The insect pathogenic bacterium Xenorhabdus innexi has attenuated virulence in multiple insect model hosts yet encodes a potent mosquitocidal toxin

Abstract

Background Xenorhabdus innexi is a bacterial symbiont of Steinernema scapterisci nematodes, which is a cricket-specialist parasite and together the nematode and bacteria infect and kill crickets. Curiously, X. innexi expresses a potent extracellular mosquitocidal toxin activity in culture supernatants. We sequenced a draft genome of X. innexi and compared it to the genomes of related pathogens to elucidate the nature of specialization.ResultsUsing green fluorescent protein-expressing X. innexi we confirm previous reports using culture-dependent techniques that X. innexi colonizes its nematode host at low levels (~3–8 cells per nematode), relative to other Xenorhabdus-Steinernema associations. We found that compared to the well-characterized entomopathogenic nematode symbiont X. nematophila, X. innexi fails to suppress the insect phenoloxidase immune pathway and is attenuated for virulence and reproduction in the Lepidoptera Galleria mellonella and Manduca sexta, as well as the dipteran Drosophila melanogaster. To assess if, compared to other Xenorhabdus spp., X. innexi has a reduced capacity to synthesize virulence determinants, we obtained and analyzed a draft genome sequence. We found no evidence for several hallmarks of Xenorhabdus spp. toxicity, including Tc and Mcf toxins. Similar to other Xenorhabdus genomes, we found numerous loci predicted to encode non-ribosomal peptide/polyketide synthetases. Anti-SMASH predictions of these loci revealed one, related to the fcl locus that encodes fabclavines and zmn locus that encodes zeamines, as a likely candidate to encode the X. innexi mosquitocidal toxin biosynthetic machinery, which we designated Xlt. In support of this hypothesis, two mutants each with an insertion in an Xlt biosynthesis gene cluster lacked the mosquitocidal compound based on HPLC/MS analysis and neither produced toxin to the levels of the wild type parent.ConclusionsThe X. innexi genome will be a valuable resource in identifying loci encoding new metabolites of interest, but also in future comparative studies of nematode-bacterial symbiosis and niche partitioning among bacterial pathogens.Electronic supplementary materialThe online version of this article (doi: 10.1186/s12864-017-4311-4) contains supplementary material, which is available to authorized users.

Host-Specific Activation of Entomopathogenic Nematode Infective Juveniles

“…Infection by EPNs is the result of a complex process that includes long- and short-range volatile chemical and contact cues, penetration into the host, activation (also known as IJ recovery), release of the symbiotic bacteria, and the release of excreted/secreted nematode proteins (ESPs), which culminates in the death of the host [ 9 , 10 , 19 ]. Activation is a key aspect of the EPN infection process and is required for efficient release of the pathogenic bacteria they carry along with any nematode-derived bioactive molecules that function in killing the host or modulating its immune response [ 7 , 20 , 21 ]. Despite the importance of activation, it has not been well studied among EPNs, nor has it been previously considered as a means of predicting EPN virulence.…”

“…EPN virulence is dependent on its own toxic secretions and/or bacterial symbiont, depending on the species. Certain EPN symbionts have shown attenuated virulence when injected into different insect hosts [ 20 , 21 , 30 ], and the total number of bacterial cells carried by each individual IJ varies from species to species, where, for example, S. carpocapsae IJs carry approximately 40 colony forming units (CFUs) each while S. scapterisci IJs carry approximately 8 each or fewer [ 21 , 31 ]. Therefore, a well-activated nematode is not necessarily lethal to its host, rather the overall virulence depends on the quantity and toxicity of the bacterial symbiont as well as the potency of host immunity, and the toxicity of the nematode-derived excreted/secreted proteins, among other factors.…”

Secretion Systems and Secreted Proteins in Gram-Negative Entomopathogenic Bacteria: Their Roles in Insect Virulence and Beyond

“…The TpsA partner generally has a recognizable N-terminal TPS domain that targets it for secretion through TpsB [ 57 , 60 ]. Kim et al identified three groups of TPS homologues that are widespread in entomopathogens, including PhlA-type hemolysins, contact-dependent inhibition systems, and a third group with an as yet unknown function [ 77 ].…”

“…Mcf promotes apoptosis in both insect hemocytes [ 67 ] and mammalian cells [ 68 ], suggesting the mechanism of its toxicity. Several Xenorhabdus species also encode a homolog of McF, which is likely moved through a T1SS [ 77 ].…”

“…Photorhabdus has four predicted T6SS gene clusters, which are located in pathogenicity islands [ 44 ]. Xenorhabdus genomes generally encode at least one T6SS [ 61 ], with some strains carrying a second, or even third [ 77 ]. Little is known about the functions of these secretion systems, though X. bovienii strains encoding multiple T6SSs tend to have a competitive advantage over strains that only encode one, suggesting a role in interbacterial competition [ 92 ].…”