The genome of a steinernematid-associated Pseudomonas piscis bacterium encodes the biosynthesis of insect toxins

Steinernema soil nematodes are a solution to the scourge of crop pests, due to their natural capacity to kill insects. Although this insect killing nature is established to stem from the nematodes’ Xenorhabdus bacterial endosymbionts, the role of other steinernematid-associated bacteria in the nematode lifecycle remains unclear. Thus, this genome study aimed to determine the potential of Pseudomonas piscis to contribute to the entomopathogenicity of its Steinernema host. Insect larvae were infected with one of three Steinernema cultures. From each of the three treatments, the prevalent bacteria in the haemocoel of cadavers, four days post infection were isolated, morphologically characterised, and their genomes sequenced and assembled. Assemblies were used for species delineation and identification of genes that encode production of secondary metabolites and antibiotic resistance. All genomes had at least 98% of their bases on a contig. Through digital DNA-DNA hybridisation analyses, we ascertained that the haemocoels of insect cadavers previously infected with Steinernema sp. Kalro, Steinernema sp. 75, and Steinernema sp. 97 were dominated with Xenorhabdus griffiniae Kalro, Pseudomonas piscis 75 and X. griffiniae 75, respectively. P. piscis 75 clustered with pseudomonads that are characterised by high insecticidal activity. The P. piscis 75 genome encoded the production of insect toxins such as hydrogen cyanide, orfamides and rhizoxins, antifungals such as pyrrolnitrin and pyoluteorin and a broad-spectrum antimicrobial. Its genome encoded resistance to over ten classes of antibiotics, including cationic lipopeptides. Thus, steinernematid-associated P. piscis bacteria have the biosynthetic potential to contribute to nematode entomopathogenicity.