Increased Survival in the Red Flour Beetle after Oral Priming with Bacteria-Conditioned Media

Abstract

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.

Oral immune priming with Bacillus thuringiensis induces a shift in the gene expression of Tribolium castaneum larvae

“…We analysed host gene expression profiles after exposure to bacterial cues (culture supernatants) of two different strains; an orally pathogenic strain ( Btt ) inducing a priming response and conferring survival benefits upon challenge, and a non-pathogenic strain ( Bt407 - ) where this effect remains absent. Consistent with phenotypic results [15], we found strong differences in host gene expression profiles between the two bacterial strains. Whereas priming with Bt407 - showed only minor differences compared to the control-priming group in terms of up-regulated genes, priming with Btt induced a large set of differentially regulated genes (Figs.…”

“…First, as with other immune defence, priming with Btt seems to be costly; primed larvae grow and develop more slowly than the controls [15]. It thus may depend on the epidemiology as to whether it is worth paying these costs.…”

Infection routes matter in population-specific responses of the red flour beetle to the entomopathogen Bacillus thuringiensis

“…It is possible that Cro1 is more resistant because it shows developmental adjustment when exposed to B. thuringiensis . This hypothesis is supported by our recent demonstration of oral priming for resistance in T. castaneum , which also entailed developmental retardation [28]. By contrast, T. castaneum that were prick-primed with heat-killed bacteria were previously shown to speed up development and thereby potentially escape a perceived risk of infection [17].…”

Infection of Tribolium castaneum with Bacillus thuringiensis: Quantification of Bacterial Replication within Cadavers, Transmission via Cannibalism, and Inhibition of Spore Germination

“…3). Since mortality largely occurs no later than the second day of exposure (13), larvae surviving this critical period continue ingesting the infectious diet for up to 2 weeks (personal observation), indicating that they might have achieved immunity through priming of the immune system (45).…”

“…tenebrionis spore-toxin mixtures showed increased survival if they were previously exposed to sterile B. thuringiensis bv. tenebrionis spore culture supernatants (45). Such oral priming of host resistance might also be derived from natural spore sources, such as cannibalized cadavers.…”