Cellular Immune Response of Protaetia brevitarsis seulensis Larvae to Metabolites Produced by Pathogenic and Symbiotic Bacteria

Studies of immune responses in insects have focused on mechanisms that interact directly with invading microorganisms. However, few studies have examined the immune response to various metabolites produced by microorganisms after they enter the host. Here, we examined immune responses in Protaetia brevitarsis seulensis larvae induced by metabolites produced by symbiotic and pathogenic bacteria. The two types of bacteria were cultured under the same conditions. The bacteria were then removed and the remaining culture supernatant was injected into the larvae. The larvae injected with culture medium (Ch-medium) from symbiotic bacteria remained relatively healthy and did not develop an immune response, whereas more than 60% of the larvae injected with pathogen culture medium (Ec-medium) died after 150 hours and dark brown patches of melanin were observed at the injection site. This immune response was confirmed by the finding of activated lysosomes in insect granulocytes. More than 50% of lysosomes in larvae injected with pathogen culture medium were strongly stained after 12 h, but less than 5% of those injected with symbiotic culture media were stained. Therefore, it is assumed that symbiotic bacteria produce few (if any) substances that induce host immune responses.