Brevibacillus laterosporus pathogenesis and local immune response regulation in the house fly midgut

The insect midgut represents the primary site of action of the entompathogenic bacterium Brevibacillus laterosporus. While most studies on this microorganism focus on the identification and characterization of possible virulence factors and toxins, little is known about the insect immune defense mechanisms that are activated against this pathogen. In this study we have investigated the local immune response of different house fly stages to B. laterosporus at the transcriptional level, and we tested the hypothesis that an improvement in entomopathogenicity can be achieved by impairing host innate immunity. Gene expression analyses showed that immediately after spore ingestion (6-12h) both larvae and adults increased the transcription rate of immune related genes in the midgut tissues, with special regard to those encoding for the main house fly antimicrobial peptides (AMPs) (i.e., attacin, cecropin, defensin, diptericin, domesticin, muscin) and for prophenoloxydase that is normally involved in the cascade of events leading to the generation of reactive oxygen species (ROS) and other factors with antibacterial properties. In experiments evaluating the use of an immunosuppressive agent to enhance the virulence of B. laterosporus against adult house flies, a significant downregulation of the same genes was observed 12-24h after the administration of sub-lethal doses of the botanical compound azadirachtin. Consequently, a significant increase in B. laterosporus entomopathogenic action was observed when flies were preliminarily or simultaneously exposed to a sub-lethal dose of azadirachtin. These results provide an important contribution to the prospect of employing immune-impairing tools to implement pest management strategies.