The bacterial symbionts of Entomopathogenic nematodes and their role in symbiosis and pathogenesis

Abstract

Entomopathogenic bacteria in the genera Xenorhabdus and Photorhabdus are mutualistically associated with entomopathogenic nematodes (EPN) Steinernema and Heterorhabditis, respectively. Together they form an insecticidal partnership which has been shown to kill a wide range of insect species. The spectrum of dependence in this symbiotic partnership is diverse, ranging from a tight, obligate relationship to a facultative one. A body of evidence suggests that the reproductive fitness of the nematode-bacterium partnership is tightly associated and interdependent. Furthermore, maintenance of their virulence is also critical to the conversion of the insect host as a suitable environment where this partnership can be perpetuated. Disruption of the symbiotic partnership can have detrimental effects on the fitness of both partners. The nematode-bacterial symbiont-insect partnership represents a model system in ecology and evolutionary biology and amenable to investigate beneficial and antagonistic interactions between invertebrates and microbes. Furthermore, the EPN’s bacterial symbionts are also viewed as a model system to study the biosynthesis, structure and function of various natural products. Their ability to produce up to 25 different natural product classes is outstanding among the Morganellaceae. These natural products show biological activity, most likely originating from important functions during the life cycle of both the nematodes and their symbionts. Tools and high throughput technologies have been developed to identify ubiquitous and rare molecules and study their function and assess their potential as novel biological activities. We herein summarize the symbiotic relationship between EPN and their bacterial symbionts, focusing on their fitness and their ability to successfully access and utilize an insect host. We also recapitulate the history of natural products research highlighting recent findings and the synthetic biology approaches that are currently implemented to identify non-natural derivatives from Xenorhabdus and Photorhabdus with improved biological activity.