Synergistic actions of symbiotic bacteria modulate the insecticidal potency of entomopathogenic nematode Steinernema monticolum KHA701.

Entomopathogenic nematodes (EPNs), primarily Steinernema and Heterorhabditis, form symbiotic relationships with bacteria from the genera Xenorhabdus and Photorhabdus, respectively. These bacteria exhibit insecticidal activity and suppress competing microorganisms, allowing EPNs and their symbionts to dominate insect cadavers. While monoxenic associations are fundamental to EPN-bacteria interactions, recent studies suggest that EPNs may harbor a diverse array of symbiotic bacteria with consistent associations. However, the role of these additional symbiotic bacteria in EPN pathogenesis and the complexity of their interactions remain unclear. In this study, Steinernema monticolum KHA701 was newly isolated using the Galleria mellonella bait method. Compared to the highly pathogenic Heterorhabditis bacteriophora TT01, S. monticolum KHA701 demonstrated superior insecticidal activity against G. mellonella larvae and exhibited a broad host range, targeting 63 arthropod species across 18 orders and 41 families. Microbiota analysis of S. monticolum KHA701 infective juveniles identified 34 bacterial species, including Xenorhabdus hominickii, from the nematode body. Five bacteria-Elizabethkingia miricola, Serratia marcescens, Pseudomonas protegens, Staphylococcus sp., and X. hominickii-were confirmed to be highly pathogenic to Zophobas morio and Periplaneta fuliginosa larvae. Notably, the combination of X. hominickii with any of the other four bacteria significantly enhanced the insecticidal activity of S. monticolum KHA701 against G. mellonella. These findings suggest that S. monticolum KHA701 utilizes a diverse community of bacterial symbionts to enhance its insecticidal efficacy, providing novel insights into the ecological strategies of EPNs.