Chen, V.; Marken, J. P.; Murray, R. M.; Cao, M.

Steinernema species are soil-dwelling and insect-parasitic nematodes that associate with symbiotic Xenorhabdus bacteria. During the infective juvenile (IJ) stage, Steinernema nematodes package species-specific Xenorhabdus bacteria in the anterior intestinal pockets. The nematodes can survive in the soil for months as they seek insect prey. The mechanisms of how these nematodes associate with environmental microbes other than their Xenorhabdus symbionts is barely known. Here, we report a new mechanism of E. coli Nissle (EcN) association with the nematode Steinernema hermaphroditum. We show that EcN cells are enclosed and lysed in at least four pairs of coelomocytes, suggesting these immune cells respond to bacterial invasion. During the IJ stage of nematode development, EcN cells localize to posterior intestinal vacuoles and enter the inter-cuticular space, where they proliferate, aggregate, then lyse. EcN cells expressed proteins in the cell lysates were maintained in the nematode cuticle over eight weeks in non-sterile soil. These observations suggest sequential steps of EcN colonization in the host nematodes involving an immune response that is distinctive from interactions with mutualistic symbiont. Our work establishes a novel framework of nematode-bacteria interaction with potential applications in environmental bioengineering.