Abstract
Entomopathogenic bacteria from the genera Photorhabdus and Xenorhabdus are closely related Gram-negative bacilli from the family Enterobacteriaceae (γ-Proteobacteria). They establish obligate mutualistic associations with soil nematodes from the genera Steinernema and Heterorhabditis to facilitate insect pathogenesis. The research of these two bacterial genera is focused mainly on their unique interactions with two different animal hosts, i.e. nematodes and insects. So far, studies of the mutualistic bacteria of nematodes collected from around the world have contributed to an increase in the number of the described Xenorhabdus and Photorhabdus species. Recently, the classification system of entomopatogenic nematode microsymbionts has undergone profound revision and now 26 species of the genus Xenorhabdus and 19 species of the genus Photorhabdus have been identified. Despite their similar life style and close phylogenetic origin, Photorhabdus and Xenorhabdus bacterial species differ significantly in e.g. the nematode host range, symbiotic strategies for parasite success, and arrays of released antibiotics and insecticidal toxins. As the knowledge of the diversity of entomopathogenic nematode microsymbionts helps to enable the use thereof, assessment of the phylogenetic relationships of these astounding bacterial genera is now a major challenge for researchers. The present article summarizes the main information on the taxonomy and evolutionary history of Xenorhabdus and Photorhabdus, entomopathogenic nematode symbionts.
Highlights
Enthomopathogenic bacteria are widespread in nature and include mainly members of the genera Bacillus, Peanibacillus, Brevibacillus, Serratia, Pseudomonas, Xenorhabdus, and Photorhabdus
Photorhabdus and Xenorhabdus bacteria exhibit another essential feature: as obligate symbionts of infective juveniles (IJs) of entomopathogenic nematodes (EPNs) from the genera Steinernema and Heterorhabditis, respectively, they spend some part of their lives inside nematodes using them as a vector for efficient infection of insects (Goodrich-Blair and Clarke 2007; Koppenhöfer and Gaugler 2009; Stock 2015)
The symbiotic bacteria of entomopathogenic nematodes emerged as a relevant model to study microorganism-host interactions
Summary
Enthomopathogenic bacteria are widespread in nature and include mainly members of the genera Bacillus, Peanibacillus, Brevibacillus, Serratia, Pseudomonas, Xenorhabdus, and Photorhabdus. EPNs together with their bacterial symbionts are well known for their long-term use in biological and integrated pest management since they display insecticidal activity against a wide range of soil-dwelling insect and other arthropods (Poinar and Grewal 2012; Hiltpold 2015). Investigations of the virulence mechanisms and secondary metabolites of the Xenorhabdus and Photorhabdus bacteria have been aimed at their potential to be used for management of agricultural pests (Hinchliffe et al 2010; Zhang et al 2012; Kumari et al 2015; Stock et al 2017), as mosquito control repellents and feeding-deterrents (Yooyangket et al 2018; Kajla et al 2019), and in medical applications as a response to the need for novel antibiotics (Tobias et al 2018; Xue et al 2018). Some Photorhabdus bacteria have been identified as opportunistic pathogens of humans; the
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