Dynamic structural adaptations enable the endobiotic predation ofbdellovibrio bacteriovorus

Abstract

Bdellovibrio bacteriovorusis an endobiotic microbial predator that offers promise as a living antibiotic for its ability to kill Gram-negative bacteria, including human pathogens. Even after six decades of study, fundamental details of its predation cycle remain mysterious. Here, we used cryo-electron tomography to comprehensively image the lifecycle ofB. bacteriovorusat nanometer-scale resolution. In addition to providing the first high-resolution images of predation in a native (hydrated, unstained) state, we also discover several surprising features of the process, including novel macromolecular complexes involved in prey attachment/invasion and a flexible portal structure lining a hole in the prey peptidoglycan that tightly seals the prey outer membrane around the predator during entry. Unexpectedly, we find thatB. bacteriovorusdoes not shed its flagellum during invasion, but rather resorbs it into its periplasm for degradation. Finally, following replication and division in the bdelloplast, we observe a transient and extensive ribosomal lattice on the condensedB. bacteriovorusnucleoid.Graphical abstract

Advances in cellular and molecular predatory biology of Bdellovibrio bacteriovorus six decades after discovery

“…Electron microscopy has been used to resolve key structures like the flagellar motor in B. bacteriovorus ( Morehouse et al, 2011 ; Chaban et al, 2018 ). Further advancements in electron microscopy techniques such as cryo-electron tomography (cryo-ET) supported the elucidation of novel structures within predator cells, to a resolution of individual proteins ( Xu et al, 2019 ; Kaplan et al, 2022 , Preprint). Alongside visualizing protein structures, the use of X-ray crystallography has generated models of individual B. bacteriovorus proteins, allowing elucidation of protein functions by evaluating interactions ( Lambert et al, 2015 ; Cho et al, 2019 ; Meek et al, 2019 ).…”

“…Transposon mutants of mcps took longer to reduce prey cell numbers by half in liquid culture ( Lambert et al, 2003 ), were unable to produce lytic plaques on prey cell lawns, and also had a significantly reduced ability to clear preformed biofilms ( Medina et al, 2008 ). Cryo-ET data identifies chemosensory arrays at the flagellated pole of B. bacteriovorus in the attack and attachment phases ( Borgnia et al, 2008 ; Butan et al, 2011 ; Kaplan et al, 2022 , Preprint), which seem to be degraded and therefore absent in later stages ( Kaplan et al, 2022 , Preprint). While there clearly is a role for chemotaxis, it is not yet fully understood which exact part of the predatory cycle it impacts.…”

Moving toward a better understanding of the model bacterial predator Bdellovibrio bacteriovorus

“…Prior cryo-electron tomography of isolated B. bacteriovorus cells established aspects of Bdellovibrio physiology -deformable cells, condensed nucleoid, chemotaxis arrays, cryptic filaments and storage granules [7], but was unable to inform on any of the stages of predation. This has been supplemented by recent analysis of Bdellovibrio invading three types of prey -V. cholerae, E. coli, and E. coli minicells [64]. The ability to follow predation events is transformative, revealing for the first time that the predator retracts [22] and later identified in a study on 3',3'-cyclic-GMP-AMP hydrolysis [71]).…”

“…This has been supplemented by recent analysis of Bdellovibrio invading three types of prey – V. cholerae, E. coli , and E. coli minicells [64]. The ability to follow predation events is transformative, revealing for the first time that the predator retracts and likely recycles its own flagellum during invasion, disproving ideas that it is shed prior to entry.…”

“…An electron-dense attachment plaque spans from the predator outer membrane to the prey cell wall, next to rose-like complexes of undetermined composition. Other observable features include surface fibres distinct from pili, a protruding ‘bubble’ at the sealed entry pore, and a partially ordered hexagonal lattice of ribosomes on the nucleoid of fed late-stage cells [64]. Features not observed are also telling, with no evidence for type IV pili in providing the entry force to overcome the pushback from periplasmic pressure – the means by which the predator translocates inward into prey remains enigmatic.…”