Increase in midgut microbiota load induces an apparent immune priming and increases tolerance to Bacillus thuringiensis.

The insect immune system is comprised of both humoral and cellular components that are mobilized in response to parasitic or pathogenic infections. Activation of the immune response implies a considerable expenditure of energy and that is why insects rely on inducible pathways that are activated after coming into contact with the pathogenic agent. Known as immune priming, insects can prolong the activation of the immune response and transmit their immune status to the next generation. Starting from a laboratory colony of the lepidopteran Spodoptera exigua and using the lytic zone assay as a measure of the immune status, we selected for a sub-colony with high levels of immune activity in the absence of external challenging with bacteria. Immune-activated insect showed characteristics that are typical reported for immune primed insects, such as increased tolerance to pathogens (Bacillus thuringiensis in our case), fitness-cost associated to the immune status, and maternal transmission of the immune status. However, additional analysis revealed that the selection for the immune-activated insects was based on the selection of insects carrying a higher bacterial load in the midgut. Our results suggest that activation of the immune system in S. exigua may not only occur as consequence of the immune priming but also from an increase in midgut microbiota load.