Microbiome Analysis of Malacopathogenic Nematodes Suggests No Evidence of a Single Bacterial Symbiont Responsible for Gastropod Mortality.

Nematodes and bacteria are prevalent in soil ecosystems, and some have evolved symbiotic relationships. In some cases, symbionts carry out highly specialized functions: a prime example being entomopathogenic nematodes (EPNs), which vector bacteria ( or ) into insect hosts, killing them to provide a food source for the nematodes. It is thought that the commercially available malacopathogenic (kills slugs and snails) biocontrol nematode vectors a bacterium () into slugs to kill them. To investigate this further we used a metagenomic approach to profile the bacteria present in the commercial strain of , a wild strain of and two other species ( and ), after they had killed their slug host (). We show that these nematodes do not exclusively associate with one bacterium but a range of species, with members of the phyla Pseudomonadota, Bacillota, Actinobacteriota and Bacteroidota the most prevalent. The commercial strain of had the least diverse bacterial community. Furthermore, we found that the bacterium has been cultured on for 25 years is not the expected species but is spp. and the only strain of the species to associate with spp. was the commercial strain of . In summary, we found no evidence to show that rely exclusively on one bacterium to cause host mortality but found variable and diverse bacterial communities associated with these nematodes in their slug hosts.