ABSTRACT
Host-microbe symbioses range from bene cial (mutualistic) interactions that enhance the tness of both partners to detrimental (pathogenic) associations that compromise the tness of the host. While pathogenic interactions have been studied extensively, mutualistic associations are less understood. Mutualistic interactions can vary considerably. Obligate associations, such as the intracellular bacteria Buchnera sp. and their aphid hosts, involve provisioning of essential nutrients to the host (Dale and Moran, 2006). In defensive mutualisms, as represented by fungal endophytes such as Epichloë/Neotyphodium that inhabit intercellular spaces of leaf structures of ryegrass, secondary metabolites are produced to repel herbivorous insects (Tanaka et al., 2005). The bioluminescent bacterium Vibrio fi scheri is an example of a free-living microbe that colonizes its host, the bobtail squid, and produces light to counterilluminate shadows to help the squid avoid predation (Nyholm and McFall-Ngai, 2004). Bacterial species of the genus Xenorhabdus are unique in their ability to engage in both pathogenic and mutualistic associations with two different host organisms. Xenorhabdus sp. form mutualistic relationships with entomopathogenic nematodes (EPNs) and are pathogenic to a diverse array of insects. Xenorhabdus sp. are considered to be semi-obligate mutualists since they have not been isolated outside the host organisms but can be grown independently in the laboratory. During their mutualistic stage, Xenorhabdus sp. are carried in the intestine of the nematode. The transition to the pathogenic stage occurs when the bacteria are released into the hemocoel after the nematode enters the insect host (Forst and Clarke, 2002; Herbert and Goodrich-Blair, 2007). The nematode uses Xenorhabdus sp. for its pathogenic potential to kill the
host as well as to provide nutrient sources for growth and development. The bacteria bene t by being vectored to the nutrient-rich source of the insect hemolymph. This chapter focuses on the competitive interactions that occur within the insect host and the mechanisms Xenorhabdus sp. employ to secure nutrient resources for themselves and their nematode partner. The contribution that the nematode makes to interspecies competition is less understood and therefore will not be covered in this chapter. For a detailed discussion of the molecular pathways involved in pathogenesis and mutualism in Xenorhabdus sp. and the sister taxa, Photorhabdus sp., the reader is referred to several excellent recent reviews on these topics (Joyce et al., 2006; Goodrich-Blair, 2007; Goodrich-Blair and Clarke, 2007; Herbert and Goodrich-Blair, 2007).
