Without defense reactions, any organism would perish from the mere growth of harmless microbes, as microbes colonize all living matter. A rapid innate immune system can stop such microbes. Insects rely solely on an innate response as they lack antibodies.
The immune system must first recognize the invading microbe as foreign to start the innate defense. We are studying the nature of the interaction between the microbial signature molecules and their receptors. In insects, receptors that recognize peptidoglycan seem to be of outmost importance for the recognition of bacteria.
We have also found that some members of this family of peptidoglycan recognition proteins (PGRPs) are enzymes cleaving peptidoglycan into pieces that do not stimulate the immune system. Thus proteins belonging to the same family are either positive or negative regulators of innate immunity.

It has been much debated how the Drosophila immune system can recognize bacterial peptidoglycan that is hidden in Gram negative bacteria. We have shown that bacteria separated from Drosophila S2cells by a semipermeable membrane can upregulate the Imd pathway and made it likely that the shed elicitors are of peptidoglycan nature. I now want to further characterize the structure and origin of the elicitors.