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The Department of Plant Pathology and Microbiology
Robert H. Smith Faculty of Agriculture, Food and Environment
The Hebrew University of Jerusalem

Martins MSc thesis experiments and results Martins MSc thesis experiments and results

Predatory bacteria

Predatory interactions are ubiquitous in nature- as such they also exist between prokaryotes. Predatory bacteria are widely distributed, they are diverse, and they exhibit a variety of hunting strategies. Among them, the Bdellovibrio and like organisms (BALOs) form a fascinating group of organisms: they are obligate predatory bacteria of gram negative cells and have unusual life cycles. We study the ecology and the life cycle of these microbial hunters.

Ecology and evolution of bacterial predators

Bdellovibrio and like organisms (BALOs) are found in many habitats. They have been isolated from and detected in soil, in the rhizosphere, in fresh, brackish and marine water, in wastewater, in extreme environments, and in the guts of some animals.

Our research goals

Predation is one of the numerous functions affecting bacterial communities; yet we know little of the players involved, how and with whom they interact. Do predatory bacteria affect ecological feature of the bacterial community (e.g. its structure, dynamics, functionality and else)? How is predation affected by environmental changes and habitat structure? Are ecological feature of bacterial predation related to the organization of the predator’s life cycle?

In order to address such questions, we study BALO diversity, and phylogeny, examine the dynamics of the predators in various environments, combining field observations with laboratory manipulations in microcosms.

Recent and ongoing research

The effect of spatial fragmentation on predatory dynamics.  Microcosms of variable connectivity were constructed using soils of different textures kept at different wetness levels. Connectivity strongly affected predatory regimes, from a single cycle of predation with both prey and predator populations crashing, to long -term stability of both predator and prey (Rita Petrenko; collaborators: Amit Huppert)

BALOs in wastewater treatment schemes. BALOs are found at consequent levels (0.1-1% of the total population) in various types of wastewater treatment systems. In order to understand the BALO interaction network and how the environment affects it  we characterize bacterial communities and predatory guilds with high throughput sequence analyses and specific quantification in -situ and in wastewater microcosms seeded with defined predators and prey (Yossi Cohen; Collaborators: Antonis Chazinotas. Funding DFG. Diana Rasuluniriana; Collaborators: Jaap van Rijn, Amit Huppert. Funding: ISF).

Bacterial predators against phytopathogens. Acidovorax citrulli attacks cucurbits, causing great losses. A screen of BALO strains has shown differential predation on various strains of the pathogens, some being sensitive others immune to predation (Einav Aharon; Collaborators: Saul Burdman. Funding: Minerva Foundation).

Figures

Top left: A Micavibrio epibiotic predator dividing, and an empty Pseudomonas corrugata prey cell; Top right: Bdellovibrio bacteriovorus  growing in the periplasm of an E. coli prey cell. Middle right: Fluorescent in-situ hybridization with a Bdellovibrio specific 16S rRNA targeted probe in a dual culture of B. bacteriovorus and E. coli. E. coli cells (red) are invaded by the predator (green/yellow). Middle left: A Bdellovibrio-invaded prey cell in wastewater, detected using a16S rRNA fluorescent probe targeting Bdellovibrio.

The obligate predatory lifestyle

Bdellovibrio and like organisms (BALOs) have a dimorphic cell cycle constituted of highly motile attack phase (AP) cells searching for a prey, and of a growth phase (GP) during which the prey is consumed and the predatory cell grows, to finally split into progeny AP cells.  BALOs differ in prey range and in predatory strategies: some BALOs are epibiotic, attaching to their prey and consuming them from outside; others penetrate their prey (periplasmic predation). 

Our research goals

We aim at understanding what makes a bacterium an obligate predator. What mediates prey sensing? What triggers growth and differentiation of a predatory cell? How is prey degradation modulated?

Recent and ongoing research

Early determinants of the predatory interaction. Based on transcriptomics and genomics work, a set of potential “early predatory genes” has been defined, including a set of genes encoding for the synthesis and excretion of specific pili. These appendages are shown to be essential for predation, and conserved among periplasmic and epibiotic BALOs (Ofir Avidan; Collaborators: Shmuel Pietrokovski. Michael Linscheid. Funding: GIF).

An ex-vivo system for growing wild type BALOs. Host independent mutants can grow axenically, providing a strong tool for cellular and expression studies. However they are deregulated in their ghost transition from AP to GP. An ex-vivo system using ghost prey cells that can be filled with different media was developed. It supports the growth of the wild type  periplasmic predator Bdellovibrio bacteriovorus and enables the dissection of signals driving the AP to GP shift (Or Rotem. Funding: ISF).

Cyclic di-GMP in BALOs. c-di-GMP greatly affect bacterial phenotypes. We identified a putative riboswitch of the AP as well as a large set of potential novel c-di-GMP binders in Bdellovibrio bacteriovorus (Or Rotem, Rita Petrenko; Collaborators, Shmuel Pietrokovski, Urs Jenal).

BALOmics. A transcriptomic analysis shows that B. bacteriovorus expresses two almost completely separated sets of genes in AP and in GP. Comparative analyses of the genomes of facultative and obligate predators reveal specific sets of genes implicated in adhesion, as well as in isoprenoid biosynthesis. These genes are unique or highly enriched in predatory bacteria vs. non-predators.  Further, epibiotic predators have significantly reduced genomes in comparison to periplasmic predators.  However, epibiotic predators belonging to different clades show conserved functions (Or Rotem, Zohar Pasternak; Collaborators: Shmuel Pietrokovski, Uri Gophna).

Figures

Right: A periplasmic BALO life cycle. 1. Attack phase; 2. Attachment; 3. Penetration; 4. Growth initiation; 5. Growth; 6, 7: division and differentiation; 8. Progeny release.

Left:Bdellovibrio bacteriovorus in a ghost prey cell.