Characterization of type IV pili in the life cycle of the predator bacterium Bdellovibrio

Bdellovibrio and like organisms (BALOs) are obligate prokaryotic predators of other Gram-negative bacteria. Bdellovibrio bacteriovorus is the most studied organism among BALOs. It has a periplasmic life cycle with two major stages: a motile, non-replicative stage spent searching for prey (the attack phase) and a stage spent inside the periplasm of the Gram-negative prey cell (the growth phase) after forming an osmotically stable body termed the bdelloplast. Within Bdellovibrio, there are also strains exhibiting an epibiotic life cycle. The genome sequence of the type strain B. bacteriovorus HD100(T) revealed the presence of multiple dispersed pil genes encoding type IV pili. Type IV pili in other bacteria are involved in adherence to and invasion of host cells and therefore can be considered to play a role in invasion of prey cells by Bdellovibrio. In this study, genes involved in producing type IV pili were identified in the periplasmic strain B. bacteriovorus 109J and an epibiotic Bdellovibrio sp. strain JSS. The presence of fibres on attack-phase cells was confirmed by examining negative stains of cells fixed with 10% buffered formalin. Fibres were at the non-flagellated pole on approximately 25% of attack-phase cells. To confirm that these fibres were type IV pili, a truncated form of PilA lacking the first 35 amino acids was designed to facilitate purification of the protein. The truncated PilA fused to a His-tag was overexpressed in Escherichia coli BL21(DE3) plysS. The fusion protein, accumulated in the insoluble fraction, was purified under denaturing conditions and used to produce polyclonal antisera. Immunoelectron microscopy showed that polar fibres present on the cell surface of the predator were composed of PilA, the major subunit of type IV pili. Immunofluorescence microscopy showed the presence of pilin on attack-phase cells of B. bacteriovorus 109J during attachment to prey cells and just after penetration, inside the bdelloplast. Antibodies against PilA delayed and inhibited predation in co-cultures of Bdellovibrio. This study confirms that type IV pill play a role in invasion of prey cells by Bdellovibrio.