A Model of Bacterial Intestinal Infections in Drosophila melanogaster

Abstract

Serratia marcescens is an entomopathogenic bacterium that opportunistically infects a wide range of hosts, including humans. In a model of septic injury, if directly introduced into the body cavity of Drosophila, this pathogen is insensitive to the host's systemic immune response and kills flies in a day. We find that S. marcescens resistance to the Drosophila immune deficiency (imd)-mediated humoral response requires the bacterial lipopolysaccharide O-antigen. If ingested by Drosophila, bacteria cross the gut and penetrate the body cavity. During this passage, the bacteria can be observed within the cells of the intestinal epithelium. In such an oral infection model, the flies succumb to infection only after 6 days. We demonstrate that two complementary host defense mechanisms act together against such food-borne infection: an antimicrobial response in the intestine that is regulated by the imd pathway and phagocytosis by hemocytes of bacteria that have escaped into the hemolymph. Interestingly, bacteria present in the hemolymph elicit a systemic immune response only when phagocytosis is blocked. Our observations support a model wherein peptidoglycan fragments released during bacterial growth activate the imd pathway and do not back a proposed role for phagocytosis in the immune activation of the fat body. Thanks to the genetic tools available in both host and pathogen, the molecular dissection of the interactions between S. marcescens and Drosophila will provide a useful paradigm for deciphering intestinal pathogenesis.

Methods to study Drosophila immunity

“…Alternative Gram-negative bacteria are Providencia [18], Serratia marcescens (notably the strain Sm DB11-40, [19]), or Pseudomonas aeruginosa [20,21], which can be found associated with flies in the wild. Laboratory strains of Escherichia coli, although not associated with wild flies, have also been used to probe the immune system, notably the Imd pathway which is activated by DAP-type PGN.…”

“…Other methods include placing flies on paper towels soaked with bacteria. In this case, fresh bacterial or sucrose solution should be frequently added to avoid dehydration (resulting in continuous infection) [19]. The method of infection (for example with or without prior starvation, continuous or one-shot delivery of bacteria) can have a strong influence on the outcome of the infection.…”

Bacterial strategies to overcome insect defences

“…AMP production in the gut epithelium can be triggered by E. c. carotovora 15 (REF. 36), P. entomophila 31 and S. marcescens 33 . This local response is mediated by the nuclear factor (NF)-κB signalling pathway called Imd 18,45 , through the recognition of Gram-negative peptidoglycan by the peptidoglycan recognition proteins (PGRPs), such as PGRP-LC 46 …”

Lepidopteran species have a variety of defence strategies against bacterial infections

“…The nonpigmented ENT strain was originally isolated from a moribund fruit fly (Flyg et al, 1980) and has been widely used as a model entomopathogen (e.g. Nehme et al 2007).…”