Increased survival in the red flour beetle after oral priming with bacteria-conditioned media

Immune priming is defined as enhanced protection upon secondary exposure to a pathogen. Such enhanced resistance after prior exposure has been demonstrated for a number of insect species including the red flour beetle, Tribolium castaneum. In testing this phenomenon, the majority of studies have focused on introducing the pathogen into the insect's hemocoel via septic wounding through the cuticle. Although such septic injury can occur in nature, many pathogens enter their hosts via the oral route, i.e. by ingestion. Bacillus thuringiensis bacteria are well-known insect pathogens that infect their host orally. We found that T. castaneum larvae showed increased survival after oral exposure to B. thuringiensis, when they had been orally primed with filter-sterilized media in which spores of B. thuringiensis had been raised. Such priming was achieved only with a naturally pathogenic strain of B. thuringiensis and a strain that was made pathogenic by transfer of plasmids. Moreover, primed larvae were smaller in size 24 h after priming and had a longer developmental time, indicating that investment in such a response comes at a cost. However, the increased survival in primed larvae was not caused by larval size differences upon challenge.