Abstract
In the present study, we examined whether microorganisms collaterally ingested by insects with their food activate the innate immune system to confer systemic resistance against subsequent bacterial invasion. Silkworms orally administered heat-killed Pseudomonas aeruginosa cells showed resistance against intra-hemolymph infection by P. aeruginosa. Oral administration of peptidoglycans, cell wall components of P. aeruginosa, conferred protective effects against P. aeruginosa infection, whereas oral administration of lipopolysaccharides, bacterial surface components, did not. In silkworms orally administered heat-killed P. aeruginosa cells, P. aeruginosa growth was inhibited in the hemolymph, and mRNA amounts of the antimicrobial peptides cecropin A and moricin were increased in the hemocytes and fat body. Furthermore, the amount of paralytic peptide, an insect cytokine that activates innate immune reactions, was increased in the hemolymph of silkworms orally administered heat-killed P. aeruginosa cells. These findings suggest that insects sense bacteria present in their food by peptidoglycan recognition, which activates systemic immune reactions to defend the insects against a second round of infection.
Highlights
Insects have hard exoskeletons that protect them against microbial invasion
These findings suggest that oral administration of heat-killed P. aeruginosa resulted in silkworm resistance against P. aeruginosa infection in the hemolymph
We examined whether oral administration of heat-killed microorganisms other than P. aeruginosa confers infection resistance of silkworms against P. aeruginosa infection in the hemolymph
Summary
Insects have hard exoskeletons that protect them against microbial invasion. The insect digestive tract, continuously comes into contact with various microorganisms present in the foods they ingest [1,2,3,4]. In Bombyx mori silkworm larvae, oral administration of Escherichia coli induces the production of antimicrobial peptides in the gut [15] These findings indicate that activation of immune reactions in the gut functions to defend insects against. We recently revealed that silkworms have a primed immune system that recognizes bacterial peptidoglycans and confers persistent infection resistance by increasing the production of antimicrobial peptides [23]. Based on these findings, we hypothesized that systemic activation of immune reactions induced by microorganisms in the silkworm food confers resistance against a second round of bacterial invasion in the hemolymph. We examined whether oral administration of heat-killed bacteria confers resistance to subsequent infection in the silkworm hemolymph
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