Increasing adult density compromises survival following bacterial infections in Drosophila melanogaster

The density-dependent prophylaxis hypothesis predicts that risk of
pathogen transmission increases with increase in population density, and
in response to this, organisms mount a prophylactic immune response when
exposed to high density. This prophylactic response is expected to help
organisms improve their chances of survival when exposed to pathogens.
Alternatively, organisms living at high densities can exhibit compromised
defense against pathogens due to lack of resources and density associated
physiological stress; the crowding stress hypothesis. We housed
adult Drosophila melanogaster flies at different
densities and measured the effect this has on their post-infection
survival and resistance to starvation. We find that flies housed at higher
densities show greater mortality after being infected with bacterial
pathogens, while also exhibiting increased resistance to starvation. Our
results are more in line with the density-stress hypothesis that
postulates a compromised immune system when hosts are subjected to high
densities.