<i>Escherichia coli</i> Nissle 1917 Occupies Previously Undocumented Host Niches in the Insect‐Parasitic Nematode <i>Steinernema hermaphroditum</i>

Steinernema species are soil-dwelling, insect-parasitic nematodes that maintain species-specific associations with Xenorhabdus symbiotic bacteria, which are packaged within anterior intestinal pockets during the infective juvenile (IJ) stage. While these nematodes can persist in soil for months while seeking insect hosts, their interactions with environmental microbes beyond their native symbionts remain poorly understood. Here, we describe a previously uncharacterized interaction between Escherichia coli Nissle 1917 (EcN) and Steinernema hermaphroditum. EcN cells are enclosed and lysed within multiple pairs of putative coelomocytes, suggesting microbial endocytosis by host cells. During the IJ stage, EcN localizes to posterior intestinal compartments and the inter-cuticular space, where cells proliferate, aggregate and subsequently lyse. Bacterially expressed proteins persist within the nematode cuticle for over 8 weeks in non-sterile soil. These findings reveal sequential stages of environmental bacterial colonization associated with host immune responses distinct from mutualistic symbiosis. This work establishes a model for understanding nematode and environmental microbe interactions and highlights opportunities to deliver bacterially expressed molecules for environmental biosensing and biocontrol applications.