Interactions between insect immunity and an insect-pathogenic nematode with symbiotic bacteria

Abstract It is well known that some insect-pathogenic nematodes always live in symbiosis with Gram negative bacteria belonging to the genus Xenorhabdus (earlier referred to as Achromobacter). We have here used diapausing pupae of Hyalophora cecropia as a model system and investigated the Mexican strain of Neoaplectana carpocapsae and its bacterium Xenorhabdus nematophilus for their ability to withstand insect immunity separately and in the symbiotic unit. We have prepared a reconstituted nematode containing a streptomycin-resistant mutant of X. nematophilus. In normal pupae LD50 (the injected dose that causes death of 50% of the population) of the bacteria was about 500 cells; for immunized pupae it was 5 x 105 cells. Cecropia immunity did not affect the nematodes. Fewer than ten nematodes with bacteria were lethal, while LD50 of axenic nematodes was about 500. Immune haemolymph caused lysis of X. nematophilus. Immune proteins P9A and P9B were identified as the active components in Cecropia immunity against X. nematophilus. It was found that the nematodes help the bacteria by excreting an immune inhibitor that selectively destroys both forms of P9 as well as immune protein P5. These results contribute to our understanding of the symbiotic relationship between nematodes and their bacteria and its survival value against induced insect immunity.