PREDATOR APPENDAGES AND PREY CUES ARE NECESSARY FOR OBLIGATION PREDATION BY BDELLOVIBRIO BACTERIOVORUS

The obligate bacterial predator Bdellovibrio bacteriovorus invades the periplasm of its gram negative prey, forming a bdelloplast. Invasion triggers filamentous growth of the predatory cell, culminating in the formation and release of progeny cells from the bdelloplast.

In order to identify predator-associated factors involved in the early stages of the interaction, a screen of Bdellovibrio genomic and transcriptomic data was performed. Among the identified 43 genes, a tight adherence (TAD) operon, encoding for two type IVb fimbriae-like pili (Flp1 and Flp2) and their machinery was further analysed. Two additional Flp pilus-encoding genes (flp3 and flp4), unlinked to the TAD locus were identified. flp1, flp2 and flp4 were expressed while flp3 was not, under any of the tested conditions. Knockout of each of the flp genes except flp3 totally abolished predation. However, adherence to the prey cell was unaffected. Differential regulation of the expressed flp genes was demonstrated as the TAD operon is controlled by the flagellar sigma factor, FliA, while flp4 is not.

For the characterization of prey-associated, predation-triggering factors, a novel ex-vivo system that enables the growth of the predator on empty prey ghosts amended with a medium of choice was developed. It was shown that the predator recognizes a prey envelope-associated cue that primes it for growth. However, growth was only initiated by the sensing of a temporally separated soluble cue from the prey’s cytosol. This novel transition phase exhibits a specific gene expression profile and functionally acts as a cell cycle checkpoint of prey quality.