Abstract
Xenorhabdus hominickii ANU1 is known to be an entomopathogenic bacterium symbiotic to nematode Steinernema monticolum. Another bacterial strain X. hominickii DY1 was isolated from a local population of S. monticolum. This bacterial strain X. hominickii DY1 was found to exhibit high insecticidal activities against lepidopteran and coleopteran species after hemocoelic injection. However, these two X. hominickii strains exhibited significant variations in insecticidal activities, with ANU1 strain being more potent than DY1 strain. To clarify their virulence difference, bacterial culture broths of these two strains were compared for secondary metabolite compositions. GC-MS analysis revealed that these two strains had different compositions, including pyrrolopyrazines, piperazines, cyclopeptides, and indoles. Some of these compounds exhibited inhibitory activities against phospholipase A2 to block eicosanoid biosynthesis and induce significant immunosuppression. They also exhibited significant insecticidal activities after oral feeding, with indole derivatives being the most potent. More kinds of indole derivatives were detected in the culture broth of ANU1 strain. To investigate variations in regulation of secondary metabolite production, expression level of leucine-responsive regulatory protein (Lrp), a global transcription factor, was compared. ANU1 strain exhibited significantly lower Lrp expression level than DY1 strain. To assess genetic variations associated with secondary metabolite synthesis, bacterial loci encoding non-ribosomal protein synthase and polyketide synthase (NRPS-PKS) were compared. Three NRPS and four PKS loci were predicted from the genome of X. hominickii. The two bacterial strains exhibited genetic variations (0.12∼0.67%) in amino acid sequences of these NRPS-PKS. Most NRPS-PKS genes exhibited high expression peaks at stationary phase of bacterial growth. However, their expression levels were significantly different between the two strains. These results suggest that differential virulence of the two bacterial strains is caused by the difference in Lrp expression level, leading to difference in the production of indole compounds and other NRPS-PKS-associated secondary metabolites.
Highlights
Gram-negative and motile bacteria Xenorhabdus spp. belong to the family of Morganellaceae in the order of Enterobacterales
When the hemocoel was open, reproduced infective juveniles (IJs) were released (Figure 1B). These IJs were used to identify the nematode using morphological characters such as total body length, excretory pore size, and tail length (Figure 1C). When these morphological characters of the IJs were compared with those of Steinernema spp., they matched to characters of S. monticolum (Supplementary Table 2)
This study reports that two strains of X. hominickii have different virulence
Summary
Gram-negative and motile bacteria Xenorhabdus spp. belong to the family of Morganellaceae in the order of Enterobacterales. Mutualistic symbionts of EPNs have a similar life cycle: a phoretic form in the nematode host, a pathogenic form in the insect body, and a saprophytic form in the insect cadaver (Herbert and Goodrich-Blair, 2007; Stock and Goodrich-Blair, 2008; Koppenhofen and Gaugler, 2009). These symbiotic bacteria are localized in a specialized vesicle in the anterior part of the gut called receptacle (Snyder et al, 2007) of infective juveniles (IJs) at the third instar larval stage. When nutrients within the insect cadaver are consumed and nematode density reaches a carrying capacity, nematodes will develop into IJs and exit the cadaver to repeat their life cycle (Popiel et al, 1989)
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