Ascaroside-mediated modulation of host microbiota and survival in galleria mellonella by symbiotic bacteria of entomopathogenic nematode

Microbial symbionts and host pheromones are recognized as important mediators in the interactions between insect and entomopathogenic nematodes (EPNs). However, the influence of EPN-derived ascarosides and associated symbiotic bacteria on the insect's microbial community remains poorly understood. In this study, Galleria mellonella larvae were injected with nematode ascarosides (ascr#9 and ascr#11) and/or symbiotic bacteria (Photorhabdus luminescens) to investigate changes in the bacterial communities of larval haemolymph and gut. The result showed that the haemolymph was dominated by Gammaproteobacteria and Bacteroidia, while the gut microbiota was enriched with Bacilli. Burkholderiaceae and Enterococcaceae were the predominant families in the haemolymph and gut, respectively. Significant differences in bacterial community composition were observed across treatments. Notably, co-injection of ascarosides and P. luminescens significantly reduced the bacterial abundance of Ralstonia (commensal bacteria) and Photorhabdus (invader bacteria). The co-injection with ascarosides and symbiotic bacteria significantly contributed to prolonged host survival and sustained physiological stability compared to being exposed to bacteria alone. Mechanistically, ascarosides selectively upregulated the quorum sensing gene luxD in P. luminescens, suggesting that nematode pheromones modulate bacterial communication and collective behavior to regulate population dynamics. These findings indicate that ascarosides function as key microbial modulators, suppressing harmful bacterial growth and fostering a balanced community that supports host viability.