Silver nanoparticles enhance the larvicidal toxicity of Photorhabdus and Xenorhabdus bacterial toxins: an approach to control the filarial vector, Culex pipiens.

Mosquito-control is still based mostly on chemical insecticides which are toxic and cause environmental deprivation. This study investigates synthesizing silver bio-nanoparticles (AgNPs) from nematode-symbiotic bacterial toxin complexes as an alternative larvicidal bioinsecticide agent against Culex pipiens larvae. Five species/strains of nematode-symbiotic bacteria, Xenorhabdus indica, Xenorhabdus spp., Photorhabdus luminescens laumondii HP88, Photorhabdus luminescens akhurstii HRM1 and Photorhabdus luminescens akhurstii HS1 were used. AgNPs were characterized by scanning electron microscopy and x-ray diffraction analysis. Larvae were initially exposed to descending concentrations (300, 150, 75, 37.5 and 18.75 µg/ml) of each of the five bacterial toxins (as positive controls) or to the bio-AgNPs synthesized from the same bacterial toxins (200, 100, 50, 25, 12.5, 6.25, 3.12 and 1.5 µg/ml) for 48 hours. Results of toxicity bioassays showed that mortality of treated larvae was concentration-dependent, toxins from X. indica, P. luminescens laumondii HP88 and P. luminescens akhurstii HS1 showed LC of 29, 28 and 2002 µg/ml, respectively. While, toxins from P. luminescens akhurstii HRM1 and Xenorhabdus sp. showed LC of 199, 318 µg/ml, respectively. Bio-AgNPs synthesized from, X. indica or Xenorhabdus sp. toxins have significantly increased their larvicidal activities (LC of 1.6, 3.7 µg/ml ) at 48h post-treatment. Moreover, bio-AgNPs synthesized from P. luminescens laumondii HP88, P. luminescens akhurstii HRM1 or P. luminescens akhurstii HS1 toxins significantly increased their larvicidal activities (LC of 2.1, 1.5, 13.9 µg/ml, respectively) at 48h post treatment. In conclusion, the highest larval toxicity was observed when larvae were treated with bio-AgNPs synthesized from P. luminescens akhurstii HRM1 and X. indica, followed by P. luminescens laumondii HP88 and Xenorhabdus sp. Subsequently, data of the present study suggest these bio-AgNPs toxin complexes as potentially effective bio-control candidates in the battle against mosquito. However, testing other types of bio-synthesized nanomaterials, and their synergistic combinations against different mosquito species still under investigation.