The KdpD/KdpE two-component system of Photorhabdus asymbiotica promotes bacterial survival within M. sexta hemocytes

Many bacteria persist within phagocytes, deploying complex sets of tightly regulated virulence factors to manipulate and survive within host cells. So far, no single factor has been identified that is sufficient to allow intracellular persistence of an otherwise non-pathogenic bacterium. Here we report that the two-component KdpD/KdpE sensor kinase/response regulator of the insect and human pathogen Photorhabdus asymbiotica (Pa) is sufficient to allow a harmless laboratory strain of E. coli to resist phagocytic killing and persist within insect hemocytes, ultimately killing the insect. Screening of a cosmid library of Pa in E. coli by injection into the moth Manduca sexta, previously identified three overlapping clones which caused the insect to cease feeding and subsequently die. Transposon mutagenesis revealed a cosmid encoded kdp high affinity potassium pump regulon was responsible for this phenotype. Gentamycin protection assays and confocal microscopy revealed the cosmid clones were persisting inside insect hemocytes far longer than control bacteria. Cloning and expression of Pa kdpD/kdpE alone into E. coli recapitulated the phenotype. Bioassay results and transcriptional analysis of various E. coli kdp mutants harboring the Pa kdp genes confirmed that Pa KdpD/KdpE was able to induce the E. coli kdp pump structural genes in response to exposure to insect hemocytes but not blood plasma alone. The finding that Pa KdpD/KdpE can facilitate resistance of E. coli to phagocytic killing suggests a central role for potassium in this process, supporting previous work implicating potassium sensing in virulence of other bacteria and also in the normal process of protease killing of engulfed bacteria by neutrophils.