Enteric bacteria counteract lipopolysaccharide induction of antimicrobial peptide genes

The humoral immunity of Drosophila involves the production of antimicrobial peptides, which are induced by evolutionary conserved microbial molecules, like LPS. By using Drosophila mbn-2 cells, we found that live bacteria, in cluding E. coli, Salmonella typhimurium, Erwinia carotovora, and Pseudomona s aeruginosa, prevented LPS from inducing antimicrobial peptide genes, whil e Micrococcus luteus and Streptococcus equi did not. The inhibitory effect was seen at bacterial levels from 20 per mbn-2 cell, while antimicrobial pe ptides were induced at lower bacterial concentrations (less than or equal t o2 bacteria per cell) also in the absence of added LPS. Gel shift experimen t suggests that the inhibitory effect is upstream or at the level of the ac tivation of the transcription factor Relish, a member of the NF-kappaB/Rel family. The bacteria have to be in physical contact with the cells, but not phagocytosed, to prevent LPS induction. Interestingly, the inhibiting mech anism is, at least for E. coli, independent of the type III secretion syste m, indicating that the inhibitory mechanism is unrelated to the one earlier described for YopJ from Yersinia.