Repercussions of Symbiotic Bacteria Associated with Entomopathogenic Nematodes and Their Biogenic Silver Nanoparticles on Immune Responses at Root-Knot Nematode Suppression

Root-knot nematodes (RKNs) of the <i>Meloidogyne</i> genus impact various plants, including crops, fruits, and vegetables. Few chemical control options exist globally, and many nematicides are banned due to health and environmental risks. This study tested a new nematicidal agent, the symbiotic bacterium <i>Xenorhabdus indica</i>, which was molecularly identified (PV845100). Cell-free culture supernatants of <i>Xenorhabdus</i> spp. and their biogenic Ag-NPs were used in nematicidal assays. <i>Meloidogyne incognita</i> showed high mortality rates of 95.3%, 74.6%, and 72.6% after 72 h of treatment with the <i>X. indica</i> filtrate at three concentrations. At the same concentrations, biogenic Ag-NPs resulted in 82.0%, 90.0%, and 85.3% mortality rates, respectively. After 72 h, hatchability decreased by 53%, 74.6%, and 72.6% for the <i>X. indica</i> filtrate and 82.0%, 90.0%, and 85.3% for Ag-NPs. Quantitative real-time PCR (Q-PCR) revealed that Mi-Ache1 expression was lower in <i>M. incognita</i> second-stage juveniles (J2s) treated with the filtrate and Ag-NPs after 72 h compared to controls. Mi-Ache2 expression was also decreased, but only slightly. Furthermore, both the <i>X. indica</i> filtrate and biogenic Ag-NPs were safe in human lung (WI-38) and skin (HFB4) cell lines. These findings suggest that bacterial filtrates and their biogenic Ag-NPs could serve as cost-effective, environmentally friendly alternatives to commercial nematicides.