Insecticidal toxin complex from Photorhabdus luminescens and its histopathological effects on the midgut of the sand fly Phlebotomus papatasi

Abstract

Photorhabdus luminescens is a gram-negative enteric bacterium that is found in association within entomopathogenic nematodes of the family Heterorhabtidae. The nematodes infect a variety of soil-dwelling insects. Upon entering an insect host, the nematode releases P. luminescens cells from its intestinal tract, and the bacteria quickly establish a lethal septicemia. In order to examine the toxicity and histopathology of the secreated toxin against the larvae of Phlebotomus papatasi, the vector of zoonotic cutaneous leishmaniasis in Egypt. We previously infected the insect wax moth with the Egyptian isolate of the nematode Heterorhabditis indica RM1 and upon death, the hemolymph was squeezed and subjected to purification to obtain the toxin complex. The toxin was lethal to the larvae when fed and the mortality was dependent on the protein concentration in the toxin complex. The dose corresponding to 11.39, 7.4 and 2.85 mg/ml caused about 60%, 32% and 2% mortality, respectively. Those larvae which are not killed by the toxic action of the complex and successfully reached the adult stage were monitored for the number of adults emerged, the number of eggs deposited and the hatchability. All these biological parameters were negatively affected by the latent action of the toxin complex. The histopathological effects of the LC50 toxin complex on the larvae following oral ingestion, were in acceleration the blebbing of the midgut epithelium into the gut lumen and eventual lysis of the epithelium. These results show that the P. luminescens toxin complex is a highly active oral insecticide.