Isolation and characterisation of the entomopathogenic bacterium, photorhabdus temperate producing a heat stable insecticidal toxin

Entomopathogenic nematodes symbiotically associated with Photorhabdus spp. invade the larvae of susceptible insects and Photorhabdus bacteria are released into the insect hemolymph, which produce toxins and kill the insect larvae. In this study, five symbiotic bacteria isolates from soil nematodes, Heterorhabditis spp., have been reported. The bacteria were screened for insecticidal toxicity against larvae of Galleria mellonella. All were identified as different strains of Photorhabdus temperata via 16 s rDNA sequencing. The insecticidal activity was highest after 3-4 days of pure culture and the level of toxicity was higher in culture supernatant than in the cell pellet. The heat stability of the insecticidal activity was tested by heat treatments for 30 min at the range of 25°C to 100°C. Out of all isolated strains, P. temperate strains J4 and J5 produced heat-stable toxins. The supernatants extracted from the culture of these strains preserved up to 95% of insecticidal activity after heat treatment for 30 min at 80°C. The insecticidal activities of culture supernatants of all the selected strains were mostly maintained after proteinase K treatment. Based upon these findings, it can be inferred that the insecticidal toxins produced by Photorhabdus spp. were not pure protein substances.