The Genera Photorhabdus and Xenorhabdus
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The Genera Photorhabdus and Xenorhabdus NOEL BOEMARE AND RAYMOND AKHURST
Introduction The genera Photorhabdus and Xenorhabdus are members of the family Enterobacteriaceae that encompass the intestinal bacterial symbionts living in commensalism with entomopathogenic nematodes (EPNs) of the genera Heterorhabditis and Steinernema, respectively (Akhurst and Boemare, 1990; Forst et al., 1997). Most of them are pathogenic for insects when injected into the hemocoel. In addition, some nonsymbiotic strains of Photorhabdus have been identified as opportunistic pathogens for humans (Farmer et al., 1989; Peel et al., 1999). Various insect and vertebrate symbionts are members of the γ-subclass of Proteobacteria, which contains a wide spectrum of animal and human pathogens, such as members of the families Enterobacteriaceae, Legionellaceae, Pasteurellaceae, Vibrionaceae, and the genera Pseudomonas (sensu stricto) and Acinetobacter (Stackebrandt, 1999). Symbionts of EPNs are phylogenetic neighbors of an important group of endosymbionts of insects (the RDP tree of the {Bergey’s Web site}). Most of the insect endosymbionts cluster according to the phylogeny of their eukaryotic hosts, which may indicate coevolutionary events (Stackebrandt, 1999). A similar cospeciation (See Habitat/ Cospeciation in this Chapter) also was observed in the case of EPNs (Akhurst, 1983; Boemare et al., 1997a; Forst et al., 1997). However, though most of the insect symbionts are endocytobionts and not cultivable, Xenorhabdus and Photorhabdus have the distinct advantage of cultivability on standard bacteriological media. Moreover, their hosts can be easily axenized. Consequently, pure cultures of axenic nematodes and bacteria are available for combination in gnotobiological experiments that demonstrate the viability and performance (both qualitative and quantitative) of the bacterial/ helminthic symbioses (Boemare et al., 1997a). Other bacterial–animal symbioses involving cultivable Proteobacteriaceae have been described: some marine animal light-organ symbionts (Nealson et al., 1990) such as the model Vibrio
fischeri (associated with the squid Euprymna scolopes) and Aeromonas veronii bv. sobria (symbiont of the medicinal leech Hirudo medicinalis). In this latter, despite the diverse bacterial flora found along the tegument of the leech, the A. veroni bv. sobria are typically found as a pure culture in the gut, suggesting that these symbionts possess a special capability that enables them to proliferate in the medicinal leech (Graf, 2000). Vibrio harveyi, often commensal with marine animals, may be pathogenic for the black tiger prawn, Penaeus monodon (Manefield et al., 2000), as are the terrestrial luminous Photorhabdus for insects.
Taxonomy Initially only one genus, Xenorhabdus, was described as encompassing all the symbionts of the entomopathogenic nematodes. Those strains initially considered as belonging to the species Xenorhabdus luminescens clearly form a DNArelatedness group that is distinct
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