Molecular investigation of Bdellovibrio bacteriovorus envelope processes associated with bacterial predation

Abstract

Antibiotics have been used to treat bacterial infections for nearly 80 years, however, the increasing emergence of antibiotic-resistant bacteria has now led to a significant global burden of antimicrobial resistance (AMR) and a worldwide healthcare crisis. Combatting AMR requires the complementary development of novel therapeutics. Bdellovibrio bacteriovorus is a small predatory bacterium that invades Gram-negative prey, replicates within the periplasm and then lyses the host cell. This predatory ability presents B. bacteriovorus as a potential novel antimicrobial therapeutic. In this thesis, I describe two short research projects which investigate the predatory envelope processes of gliding motility and cell wall-modification in B. bacteriovorus. B. bacteriovorus uses gliding motility on surfaces to both scout for prey and to ultimately facilitate exit from the dead host. Cyclic-di-GMP (c-di-GMP) positively regulates gliding motility in B. bacteriovorus and c-di-GMP signals are transduced through PilZ domains of receptor proteins. Some B. bacteriovorus PilZ proteins contain an additional putative GYF domain. We hypothesised that PilZ: GYF hybrid proteins may bind c-di-GMP (via the PilZ domain) and interact with the gliding motor (via the GYF domain) to regulate gliding motility. We aimed to test this hypothesis by crystallising the PilZ: GYF protein Bd1996. Bd1996 homologues from two different B. bacteriovorus strains were successfully expressed and purified. Bd1996 bound c-di-GMP in vitro, however, neither protein homologue could be crystallised. Modification of predator and prey cell walls is an important predatory process that involves a repertoire of different enzymes. In my second project, I investigated the function of the two cell wall-modifying enzymes Bd1402 and Bd1075. Bd1402 was secreted into the periplasm of the prey in which it may modify the prey cell wall. In contrast, Bd1075 localised to the B. bacteriovorus predator itself and an unmarked deletion of bd1075 resulted in the formation of predator cells that were straight rods, in comparison to vibroid wild-type cells. Together, these data suggest that Bd1075 is the curvature-determinant of B. bacteriovorus HD100