema-Xenorhabdus-insect symbiosis offers numerous advantages as a model to understand the specificity and processes of beneficial and pathogenic microbial
colonization of host tissues. These include its experimental tractability: Each member of the tripartite
association can be reared separately or together in varied combinations in the laboratory, at least one
representative of each has a sequenced genome and transcriptome, and numerous genetic techniques
have been developed for Xenorhabdus bacteria. Furthermore, investigations are simplified by the fact
that Steinernema are colonized by a monoxenic culture of a specific species of Xenorhabdus bacteria,
rather than a complex microbial consortium. Thus, the system can be probed to understand the molecular basis for specificity and the impacts of bacteria on animal host behavior, reproduction, immunity,
and physiology. Finally, the Steinernema-Xenorhabdus mutualism is a conserved characteristic of both
genera, facilitating evolutionary and comparative analyses of symbiosis mechanisms. The potential of
the Steinernema-Xenorhabdus symbiosis to reveal fundamental molecular and cellular processes mediating symbiosis is inhibited by a lack of genetic and molecular tools to manipulate the nematode. Furthermore, although several C. elegans researchers have made advances in understanding several aspects of Steinernema molecular biology, much remains to be learned regarding Steinernema physiology, behavior, development, and immunity with respect to their symbiosis with Xenorhabdus.
Research in my lab focuses on understanding the molecular basis of bacterial symbiosis with
animal hosts, and what distinguishes beneficial and pathogenic interactions. I will focus my talk on the
molecular basis of species specificity between nematodes and bacteria. We have determined that X.
nematophila bacterial association with its S. carpocapsae nematode host is tissue- and species-specific
and requires the integral outer membrane protein NilB; Bacteria adhere to the anterior intestinal epithelial surface of adult and juvenile nematodes only when expressing nilB. The consequences of this surface colonization are as yet unknown, but possibilities include bacteria-host cell signaling that promotes pheromone production or immunity. The molecular function of NilB also is unknown. It is a beta
-barrel protein with surface exposed loops both necessary for nematode interactions and a predicted
small channel with an electronegative strip. These regions may be important for NilB binding to host
cell surfaces or for transport of host-derived molecules. NilB is the only characterized member of a
family of OMPs found exclusively in symbionts of animals (e.g., Neisseria, Haemophilus, Moraxella,
Actinobacillus). Therefore, understanding NilB function in Xenorhabdus-Steinernema mutualism will
provide insights into NilB homolog function in human associated microbes.
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