Feeding Dead Bacteria Protects Drosophila From Pathogenic Bacterial Infection

The fruit fly Drosophila melanogaster possesses highly effective innate immune system,two main innate immune signaling pathways in D.melanogaster are the Toll-Spatzle and Imd(immune deficiency)pathways.In order to defend against infection of pathogenic microorganisms,D.melanogaster relies on a variety of defense mechanisms,including systemic and local immune responses.In this study,we investigate whether and how feeding dead bacteria activates antimicrobial peptides in wild-type and three mutant flies at different developmental stages(larvae,pupae and adults)by qRT-PCR,and study whether feeding dead bacteria can protect D.melanogaster from pathogenic bacterial infection.Our results showed:(1)After feeding dead Escherichia coli,expression of Diptericin and Cecropin,the two genes regulated by the Imd pathway,was significantly induced in the first and third instar larvae in the wild-type D.melanogaster.Expression of the two genes was also significantly induced in the first instar larvae in the Imd-mutant flies,significantly induced to a high level in the second instar larvae in Relish-mutant flies,and significantly induced in the third instar larvae in the MyD88-mutant flies.(2)After feeding Staphylococcus aureus,the expression patterns of Drosomycin and Defensin,two genes in the Toll pathway,were different.For example,Drosomycin was highly expressed in the first instar larva of the Reish-mutant flies and in the pupae of MyD88-mutant fruit flyies,while Defensin showed low induction level in the larvae and pupae of all four fly lines.(3)Our data suggest that expression of a single antimicrobial peptide gene is regulated by more than one signaling pathways.Based on the data of the three Drosophila mutant flies,even when a key factor in a signaling pathway is absent,the antimicrobial peptide genes regulated by the pathway are still induced,suggesting that antimicrobial peptide genes may be cross-regulated by both the Toll and Imd pathways.(4)Activation of local immune responses by feeding dead bacteria can protect flies from subsequent infection by live pathogenic bacteria.It interestingly,feeding Gram-negative E.coli not only protects flies from Gram-negative Pseudomonas aeruginosa infection but also protects flies from Gram-positive Enterococcos faecalis infection.Similarly,feeding Gram-positive S.aureus protects flies from both,P.aeruginosa and E.faecalis infection.