Activity of Bdellovibrio hit locus proteins, Bd0108 and Bd0109, links Type IVa pilus extrusion/retraction status to prey-independent growth signalling

Flagellar stator genes control a trophic shift from obligate to facultative predation and biofilm formation in a bacterial predator

The bacterial predator Bdellovibrio bacteriovorus is considered to be obligatorily prey (host)-dependent (H-D), and thus unable to form biofilms. However, spontaneous host-independent (H-I) variants grow axenically and can form robust biofilms. A screen of 350 H-I mutants revealed that single mutations in stator genes fliL or motA were sufficient to generate flagellar motility-defective H-I strains able to adhere to surfaces but unable to develop biofilms. The variants showed large transcriptional shifts in genes related to flagella, prey-invasion, and cyclic-di-GMP (CdG), as well as large changes in CdG cellular concentration relative to the H-D parent. The introduction of the parental fliL allele resulted in a full reversion to the H-D phenotype, but we propose that specific interactions between stator proteins prevented functional complementation by fliL paralogs. In contrast, specific mutations in a pilus-associated protein (Bd0108) mutant background were necessary for biofilm formation, including secretion of extracellular DNA (eDNA), proteins, and polysaccharides matrix components. Remarkably, fliL disruption strongly reduced biofilm development. All H-I variants grew similarly without prey, showed a strain-specific reduction in predatory ability in prey suspensions, but maintained similar high efficiency in prey biofilms. Population-wide allele sequencing suggested additional routes to host independence. Thus, stator and invasion pole-dependent signaling control the H-D and the H-I biofilm-forming phenotypes, with single mutations overriding prey requirements, and enabling shifts from obligate to facultative predation, with potential consequences on community dynamics. Our findings on the facility and variety of changes leading to facultative predation also challenge the concept of Bdellovibrio and like organisms being obligate predators.

IMPORTANCE

The ability of bacteria to form biofilms is a central research theme in biology, medicine, and the environment. We show that cultures of the obligate (host-dependent) "solitary" predatory bacterium Bdellovibrio bacteriovorus, which cannot replicate without prey, can use various genetic routes to spontaneously yield host-independent (H-I) variants that grow axenically (as a single species, in the absence of prey) and exhibit various surface attachment phenotypes, including biofilm formation. These routes include single mutations in flagellar stator genes that affect biofilm formation, provoke motor instability and large motility defects, and disrupt cyclic-di-GMP intracellular signaling. H-I strains also exhibit reduced predatory efficiency in suspension but high efficiency in prey biofilms. These changes override the requirements for prey, enabling a shift from obligate to facultative predation, with potential consequences on community dynamics.

Bdellovibrio's prey-independent lifestyle is fueled by amino acids as a carbon source

Identifying the nutritional requirements and growth conditions of microorganisms is crucial for determining their applicability in industry and understanding their role in clinical ecology. Predatory bacteria such as Bdellovibrio bacteriovorus have emerged as promising tools for combating infections by human bacterial pathogens due to their natural killing features. Bdellovibrio's lifecycle occurs inside prey cells, using the cytoplasm as a source of nutrients and energy. However, this lifecycle supposes a challenge when determining the specific uptake of metabolites from the prey to complete the growth inside cells, a process that has not been completely elucidated. Here, following a model-based approach, we illuminate the ability of B. bacteriovorus to replicate DNA, increase biomass, and generate adenosine triphosphate (ATP) in an amino acid-based rich media in the absence of prey, keeping intact its predatory capacity. In this culture, we determined the main carbon sources used and their preference, being glutamate, serine, aspartate, isoleucine, and threonine. This study offers new insights into the role of predatory bacteria in natural environments and establishes the basis for developing new Bdellovibrio applications using appropriate metabolic and physiological methodologies. KEY POINTS: • Amino acids support axenic lifestyle of Bdellovibrio bacteriovorus. • B. bacteriovorus preserves its predatory ability when growing in the absence of prey.

Controlling the expression of heterologous genes in Bdellovibrio bacteriovorus using synthetic biology strategies

Bdellovibrio bacteriovorus HD100 is an obligate predatory bacterium that preys upon Gram-negative bacteria. It has been proposed to be applied as a "living antibiotic" in several fields such as agriculture or even medicine, since it is able to prey upon bacterial pathogens. Its interesting lifestyle makes this bacterium very attractive as a microbial chassis for co-culture systems including two partners. A limitation to this goal is the scarcity of suitable synthetic biology tools for predator domestication. To fill this gap, we have firstly adapted the hierarchical assembly cloning technique Golden Standard (GS) to make it compatible with B. bacteriovorus HD100. The chromosomal integration of the Tn7 transposon's mobile element, in conjunction with the application of the GS technique, has allowed the systematic characterization of a repertoire of constitutive and inducible promoters, facilitating the control of the expression of heterologous genes in this bacterium. PJExD/EliR proved to be an exceptional promoter/regulator system in B. bacteriovorus HD100 when precise regulation is essential, while the synthetic promoter PBG37 showed a constitutive high expression. These genetic tools represent a step forward in the conversion of B. bacteriovorus into an amenable strain for microbial biotechnology approaches.

Bdellovibrio bacteriovorus uses chimeric fibre proteins to recognize and invade a broad range of bacterial hosts

Predatory bacteria, like the model endoperiplasmic bacterium Bdellovibrio bacteriovorus, show several adaptations relevant to their requirements for locating, entering and killing other bacteria. The mechanisms underlying prey recognition and handling remain obscure. Here we use complementary genetic, microscopic and structural methods to address this deficit. During invasion, the B. bacteriovorus protein CpoB concentrates into a vesicular compartment that is deposited into the prey periplasm. Proteomic and structural analyses of vesicle contents reveal several fibre-like proteins, which we name the mosaic adhesive trimer (MAT) superfamily, and show localization on the predator surface before prey encounter. These dynamic proteins indicate a variety of binding capabilities, and we confirm that one MAT member shows specificity for surface glycans from a particular prey. Our study shows that the B. bacteriovorus MAT protein repertoire enables a broad means for the recognition and handling of diverse prey epitopes encountered during bacterial predation and invasion.

Two reasons to kill: predation and kin discrimination in myxobacteria

Myxobacteria are social microbial predators that use cell-cell contacts to identify bacterial or fungal prey and to differentiate kin relatives to initiate cellular responses. For prey killing, they assemble Tad-like and type III-like secretion systems at contact sites. For kin discrimination (KD), they assemble outer membrane exchange complexes composed of the TraA and TraB receptors at contacts sites. A type VI secretion system and Rhs proteins also mediate KD. Following cellular recognition, these systems deliver appropriate effectors into target cells. For prey, this leads to cell death and lysis for nutrient consumption by myxobacteria. In KD, a panel of effectors are delivered, and if adjacent cells are clonal cells, resistance ensues because they express a cognate panel of immunity factors; while nonkin lack complete immunity and are intoxicated. This review compares and contrasts recent findings from these systems in myxobacteria.

AFM Force Mapping Elucidates Pilus Deployment and Key Lifestyle-Dependent Surface Properties in Bdellovibrio bacteriovorus

Bdellovibrio bacteriovorus is known for predation of a wide variety of Gram-negative bacteria, making it of interest as an alternative or supplement to chemical antibiotics. However, a fraction of B. bacteriovorus follows a nonpredatory, "host-independent" (HI) life cycle. In this study, live predatory and HI B. bacteriovorus were captured on a surface and examined, in buffer, by collecting force maps using atomic force microscopy (AFM). The approach curves obtained on HI cells are similar to those on other Gram-negative cells, with a short nonlinear region followed by a linear region. In contrast, the approach curves obtained on predatory cells have a large nonlinear region, reflecting the unusual flexibility of the predatory cell. As the AFM tip is retracted, it shows virtually no adhesion to predatory B. bacteriovorus but has multiple adhesion events on HI cells and the 200-500+ nm region immediately surrounding them. Measured pull-off forces, pull-off distances, and effective spring constants are consistent with the multiple stretching events of Type IV pili, both on and especially adjacent to the cells. Exposure of the HI B. bacteriovorus to a pH-neutral 10% cranberry juice solution, which contains type A proanthocyanidins that are known to interfere with the adhesion of multiple types of pili, results in a substantial reduction in adhesion. Type IV pili are required for successful predation by B. bacteriovorus, but pili used in the predation process are located at the non-flagellated pole of the cell and can retract when not in use. Such pili are rarely observed under the conditions of this study, where the predator has not encountered a prey cell. In contrast, HI cells appear to have many pili distributed on and around the whole cell, presumably ready to be utilized for a variety of HI cell activities including attachment to surfaces.

Production of 3′,3′-cGAMP by a Bdellovibrio bacteriovorus promiscuous GGDEF enzyme, Bd0367, regulates exit from prey by gliding motility

Bacterial second messengers are important for regulating diverse bacterial lifestyles. Cyclic di-GMP (c-di-GMP) is produced by diguanylate cyclase enzymes, named GGDEF proteins, which are widespread across bacteria. Recently, hybrid promiscuous (Hypr) GGDEF proteins have been described in some bacteria, which produce both c-di-GMP and a more recently identified bacterial second messenger, 3′,3′-cyclic-GMP-AMP (cGAMP). One of these proteins was found in the predatory Bdellovibrio bacteriovorus, Bd0367. The bd0367 GGDEF gene deletion strain was found to enter prey cells, but was incapable of leaving exhausted prey remnants via gliding motility on a solid surface once predator cell division was complete. However, it was unclear which signal regulated this process. We show that cGAMP signalling is active within B. bacteriovorus and that, in addition to producing c-di-GMP and some c-di-AMP, Bd0367 is a primary producer of cGAMP in vivo. Site-directed mutagenesis of serine 214 to an aspartate rendered Bd0367 into primarily a c-di-GMP synthase. B. bacteriovorus strain bd0367S214D phenocopies the bd0367 deletion strain by being unable to glide on a solid surface, leading to an inability of new progeny to exit from prey cells post-replication. Thus, this process is regulated by cGAMP. Deletion of bd0367 was also found to be incompatible with wild-type flagellar biogenesis, as a result of an acquired mutation in flagellin chaperone gene homologue fliS, implicating c-di-GMP in regulation of swimming motility. Thus the single Bd0367 enzyme produces two secondary messengers by action of the same GGDEF domain, the first reported example of a synthase that regulates multiple second messengers in vivo. Unlike roles of these signalling molecules in other bacteria, these signal to two separate motility systems, gliding and flagellar, which are essential for completion of the bacterial predation cycle and prey exit by B. bacteriovorus.

Secondary messengers are important signalling molecules in bacteria and a recently discovered one, called cGAMP, has recently been shown to be made by some enzymes which had previously been known to produce another secondary messenger, c-di-GMP. One of these “hybrid promiscuous” enzymes (Bd0367) is found in Bdellovibrio bacteriovorus, a bacterium that preys upon other bacteria, burrowing inside them and consuming them from within. Previous gene deletion work had shown that Bd0367 was essential in signalling for Bdellovibrio to leave the remains of its prey cell by gliding motility after predation was complete and it was thought that this was due to c-di-GMP signalling. However, here, we show that this gliding motility is actually regulated by cGAMP signalling and that c-di-GMP signalling is involved in swimming motility. A single enzyme produces two different molecules, signalling to two discrete motility systems, both of which are required for successful completion of the bacterium’s predatory lifestyle in prey on solid surfaces or in liquids.

Interactions between Bdellovibrio and like organisms and bacteria in biofilms: beyond predator-prey dynamics

Bdellovibrio and like organisms (BALOs) prey on Gram-negative bacteria in the planktonic phase as well as in biofilms, with the ability to reduce prey populations by orders of magnitude. During the last few years, evidence has mounted for a significant ecological role for BALOs, with important implications for our understanding of microbial community dynamics as well as for applications against pathogens, including drug-resistant pathogens, in medicine, agriculture and aquaculture, and in industrial settings for various uses. However, our understanding of biofilm predation by BALOs is still very fragmentary, including gaps in their effect on biofilm structure, on prey resistance, and on evolutionary outcomes of both predators and prey. Furthermore, their impact on biofilms has been shown to reach beyond predation, as they are reported to reduce biofilm structures of non-prey cells (including Gram-positive bacteria). Here, we review the available literature on BALOs in biofilms, extending known aspects to potential mechanisms employed by the predators to grow in biofilms. Within that context, we discuss the potential ecological significance and potential future utilization of the predatory and enzymatic possibilities offered by BALOs in medical, agricultural and environmental applications.

Bdellovibrio and like organisms: current understanding and knowledge gaps of the smallest cellular hunters of the microbial world

Almost sixty years ago, Bdellovibrio and like organisms (BALOs) were discovered as the first obligate bacterial predators of other bacteria known to science. Since then, they were shown to be diverse and ubiquitous in the environment, and to bear astonishing ecological, physiological, and metabolic capabilities. The last decade has seen important strides made in understanding the mechanistic basis of their life cycle, the dynamics of their interactions with prey, along with significant developments towards their use in medicine, agriculture, and industry. This review details these achievements, identify current understanding and knowledge gaps to encourage and guide future BALO research.

Bacterial Predation on Cyanobacteria

Predatory bacteria gained interest in the last 20 years. Nevertheless, only a few species are well characterized. The endobiotic predator Bdellovibrio bacteriovorus invades its prey to consume it from the inside, whereas Myxococcus xanthus hunts as a whole group to overcome its prey. Both species were described to prey on cyanobacteria as well. This minireview summarizes the findings of the last 20 years of predatory bacteria of cyanobacteria and is supplemented by new findings from a screening experiment for bacterial predators of the model organism Anabaena variabilis PCC 7937. Known predatory bacteria of cyanobacteria belong to the phyla Proteobacteria, Bacteroidetes, and Firmicutes and follow different hunting strategies. The underlying mechanisms are in most cases not known in much detail. Isolates from the screening experiment were clustered after predation behaviour and analyzed with respect to their size. The effect of predation in high nitrate levels and the occurrence of nitrogen-fixing cells, called heterocysts, are addressed.

A lysozyme with altered substrate specificity facilitates prey cell exit by the periplasmic predator Bdellovibrio bacteriovorus

Lysozymes are among the best-characterized enzymes, acting upon the cell wall substrate peptidoglycan. Here, examining the invasive bacterial periplasmic predator Bdellovibrio bacteriovorus, we report a diversified lysozyme, DslA, which acts, unusually, upon (GlcNAc-) deacetylated peptidoglycan. B. bacteriovorus are known to deacetylate the peptidoglycan of the prey bacterium, generating an important chemical difference between prey and self walls and implying usage of a putative deacetyl-specific “exit enzyme”. DslA performs this role, and ΔDslA strains exhibit a delay in leaving from prey. The structure of DslA reveals a modified lysozyme superfamily fold, with several adaptations. Biochemical assays confirm DslA specificity for deacetylated cell wall, and usage of two glutamate residues for catalysis. Exogenous DslA, added ex vivo, is able to prematurely liberate B. bacteriovorus from prey, part-way through the predatory lifecycle. We define a mechanism for specificity that invokes steric selection, and use the resultant motif to identify wider DslA homologues.

The bacterial periplasmic predator Bdellovibrio bacteriovorus deacetylates the peptidoglycan of the prey bacterium early upon invasion. Here, the authors identify and characterize a Bdellovibrio lysozyme that acts specifically on deacetylated peptidoglycan and is important for periplasmic exit.

Providing new insights on the biphasic lifestyle of the predatory bacterium Bdellovibrio bacteriovorus through genome-scale metabolic modeling

In this study we analyze the growth-phase dependent metabolic states of Bdellovibrio bacteriovorus by constructing a fully compartmented, mass and charge-balanced genome-scale metabolic model of this predatory bacterium (iCH457). Considering the differences between life cycle phases driving the growth of this predator, growth-phase condition-specific models have been generated allowing the systematic study of its metabolic capabilities. Using these computational tools, we have been able to analyze, from a system level, the dynamic metabolism of the predatory bacteria as the life cycle progresses. We provide computational evidences supporting potential axenic growth of B. bacteriovorus's in a rich medium based on its encoded metabolic capabilities. Our systems-level analysis confirms the presence of "energy-saving" mechanisms in this predator as well as an abrupt metabolic shift between the attack and intraperiplasmic growth phases. Our results strongly suggest that predatory bacteria's metabolic networks have low robustness, likely hampering their ability to tackle drastic environmental fluctuations, thus being confined to stable and predictable habitats. Overall, we present here a valuable computational testbed based on predatory bacteria activity for rational design of novel and controlled biocatalysts in biotechnological/clinical applications.

Studies on Bd0934 and Bd3507, Two Secreted Nucleases from Bdellovibrio bacteriovorus, Reveal Sequential Release of Nucleases during the Predatory Cycle

Antibiotic resistance is a major global concern with few available new means to combat it. From a therapeutic perspective, predatory bacteria constitute an interesting tool. They not only eliminate the pathogen but also reduce the overall pool of antibiotic resistance genes through secretion of nucleases and complete degradation of exogenous DNA. Molecular knowledge of how these secreted DNases act will give us further insight into how antibiotic resistance, and the spread thereof, can be limited through the action of predatory bacteria.

Bdellovibrio bacteriovorus is an obligate predatory bacterium that invades and kills a broad range of Gram-negative prey cells, including human pathogens. Its potential therapeutic application has been the subject of increased research interest in recent years. However, an improved understanding of the fundamental molecular aspects of the predatory life cycle is crucial for developing this bacterium as a “living antibiotic.” During intracellular growth, B. bacteriovorus secretes an arsenal of hydrolases, which digest the content of the host cell to provide growth nutrients for the predator, e.g., prey DNA is completely degraded by the nucleases. Here, we have, on a genetic and molecular level, characterized two secreted DNases from B. bacteriovorus, Bd0934 and Bd3507, and determined the temporal expression profile of other putative secreted nucleases. We conclude that Bd0934 and Bd3507 are likely a part of the predatosome but are not essential for the predation, host-independent growth, prey biofilm degradation, and self-biofilm formation. The detailed temporal expression analysis of genes encoding secreted nucleases revealed that these enzymes are produced in a sequential orchestrated manner. This work contributes to our understanding of the sequential breakdown of the prey nucleic acid by the nucleases secreted during the predatory life cycle of B. bacteriovorus.

IMPORTANCE Antibiotic resistance is a major global concern with few available new means to combat it. From a therapeutic perspective, predatory bacteria constitute an interesting tool. They not only eliminate the pathogen but also reduce the overall pool of antibiotic resistance genes through secretion of nucleases and complete degradation of exogenous DNA. Molecular knowledge of how these secreted DNases act will give us further insight into how antibiotic resistance, and the spread thereof, can be limited through the action of predatory bacteria.

DivIVA Controls Progeny Morphology and Diverse ParA Proteins Regulate Cell Division or Gliding Motility in Bdellovibrio bacteriovorus

The predatory bacterium B. bacteriovorus grows and divides inside the periplasm of Gram-negative bacteria, forming a structure known as a bdelloplast. Cell division of predators inside the dead prey cell is not by binary fission but instead by synchronous division of a single elongated filamentous cell into odd or even numbers of progeny cells. Bdellovibrio replication and cell division processes are dependent on the finite level of nutrients available from inside the prey bacterium. The filamentous growth and division process of the predator maximizes the number of progeny produced by the finite nutrients in a way that binary fission could not. To learn more about such an unusual growth profile, we studied the role of DivIVA in the growing Bdellovibrio cell. This protein is well known for its link to polar cell growth and spore formation in Gram-positive bacteria, but little is known about its function in a predatory growth context. We show that DivIVA is expressed in the growing B. bacteriovorus cell and controls cell morphology during filamentous cell division, but not the number of progeny produced. Bacterial Two Hybrid (BTH) analysis shows DivIVA may interact with proteins that respond to metabolic indicators of amino-acid biosynthesis or changes in redox state. Such changes may be relevant signals to the predator, indicating the consumption of prey nutrients within the sealed bdelloplast environment. ParA, a chromosome segregation protein, also contributes to bacterial septation in many species. The B. bacteriovorus genome contains three ParA homologs; we identify a canonical ParAB pair required for predatory cell division and show a BTH interaction between a gene product encoded from the same operon as DivIVA with the canonical ParA. The remaining ParA proteins are both expressed in Bdellovibrio but are not required for predator cell division. Instead, one of these ParA proteins coordinates gliding motility, changing the frequency at which the cells reverse direction. Our work will prime further studies into how one bacterium can co-ordinate its cell division with the destruction of another bacterium that it dwells within.

From the Inside Out: an Epibiotic Bdellovibrio Predator with an Expanded Genomic Complement

Bdellovibrio and like organisms are abundant environmental parasitoids of prokaryotes that show diverse predation strategies. The vast majority of studied Bdellovibrio bacteria and like organisms deploy intraperiplasmic replication inside the prey cell, while few isolates with smaller genomes consume their prey from the outside in an epibiotic manner. The novel parasitoid "Candidatus Bdellovibrio qaytius" was isolated from a eutrophic freshwater pond in British Columbia, where it was a continual part of the microbial community. "Ca Bdellovibrio qaytius" was found to preferentially prey on the betaproteobacterium Paraburkholderia fungorum without entering the periplasm. Despite its epibiotic replication strategy, "Ca Bdellovibrio" encodes a large genomic complement more similar to that of complex periplasmic predators. Functional genomic annotation further revealed several biosynthesis pathways not previously found in epibiotic predators, indicating that "Ca Bdellovibrio" represents an intermediate phenotype and at the same time narrowing down the genomic complement specific to epibiotic predators. In phylogenetic analysis, "Ca Bdellovibrio qaytius" occupies a widely distributed, but poorly characterized, basal cluster within the genus Bdellovibrio This suggests that epibiotic predation might be a common predation type in nature and that epibiotic predation could be the ancestral predation type in the genus.IMPORTANCE Bdellovibrio and like organisms are bacteria that prey on other bacteria and are widespread in the environment. Most of the known Bdellovibrio species enter the space between the inner and outer prey membrane, where they consume their prey cells. However, one Bdellovibrio species has been described that consumes its prey from the outside. Here, we describe "Ca Bdellovibrio qaytius," a novel member of the genus Bdellovibrio that also remains outside the prey cell throughout its replication cycle. Unexpectedly, the genome of "Ca Bdellovibrio" is much more similar to the genomes of intracellular predators than to the species with a similar life cycle. Since "Ca Bdellovibrio" is also a basal representative of this genus, we hypothesize that extracellular predation could be the ancestral predation strategy.

Expression of attack and growth phase genes of Bdellovibrio bacteriovorus in the presence of Gram-negative and Gram-positive prey

The expression of attack phase (AP) and growth phase (GP) genes of Bdellovibrio bacteriovorus (B. bacteriovorus) was compared in the presence of Gram-negative [Escherichia coli (E. coli) and Klebsiella pneumoniae (K. pneumoniae)] and Gram-positive [Enterococcus faecium (E. faecium)] prey, using relative quantitative polymerase chain reaction (relative qPCR) assays. The genes bd0108 (pili retraction/extrusion) and merRNA (massively expressed riboswitch RNA) were highly expressed in the AP cells [3.99- to 6.06-fold (E. coli), 3.91- to 7.05-fold (K. pneumoniae) and 2.91- to 7.30-fold (E. faecium)]. The fliC1 gene (flagella filament) was also expressed at a high level in the AP cells however, after 240 min of co-culture with E. faecium the expression of fliC1 remained low (at 0.759-fold), while in the presence of the Gram-negative prey fliC1 expression increased. Additionally, the GP genes bd0816 (peptidoglycan-modifying enzyme) and groES1 (chaperone protein) were not induced in the presence of E. faecium. However, they were expressed in the early GP and GP of B. bacteriovorus after exposure to the Gram-negative prey. It can thus be concluded that B. bacteriovorus senses the presence of potential prey when exposed to Gram-positive and Gram-negative bacteria, however the GP genes are not induced in co-culture with E. faecium. The results from this study thus indicate that B. bacteriovorus does not actively grow in the presence of E. faecium and the second predatory cue (induces active growth of B. bacteriovorus) is lacking when B. bacteriovorus is co-cultured with the Gram-positive prey.

Shedding Light on the Cell Biology of the Predatory Bacterium Bdellovibrio bacteriovorus

Bdellovibrio bacteriovorus is a predatory bacterium that feeds upon and proliferates inside other Gram-negative bacteria. Upon entry into the periplasmic space of the prey envelope, B. bacteriovorus initiates an exquisite developmental program in which it digests the host resources and grows as a filament, which eventually divides in a non-binary manner, releasing a variable number of daughter cells. The progeny then escape from the prey ghost to encounter new victims and resume the predation cycle. Owing to its unique biology, B. bacteriovorus undoubtedly represents an attractive model to unravel novel mechanisms of bacterial cell cycle control and cellular organization. Yet, the molecular factors behind the sophisticated lifestyle of this micro-predator are still mysterious. In particular, the spatiotemporal dynamics of proteins that control key cellular processes such as transmission of the genetic information, cell growth and division remain largely unexplored. In this Perspective article, I highlight outstanding fundamental questions related to these aspects and arising from the original biology of this bacterium. I also discuss available insights and potential cell biology approaches based on quantitative live imaging techniques, in combination with bacterial genetics and biochemistry, to shed light on the intracellular organization of B. bacteriovorus in space and time.

High-Throughput Analysis of Gene Function in the Bacterial Predator Bdellovibrio bacteriovorus

Bdellovibrio bacteriovorus is a predatory bacterium that can kill a wide range of Gram-negative bacteria, including many human pathogens. Given the global rise of antibiotic resistance and dearth of new antibiotics discovered in the past 30 years, this predator has potential as an alternative to traditional antibiotics. For many years, B. bacteriovorus research was hampered by a lack of genetic tools, and the genetic mechanisms of predation have only recently begun to be established. Here, we comprehensively identify and characterize predator genes required for killing bacterial prey, as well as genes that interfere in this process, which may allow us to design better therapeutic predators. Based on our study, we and other researchers may ultimately be able to genetically engineer strains that have improved killing rates, target specific species of prey, or preferentially target prey in the planktonic or biofilm state.

Bdellovibrio bacteriovorus is a bacterial predator capable of killing and replicating inside most Gram-negative bacteria, including antibiotic-resistant pathogens. Despite growing interest in this organism as a potential therapeutic, many of its genes remain uncharacterized. Here, we perform a high-throughput genetic screen with B. bacteriovorus using transposon sequencing (Tn-seq) to explore the genetic requirements of predation. Two hundred one genes were deemed essential for growth in the absence of prey, whereas over 100 genes were found to be specifically required for predative growth on the human pathogens Vibrio cholerae and Escherichia coli in both planktonic and biofilm states. To further this work, we created an ordered-knockout library in B. bacteriovorus and developed new high-throughput techniques to characterize the mutants by their stage of deficiency in the predator life cycle. Using microscopy and flow cytometry, we confirmed 10 mutants defective in prey attachment and eight mutants defective in prey rounding. The majority of these genes are hypothetical and previously uncharacterized. Finally, we propose new nomenclature to group B. bacteriovorus mutants into classes based on their stage of predation defect. These results contribute to our basic understanding of bacterial predation and may be useful for harnessing B. bacteriovorus to kill harmful pathogens in the clinical setting.

Evolutionary diversification of the RomR protein of the invasive deltaproteobacterium, Bdellovibrio bacteriovorus

Bdellovibrio bacteriovorus is a predatory deltaproteobacterium that encounters individual Gram-negative prey bacteria with gliding or swimming motility, and then is able to invade such prey cells via type IVa pilus-dependent mechanisms. Movement control (pili or gliding) in other deltaproteobacteria, such as the pack hunting Myxococcus xanthus, uses a response regulator protein, RomR_Mx (which dynamically relocalises between the cell poles) and a GTPase, MglA_Mx, previously postulated as an interface between the Frz_Mx chemosensory system and gliding or pilus-motility apparatus, to produce regulated bidirectional motility. In contrast, B. bacteriovorus predation is a more singular encounter between a lone predator and prey; contact is always via the piliated, non-flagellar pole of the predator, involving MglA_Bd, but no Frz system. In this new study, tracking fluorescent RomR_Bd microscopically during predatory growth shows that it does not dynamically relocalise, in contrast to the M. xanthus protein; instead having possible roles in growth events. Furthermore, transcriptional start analysis, site-directed mutagenesis and bacterial two-hybrid interaction studies, indicate an evolutionary loss of RomR_Bd activation (via receiver domain phosphorylation) in this lone hunting bacterium, demonstrating divergence from its bipolar role in motility in pack-hunting M. xanthus and further evolution that may differentiate lone from pack predators.

Whole-Genome Sequencing and Comparative Genome Analysis Provided Insight into the Predatory Features and Genetic Diversity of Two Bdellovibrio Species Isolated from Soil

Bdellovibrio spp. are predatory bacteria with great potential as antimicrobial agents. Studies have shown that members of the genus Bdellovibrio exhibit peculiar characteristics that influence their ecological adaptations. In this study, whole genomes of two different Bdellovibrio spp. designated SKB1291214 and SSB218315 isolated from soil were sequenced. The core genes shared by all the Bdellovibrio spp. considered for the pangenome analysis including the epibiotic B. exovorus were 795. The number of unique genes identified in Bdellovibrio spp. SKB1291214, SSB218315, W, and B. exovorus JJS was 1343, 113, 857, and 1572, respectively. These unique genes encode hydrolytic, chemotaxis, and transporter proteins which might be useful for predation in the Bdellovibrio strains. Furthermore, the two Bdellovibrio strains exhibited differences based on the % GC content, amino acid identity, and 16S rRNA gene sequence. The 16S rRNA gene sequence of Bdellovibrio sp. SKB1291214 shared 99% identity with that of an uncultured Bdellovibrio sp. clone 12L 106 (a pairwise distance of 0.008) and 95-97% identity (a pairwise distance of 0.043) with that of other culturable terrestrial Bdellovibrio spp., including strain SSB218315. In Bdellovibrio sp. SKB1291214, 174 bp sequence was inserted at the host interaction (hit) locus region usually attributed to prey attachment, invasion, and development of host independent Bdellovibrio phenotypes. Also, a gene equivalent to Bd0108 in B. bacteriovorus HD100 was not conserved in Bdellovibrio sp. SKB1291214. The results of this study provided information on the genetic characteristics and diversity of the genus Bdellovibrio that can contribute to their successful applications as a biocontrol agent.

Predator Versus Pathogen: How Does Predatory Bdellovibrio bacteriovorus Interface with the Challenges of Killing Gram-Negative Pathogens in a Host Setting?

Bdellovibrio bacteriovorus is a small deltaproteobacterial predator that has evolved to invade, reseal, kill, and digest other gram-negative bacteria in soils and water environments. It has a broad host range and kills many antibiotic-resistant, clinical pathogens in vitro, a potentially useful capability if it could be translated to a clinical setting. We review relevant mechanisms of B. bacteriovorus predation and the physiological properties that would influence its survival in a mammalian host. Bacterial pathogens increasingly display conventional antibiotic resistance by expressing and varying surface and soluble biomolecules. Predators coevolved alongside prey bacteria and so encode diverse predatory enzymes that are hard for pathogens to resist by simple mutation. Predators do not replicate outside pathogens and thus express few transport proteins and thus few surface epitopes for host immune recognition. We explain these features, relating them to the potential of predatory bacteria as cellular medicines.

Controlling Bdellovibrio bacteriovorus Gene Expression and Predation Using Synthetic Riboswitches

Bdellovibrio bacteriovorus is a predatory bacterium that feeds on Gram-negative bacteria including a wide range of pathogens and thus has potential applications as a biocontrol agent. Owing to its unique life cycle, however, there are limited tools that enable genetic manipulation of B. bacteriovorus. This work describes our first steps toward engineering the predatory bacterium for practical applications by developing basic genetic parts to control gene expression. Specifically, we evaluated four robust promoters that are active during the attack phase of B. bacteriovorus. Subsequently, we tested several synthetic riboswitches that have been reported to function in Escherichia coli, and identified theophylline-activated riboswitches that function in B. bacteriovorus. Finally, we inserted the riboswitch into the bacterial chromosome to regulate expression of the flagellar sigma factor fliA, which was previously predicted to be essential for predation, and observed that the engineered strain shows a faster predation kinetics in the presence of theophylline.

Attack-Phase Bdellovibrio bacteriovorus Responses to Extracellular Nutrients Are Analogous to Those Seen During Late Intraperiplasmic Growth

Bdellovibrio bacteriovorus is a predatory bacterium which lives by invading the periplasm of gram-negative bacteria and consuming them from within. This predator was thought to be dependent upon prey for nutrients since it lacks genes encoding for critical enzymes involved in amino acid biosynthesis. This study, however, found that planktonic attack-phase predators are not just dependent upon prey for nutrients, but rather, they respond to nutrients in the surrounding medium and, subsequently, synthesize and secrete proteases in a nutrient-dependent manner. The major secreted proteases were identified through mass spectrometry analyses. Subsequent RT-qPCR analyses found that the nutrient-induced proteases are similar to those expressed within the prey periplasm during the late intraperiplasmic growth phase. Furthermore, RNA sequencing found that incubating the planktonic attack-phase cells in a nutritious environment for a short period of time (4 h) changes its gene expression pattern to a status that is akin to the late intraperiplasmic phase, with more than 94% of the genes previously identified as being late intraperiplasmic-specific also being induced by nutrient broth in this study. This strong correlation between the gene expression patterns hints that the availability of hydrolyzed prey cell components to the predator is likely the stimulus controlling the expression of late intraperiplasmic B. bacteriovorus genes during predation.

[Bdellovibrio and like organisms: outstanding predators!]

Obligate predatory bacteria, i.e. bacteria requiring a Gram negative prey cell in order to complete their cell cycle, belong to the polyphyletic group referred to as the Bdellovibrio And Like Organisms (BALO). Predatory interactions between bacteria are complex, yet their dynamics and impact on bacterial communities in the environment are becoming better understood. BALO have unique life cycles: they grow epibiotically with the predator remaining attached to the prey's envelope, dividing in a binary manner or periplasmically, i.e. by penetrating the prey's periplasm to generate a number of progeny cells. The periplasmic life cycle includes unique gene and protein patterns and unique signaling features. These ecological and cellular features, along with applications of the BALO in the medical, agricultural and environmental fields are surveyed.

Identification and Characterization of Differentially-Regulated Type IVb Pilin Genes Necessary for Predation in Obligate Bacterial Predators

Bdellovibrio bacteriovorus is an obligate predator of bacteria that grows and divides within the periplasm of its prey. Functions involved in the early steps of predation have been identified and characterized, but mediators of prey invasion are still poorly detailed. By combining omics data available for Bdellovibrio and like organisms (BALO’s), we identified 43 genes expressed in B. bacteriovorus during the early interaction with prey. These included genes in a tight adherence (TAD) operon encoding for two type IVb fimbriae-like pilin proteins (flp1 and flp2), and their processing and export machinery. Two additional flp genes (flp3 and flp4) were computationally identified at other locations along the chromosome, defining the largest and most diverse type IVb complement known in bacteria to date. Only flp1, flp2 and flp4 were expressed; their respective gene knock-outs resulted in a complete loss of the predatory ability without losing the ability to adhere to prey cells. Additionally, we further demonstrate differential regulation of the flp genes as the TAD operon of BALOs with different predatory strategies is controlled by a flagellar sigma factor FliA, while flp4 is not. Finally, we show that FliA, a known flagellar transcriptional regulator in other bacteria, is an essential Bdellovibrio gene.

Interrupting peptidoglycan deacetylation during Bdellovibrio predator-prey interaction prevents ultimate destruction of prey wall, liberating bacterial-ghosts

The peptidoglycan wall, located in the periplasm between the inner and outer membranes of the cell envelope in Gram-negative bacteria, maintains cell shape and endows osmotic robustness. Predatory Bdellovibrio bacteria invade the periplasm of other bacterial prey cells, usually crossing the peptidoglycan layer, forming transient structures called bdelloplasts within which the predators replicate. Prey peptidoglycan remains intact for several hours, but is modified and then degraded by escaping predators. Here we show predation is altered by deleting two Bdellovibrio N-acetylglucosamine (GlcNAc) deacetylases, one of which we show to have a unique two domain structure with a novel regulatory”plug”. Deleting the deacetylases limits peptidoglycan degradation and rounded prey cell “ghosts” persist after mutant-predator exit. Mutant predators can replicate unusually in the periplasmic region between the peptidoglycan wall and the outer membrane rather than between wall and inner-membrane, yet still obtain nutrients from the prey cytoplasm. Deleting two further genes encoding DacB/PBP4 family proteins, known to decrosslink and round prey peptidoglycan, results in a quadruple mutant Bdellovibrio which leaves prey-shaped ghosts upon predation. The resultant bacterial ghosts contain cytoplasmic membrane within bacteria-shaped peptidoglycan surrounded by outer membrane material which could have promise as “bacterial skeletons” for housing artificial chromosomes.

Identification and differential production of ubiquinone-8 in the bacterial predator Bdellovibrio bacteriovorus

Bdellovibrio bacteriovorus 109J, a predatory bacterium with potential as a bacterial control agent, can exist in several lifestyles that differ both in predatory capacity and color. We determined that levels of ubiquinone-8 contribute to the distinctive but variable yellow color of different types of Bdellovibrio cells. Steady-state ubiquinone-8 concentrations did not differ markedly between conventional predatory and host-independent B. bacteriovorus despite upregulation of a suite of ubiquinone-8 synthesis genes in host-independent cells. In contrast, in spatially organized B. bacteriovorus films, the yellow inner regions contain significantly higher ubiquinone-8 concentrations than the off-white outer regions. Correspondingly, RT-PCR analysis reveals that the inner region, previously shown to consist primarily of active predators, clearly expresses two ubiquinone biosynthesis genes, while the outer region, composed mainly of quiescent or stalled bdelloplasts, expresses those genes weakly or not at all. Moreover, B. bacteriovorus cells in the inner region of week-old interfacial films, which are phenotypically attack-phase, have much higher UQ8 levels than regular attack-phase bdellovibrios, most likely because their "trapped" state prevents a high expenditure of energy to power flagellar motion.

Visualizing Bdellovibrio bacteriovorus by Using the tdTomato Fluorescent Protein

Bdellovibrio bacteriovorus is a Gram-negative bacterium that belongs to the delta subgroup of proteobacteria and is characterized by a predatory life cycle. In recent years, work has highlighted the potential use of this predator to control bacteria and biofilms. Traditionally, the reduction in prey cells was used to monitor predation dynamics. In this study, we introduced pMQ414, a plasmid that expresses the tdTomato fluorescent reporter protein, into a host-independent strain and a host-dependent strain of B. bacteriovorus 109J. The new construct was used to conveniently monitor predator proliferation in real time, in different growth conditions, in the presence of lytic enzymes, and on several prey bacteria, replicating previous studies that used plaque analysis to quantify B. bacteriovorus. The new fluorescent plasmid also enabled us to visualize the predator in liquid cultures, in the context of a biofilm, and in association with human epithelial cells.

Ankyrin-mediated self-protection during cell invasion by the bacterial predator Bdellovibrio bacteriovorus

Predatory Bdellovibrio bacteriovorus are natural antimicrobial organisms, killing other bacteria by whole-cell invasion. Self-protection against prey-metabolizing enzymes is important for the evolution of predation. Initial prey entry involves the predator's peptidoglycan DD-endopeptidases, which decrosslink cell walls and prevent wasteful entry by a second predator. Here we identify and characterize a self-protection protein from B. bacteriovorus, Bd3460, which displays an ankyrin-based fold common to intracellular pathogens of eukaryotes. Co-crystal structures reveal Bd3460 complexation of dual targets, binding a conserved epitope of each of the Bd3459 and Bd0816 endopeptidases. Complexation inhibits endopeptidase activity and cell wall decrosslinking in vitro. Self-protection is vital - ΔBd3460 Bdellovibrio deleteriously decrosslink self-peptidoglycan upon invasion, adopt a round morphology, and lose predatory capacity and cellular integrity. Our analysis provides the first mechanistic examination of self-protection in Bdellovibrio, documents protection-multiplicity for products of two different genomic loci, and reveals an important evolutionary adaptation to an invasive predatory bacterial lifestyle.

Cell-cycle progress in obligate predatory bacteria is dependent upon sequential sensing of prey recognition and prey quality cues

Predators feed on prey to acquire the nutrients necessary to sustain their survival, growth, and replication. In Bdellovibrio bacteriovorus, an obligate predator of Gram-negative bacteria, cell growth and replication are tied to a shift from a motile, free-living phase of search and attack to a sessile, intracellular phase of growth and replication during which a single prey cell is consumed. Engagement and sustenance of growth are achieved through the sensing of two unidentified prey-derived cues. We developed a novel ex vivo cultivation system for B. bacteriovorus composed of prey ghost cells that are recognized and invaded by the predator. By manipulating their content, we demonstrated that an early cue is located in the prey envelope and a late cue is found within the prey soluble fraction. These spatially and temporally separated cues elicit discrete and combinatory regulatory effects on gene transcription. Together, they delimit a poorly characterized transitory phase between the attack phase and the growth phase, during which the bdelloplast (the invaded prey cell) is constructed. This transitory phase constitutes a checkpoint in which the late cue presumably acts as a determinant of the prey's nutritional value before the predator commits. These regulatory adaptations to a unique bacterial lifestyle have not been reported previously.

An Extended Cyclic Di-GMP Network in the Predatory Bacterium Bdellovibrio bacteriovorus

Over the course of the last 3 decades the role of the second messenger cyclic di-GMP (c-di-GMP) as a master regulator of bacterial physiology was determined. Although the control over c-di-GMP levels via synthesis and breakdown and the allosteric regulation of c-di-GMP over receptor proteins (effectors) and riboswitches have been extensively studied, relatively few effectors have been identified and most are of unknown functions. The obligate predatory bacterium Bdellovibrio bacteriovorus has a peculiar dimorphic life cycle, in which a phenotypic transition from a free-living attack phase (AP) to a sessile, intracellular predatory growth phase (GP) is tightly regulated by specific c-di-GMP diguanylate cyclases. B. bacteriovorus also bears one of the largest complement of defined effectors, almost none of known functions, suggesting that additional proteins may be involved in c-di-GMP signaling. In order to uncover novel c-di-GMP effectors, a c-di-GMP capture-compound mass-spectroscopy experiment was performed on wild-type AP and host-independent (HI) mutant cultures, the latter serving as a proxy for wild-type GP cells. Eighty-four proteins were identified as candidate c-di-GMP binders. Of these proteins, 65 did not include any recognized c-di-GMP binding site, and 3 carried known unorthodox binding sites. Putative functions could be assigned to 59 proteins. These proteins are included in metabolic pathways, regulatory circuits, cell transport, and motility, thereby creating a potentially large c-di-GMP network. False candidate effectors may include members of protein complexes, as well as proteins binding nucleotides or other cofactors that were, respectively, carried over or unspecifically interacted with the capture compound during the pulldown. Of the 84 candidates, 62 were found to specifically bind the c-di-GMP capture compound in AP or in HI cultures, suggesting c-di-GMP control over the whole-cell cycle of the bacterium. High affinity and specificity to c-di-GMP binding were confirmed using microscale thermophoresis with a hypothetical protein bearing a PilZ domain, an acyl coenzyme A dehydrogenase, and a two-component system response regulator, indicating that additional c-di-GMP binding candidates may be bona fide novel effectors.

IMPORTANCE

In this study, 84 putative c-di-GMP binding proteins were identified in B. bacteriovorus, an obligate predatory bacterium whose lifestyle and reproduction are dependent on c-di-GMP signaling, using a c-di-GMP capture compound precipitation approach. This predicted complement covers metabolic, energy, transport, motility and regulatory pathways, and most of it is phase specific, i.e., 62 candidates bind the capture compound at defined modes of B. bacteriovorus lifestyle. Three of the putative binders further demonstrated specificity and high affinity to c-di-GMP via microscale thermophoresis, lending support for the presence of additional bona fide c-di-GMP effectors among the pulled-down protein repertoire.

The lifestyle switch protein Bd0108 of Bdellovibrio bacteriovorus is an intrinsically disordered protein

Bdellovibrio bacteriovorus is a δ-proteobacterium that preys upon Salmonella spp., E. coli, and other Gram-negative bacteria. Bdellovibrio can grow axenically (host-independent, HI, rare and mutation-driven) or subsist via a predatory lifecycle (host-dependent, HD, the usual case). Upon contact with prey, B. bacteriovorus enters the host periplasm from where it slowly drains the host cytosol of nutrients for its own replication. At the core of this mechanism is a retractile pilus, whose architecture is regulated by the protein Bd0108 and its interaction with the neighboring gene product Bd0109. Deletion of bd0108 results in negligible pilus formation, whereas an internal deletion (the one that instigates host-independence) causes mis-regulation of pilus length. These mutations, along with a suite of naturally occurring bd0108 mutant strains, act to control the entry to HI growth. To further study the molecular mechanism of predatory regulation, we focused on the apparent lifecycle switch protein Bd0108. Here we characterize the solution structure and dynamics of Bd0108 using nuclear magnetic resonance (NMR) spectroscopy complemented with additional biophysical methods. We then explore the interaction between Bd0108 and Bd0109 in detail utilizing isothermal titration calorimetry (ITC) and NMR spectroscopy. Together our results demonstrate that Bd0108 is an intrinsically disordered protein (IDP) and that the interaction with Bd0109 is of low affinity. Furthermore, we observe that Bd0108 retains an IDP nature while binding Bd0109. From our data we conclude that Bdellovibrio bacteriovorus utilizes an intrinsically disordered protein to regulate its pilus and control predation signaling.

Spatially Organized Films from Bdellovibrio bacteriovorus Prey Lysates

Bdellovibrio bacteriovorus is a Gram-negative predator of other Gram-negative bacteria. Interestingly, Bdellovibrio bacteriovorus 109J cells grown in coculture with Escherichia coli ML-35 prey develop into a spatially organized two-dimensional film when located on a nutrient-rich surface. From deposition of 10 μl of a routine cleared coculture of B. bacteriovorus and E. coli cells, the cells multiply into a macroscopic community and segregate into an inner, yellow circular region and an outer, off-white region. Fluorescence in situ hybridization and atomic force microscopy measurements confirm that the mature film is spatially organized into two morphologically distinct Bdellovibrio populations, with primarily small, vibroid cells in the center and a complex mixture of pleomorphic cells in the outer radii. The interior region cell population exhibits the hunting phenotype while the outer region cell subpopulation does not. Crowding and high nutrient availability with limited prey appear to favor diversification of the B. bacteriovorus population into two distinct, thriving subpopulations and may be beneficial to the persistence of B. bacteriovorus in biofilms.

Bdellovibrio bacteriovorus HD100 guards against Pseudomonas tolaasii brown-blotch lesions on the surface of post-harvest Agaricus bisporus supermarket mushrooms

BACKGROUND

Pseudomonas tolaasii is a problematic pathogen of cultured mushrooms, forming dark brown 'blotches' on mushroom surfaces and causing spoilage during crop growth and post-harvest . Treating P. tolaasii infection is difficult, as other, commensal bacterial species such as Pseudomonas putida are necessary for mushroom growth, so treatments must be relatively specific.

RESULTS

We have found that P. tolaasii is susceptible to predation in vitro by the δ-proteobacterium Bdellovibrio bacteriovorus. This effect also occurred in funga, where B. bacteriovorus was administered to post-harvest mushroom caps before and after administration of the P. tolaasii pathogen. A significant, visible improvement in blotch appearance, after incubation, was observed on administration of Bdellovibrio. A significant reduction in viable P. tolaasii cell numbers, recovered from the mushroom tissue, was detected. This was accompanied by a more marked reduction in blotch severity on Bdellovibrio administration. We found that there was in some cases an accompanying overgrowth of presumed-commensal, non-Pseudomonas bacteria on post-harvest mushroom caps after Bdellovibrio-treatment. These bacteria were identified (by 16SrRNA gene sequencing) as Enterobacter species, which were seemingly resistant to predation. We visualised predatory interactions occuring between B. bacteriovorus and P. tolaasii on the post-harvest mushroom cap surface by Scanning Electron Microscopy, seeing predatory invasion of P. tolaasii by B. bacteriovorus in funga. This anti-P. tolaasii effect worked well in post-harvest supermarket mushrooms, thus Bdellovibrio was not affected by any pre-treatment of mushrooms for commercial/consumer purposes.

CONCLUSIONS

The soil-dwelling B. bacteriovorus HD100 preys upon and kills P. tolaasii, on mushroom surfaces, and could therefore be applied to prevent spoilage in post-harvest situations where mushrooms are stored and packaged for sale.

Ras GTPase-like protein MglA, a controller of bacterial social-motility in Myxobacteria, has evolved to control bacterial predation by Bdellovibrio

Bdellovibrio bacteriovorus invade Gram-negative bacteria in a predatory process requiring Type IV pili (T4P) at a single invasive pole, and also glide on surfaces to locate prey. Ras-like G-protein MglA, working with MglB and RomR in the deltaproteobacterium Myxococcus xanthus, regulates adventurous gliding and T4P-mediated social motility at both M. xanthus cell poles. Our bioinformatic analyses suggested that the GTPase activating protein (GAP)-encoding gene mglB was lost in Bdellovibrio, but critical residues for MglA_Bd GTP-binding are conserved. Deletion of mglA_Bd abolished prey-invasion, but not gliding, and reduced T4P formation. MglA_Bd interacted with a previously uncharacterised tetratricopeptide repeat (TPR) domain protein Bd2492, which we show localises at the single invasive pole and is required for predation. Bd2492 and RomR also interacted with cyclic-di-GMP-binding receptor CdgA, required for rapid prey-invasion. Bd2492, RomR_Bd and CdgA localize to the invasive pole and may facilitate MglA-docking. Bd2492 was encoded from an operon encoding a TamAB-like secretion system. The TamA protein and RomR were found, by gene deletion tests, to be essential for viability in both predatory and non-predatory modes. Control proteins, which regulate bipolar T4P-mediated social motility in swarming groups of deltaproteobacteria, have adapted in evolution to regulate the anti-social process of unipolar prey-invasion in the “lone-hunter” Bdellovibrio. Thus GTP-binding proteins and cyclic-di-GMP inputs combine at a regulatory hub, turning on prey-invasion and allowing invasion and killing of bacterial pathogens and consequent predatory growth of Bdellovibrio.

Bacterial cell polarity control is important for maintaining asymmetry of polar components such as flagella and pili. Bdellovibrio bacteriovorus is a predatory deltaproteobacterium which attaches to, and invades, other bacteria using Type IV pili (T4P) extruded from the specialised, invasive, non-flagellar pole of the cell. It was not known how that invasive pole is specified and regulated. Here we discover that a regulatory protein-hub, including Ras-GTPase-like protein MglA and cyclic-di-GMP receptor-protein CdgA, control prey-invasion. In the deltaproteobacterium, Myxococcus xanthus, MglA, with MglB and RomR, was found by others to regulate switching of T4P in social ‘swarming’ surface motility by swapping the pole at which T4P are found. In contrast, in B. bacteriovorus MglA regulates the process of prey-invasion and RomR, which is required for surface motility regulation in Myxococcus, is essential for growth and viability in Bdellovibrio. During evolution, B. bacteriovorus has lost mglB, possibly as T4P-pole-switching is not required; pili are only required at the invasive pole. A previously unidentified tetratricopeptide repeat (TPR) protein interacts with MglA and is essential for prey-invasion. This regulatory protein hub allows prey-invasion, likely integrating cyclic-di-GMP signals, pilus assembly and TamAB secretion in B. bacteriovorus.

Pretreatment with alum or powdered activated carbon reduces bacterial predation-associated irreversible fouling of membranes

This study evaluated the co-application of bacterial predation by Bdellovibrio bacteriovorus and either alum coagulation or powdered activated carbon adsorption to reduce fouling caused by Escherichia coli rich feed solutions in dead-end microfiltration tests. The flux increased when the samples were predated upon or treated with 100 ppm alum or PAC, but co-treatment with alum and predation gave the best flux results. The total membrane resistance caused by the predated sample was reduced six-fold when treated with 100 ppm PAC, from 11.8 to 1.98 × 10(11) m(-1), while irreversible fouling (Rp) was 2.7-fold lower. Treatment with 100 ppm alum reduced the total resistance 14.9-fold (11.8 to 0.79 × 10(11) m(-1)) while the Rp decreased 4.25-fold. SEM imaging confirmed this, with less obvious fouling of the membrane after the combined process. This study illustrates that the combination of bacterial predation and the subsequent removal of debris using coagulation or adsorption mitigates membrane biofouling and improves membrane performance.