Changes in cell composition and viability of Bdellovibrio bacteriovorus during starvation

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.

Two-state swimming: Strategy and survival of a model bacterial predator in response to environmental cues

Bdellovibrio bacteriovorus is a predatory bacterium preying upon Gram-negative bacteria. As such, B. bacteriovorus has the potential to control antibiotic-resistant pathogens and biofilm populations. To survive and reproduce, B. bacteriovorus must locate and infect a host cell. However, in the temporary absence of prey, it is largely unknown how B. bacteriovorus modulate their motility patterns in response to physical or chemical environmental cues to optimize their energy expenditure. To investigate B. bacteriovorus' predation strategy, we track and quantify their motion by measuring speed distributions as a function of starvation time. While an initial unimodal speed distribution relaxing to one for pure diffusion at long times may be expected, instead we observe a bimodal speed distribution with one mode centered around that expected from diffusion and the other centered at higher speeds. What is more, for an increasing amount of time over which B. bacteriovorus is starved, we observe a progressive reweighting from the active swimming state to an apparent diffusive state in the speed distribution. Distributions of trajectory-averaged speeds for B. bacteriovorus are largely unimodal, indicating switching between a faster swim speed and an apparent diffusive state within individual observed trajectories rather than there being distinct active swimming and apparent diffusive populations. We also find that B. bacteriovorus' apparent diffusive state is not merely caused by the diffusion of inviable bacteria as subsequent spiking experiments show that bacteria can be resuscitated and bimodality restored. Indeed, starved B. bacteriovorus may modulate the frequency and duration of active swimming as a means of balancing energy consumption and procurement. Our results thus point to a reweighting of the swimming frequency on a trajectory basis rather than a population level basis.

The role of mathematical modelling in understanding prokaryotic predation

With increasing levels of antimicrobial resistance impacting both human and animal health, novel means of treating resistant infections are urgently needed. Bacteriophages and predatory bacteria such as Bdellovibrio bacteriovorus have been proposed as suitable candidates for this role. Microbes also play a key environmental role as producers or recyclers of nutrients such as carbon and nitrogen, and predators have the capacity to be keystone species within microbial communities. To date, many studies have looked at the mechanisms of action of prokaryotic predators, their safety in in vivo models and their role and effectiveness under specific conditions. Mathematical models however allow researchers to investigate a wider range of scenarios, including aspects of predation that would be difficult, expensive, or time-consuming to investigate experimentally. We review here a history of modelling in prokaryote predation, from simple Lotka-Volterra models, through increasing levels of complexity, including multiple prey and predator species, and environmental and spatial factors. We consider how models have helped address questions around the mechanisms of action of predators and have allowed researchers to make predictions of the dynamics of predator-prey systems. We examine what models can tell us about qualitative and quantitative commonalities or differences between bacterial predators and bacteriophage or protists. We also highlight how models can address real-world situations such as the likely effectiveness of predators in removing prey species and their potential effects in shaping ecosystems. Finally, we look at research questions that are still to be addressed where models could be of benefit.

A Comparative Study of the Dynamics and Diversity of Bdellovibrio and Like Organisms in Lakes Annecy and Geneva

Bdellovibrio and like organisms (BALOs) are obligate bacterial predators of other Gram-negative bacteria. Here, we used quantitative PCR (qPCR) and recently developed specific primers which target the 16S rRNA gene to explore the abundance and distribution of three families of BALO belonging to the Oligoflexia class (i.e., Bdellovibrionaceae, Peredibacteraceae and Bacteriovoracaceae) over one year in the epilimnion and hypolimnion of Lakes Annecy and Geneva. Peredibacteraceae was the dominant group at all sampling points except at the bottom of Lake Geneva, where Bdellovibrionaceae was found in higher number. In addition, the abundance of BALOs increased significantly during the warmer months. Using high-throughput sequencing (Illumina Miseq), hundreds of OTUs were identified for Bdellovibrionaceae and Peredibacteraceae. Phylogenetic analysis suggests that Bdellovibrionaceae are more diverse than Peredibacteraceae and that some OTUs belong to new species of Bdellovibrionaceae. We also found that dominant OTUs were present simultaneously in the two lakes, while some others were specific to each lake, suggesting an adaptive pattern. Finally, both abundance and diversity of BALOs were poorly associated with abiotic factors except temperature, suggesting the importance of studying biotic relationships, assumed to play a greater role than physico-chemical variables in BALOs' dynamics and distribution.

Predation strategies of the bacterium Bdellovibrio bacteriovorus result in overexploitation and bottlenecks

With increasing antimicrobial resistance, alternatives for treating infections or removing resistant bacteria are urgently needed, such as the bacterial predator Bdellovibrio bacteriovorus or bacteriophage. Therefore, we need to better understand microbial predator-prey dynamics. We developed mass-action mathematical models of predation for chemostats, which capture the low substrate concentration and slow growth typical for intended application areas of the predators such as wastewater treatment, aquaculture or the gut. Our model predicted that predator survival required a minimal prey cell size, explaining why Bdellovibrio is much smaller than its prey. A too good predator (attack rate too high, mortality too low) overexploited its prey leading to extinction (tragedy of the commons). Surprisingly, a predator taking longer to produce more offspring outcompeted a predator producing fewer offspring more rapidly (rate versus yield trade-off). Predation was only efficient in a narrow region around optimal parameters. Moreover, extreme oscillations under a wide range of conditions led to severe bottlenecks. These could be avoided when two prey species became available in alternating seasons. A bacteriophage outcompeted Bdellovibrio due to its higher burst size and faster life cycle. Together, results suggest that Bdellovibrio would struggle to survive on a single prey, explaining why it must be a generalist predator and suggesting it is better suited than phage to environments with multiple prey. Importance The discovery of antibiotics led to a dramatic drop in deaths due to infectious disease. Increasing levels of antimicrobial resistance, however, threaten to reverse this progress. There is thus a need for alternatives, such as therapies based on phage and predatory bacteria that kill bacteria regardless of whether they are pathogens or resistant to antibiotics. To best exploit them, we need to better understand what determines their effectiveness. By using a mathematical model to study bacterial predation in realistic slow growth conditions, we found that the generalist predator Bdellovibrio is most effective within a narrow range of conditions for each prey. For example, a minimum prey cell size is required, and the predator should not be too good as this would result in over-exploitation risking extinction. Together these findings give insights into the ecology of microbial predation and help explain why Bdellovibrio needs to be a generalist predator.

Effects of the prey landscape on the fitness of the bacterial predators Bdellovibrio and like organisms

Bdellovibrio and like organisms (BALOs) are obligate predatory bacteria commonly encountered in the environment. In dual predator-prey cultures, prey accessibility ensures optimal feeding and replication and rapid BALO population growth. However, the environmental prey landscape is complex, as it also incorporates non-prey cells and other particles. These may act as decoys, generating unproductive encounters which in turn may affect both predator and prey population dynamics. In this study, we hypothesized that increasing decoy:prey ratios would bring about increasing costs on the predator's reproductive fitness. We also tested the hypothesis that different BALOs and decoys would have different effects. To this end, we constructed prey landscapes including periplasmic or epibiotic predators including two types of decoy under a large range of initial decoy:prey ratio, and mixed cultures containing multiple predators and prey. We show that as decoy:prey ratios increase, the maximal predator population sizes is reduced and the time to reach it significantly increases. We found that BALOs spent less time handling non-prey (including superinfection-immune invaded prey) than prey cells, and did not differentiate between efficient and less efficient prey. This may explain why in multiple predator and prey cultures, less preferred prey appear to act as decoy.

To hunt or to rest: prey depletion induces a novel starvation survival strategy in bacterial predators

The small size of bacterial cells necessitates rapid adaption to sudden environmental changes. In Bdellovibrio bacteriovorus, an obligate predator of bacteria common in oligotrophic environments, the non-replicative, highly motile attack phase (AP) cell must invade a prey to ensure replication. AP cells swim fast and respire at high rates, rapidly consuming their own contents. How the predator survives in the absence of prey is unknown. We show that starvation for prey significantly alters swimming patterns and causes exponential decay in prey-searching cells over hours, until population-wide swim-arrest. Swim-arrest is accompanied by changes in energy metabolism, enabling rapid swim-reactivation upon introduction of prey or nutrients, and a sweeping change in gene expression and gene regulation that largely differs from those of the paradigmatic stationary phase. Swim-arrest is costly as it imposes a fitness penalty in the form of delayed growth. We track the control of the swim arrest-reactivation process to cyclic-di-GMP (CdG) effectors, including two motility brakes. CRISPRi transcriptional inactivation, and in situ localization of the brakes to the cell pole, demonstrated their essential role for effective survival under prey-induced starvation. Thus, obligate predators evolved a unique CdG-controlled survival strategy, enabling them to sustain their uncommon lifestyle under fluctuating prey supply.

Bradymonabacteria, a novel bacterial predator group with versatile survival strategies in saline environments

BACKGROUND

Bacterial predation is an important selective force in microbial community structure and dynamics. However, only a limited number of predatory bacteria have been reported, and their predatory strategies and evolutionary adaptations remain elusive. We recently isolated a novel group of bacterial predators, Bradymonabacteria, representative of the novel order Bradymonadales in δ-Proteobacteria. Compared with those of other bacterial predators (e.g., Myxococcales and Bdellovibrionales), the predatory and living strategies of Bradymonadales are still largely unknown.

RESULTS

Based on individual coculture of Bradymonabacteria with 281 prey bacteria, Bradymonabacteria preyed on diverse bacteria but had a high preference for Bacteroidetes. Genomic analysis of 13 recently sequenced Bradymonabacteria indicated that these bacteria had conspicuous metabolic deficiencies, but they could synthesize many polymers, such as polyphosphate and polyhydroxyalkanoates. Dual transcriptome analysis of cocultures of Bradymonabacteria and prey suggested a potential contact-dependent predation mechanism. Comparative genomic analysis with 24 other bacterial predators indicated that Bradymonabacteria had different predatory and living strategies. Furthermore, we identified Bradymonadales from 1552 publicly available 16S rRNA amplicon sequencing samples, indicating that Bradymonadales was widely distributed and highly abundant in saline environments. Phylogenetic analysis showed that there may be six subgroups in this order; each subgroup occupied a different habitat.

CONCLUSIONS

Bradymonabacteria have unique living strategies that are transitional between the "obligate" and the so-called facultative predators. Thus, we propose a framework to categorize the current bacterial predators into 3 groups: (i) obligate predators (completely prey-dependent), (ii) facultative predators (facultatively prey-dependent), and (iii) opportunistic predators (prey-independent). Our findings provide an ecological and evolutionary framework for Bradymonadales and highlight their potential ecological roles in saline environments. Video abstract.

Relative Contributions of Halobacteriovorax and Bacteriophage to Bacterial Cell Death under Various Environmental Conditions

The role of protists and bacteriophages in bacterial predation in the microbial food web has been well studied. There is mounting evidence that Bdellovibrio and like organisms (BALOs) also contribute to bacterial mortality and, in some cases, more so than bacteriophages. A full understanding of the ecologic function of the microbial food web requires recognition of all major predators and the magnitude of each predator’s contribution. Here we investigated the contribution of Halobacteriovorax, one of the BALOs, and bacteriophages when incubated with their common prey, Vibrio vulnificus, in a seawater microcosm. We observed that Halobacteriovorax was the greatest responder to the prey, increasing 18-fold with a simultaneous 4.4-log-unit reduction of V. vulnificus at 40 h, whereas the bacteriophage population showed no significant increase. In subsequent experiments to formulate a medium that would support the predatory activities and replication of both predators, low-nutrient media favored the predation and replication of the Halobacteriovorax, whereas higher-nutrient media enhanced phage growth. The greatest prey reduction and replication of both Halobacteriovorax and phage were observed in media with moderate nutrient levels. Additional experiments show that the predatory activities of both predators were influenced by environmental conditions, specifically, temperature and salinity. The two predators combined exerted greater control on V. vulnificus, a synergism that may be exploited for practical applications to reduce bacterial populations. These findings suggest that along with bacteriophage and protists, Halobacteriovorax has the potential to have a prominent role in bacterial mortality and cycling of nutrients, two vital ecologic functions.

Although much has been reported about the marine microbial food web and the role of micropredators, specifically viruses and protists, the contribution of Bdellovibrio-like predators has largely been ignored, posing a major gap in understanding food web processes. A complete scenario of the microbial food web cannot be developed until the roles of all major micropredators and the magnitude of their contributions to bacterial mortality, structuring of microbial communities, and cycling of nutrients are assessed. Here we show compelling evidence that Halobacteriovorax, a predatory bacterium, is a significant contributor to bacterial death and, in some cases, may rival viruses as agents of bacterial mortality. These results advance current understanding of the microbial loop and top-down control on the bacterial community.

Abundance, diversity and seasonal dynamics of predatory bacteria in aquaculture zero discharge systems

Standard aquaculture generates large-scale pollution and strains water resources. In aquaculture using zero discharge systems (ZDS), highly efficient fish growth and water recycling are combined. The wastewater stream is directed through compartments in which beneficial microbial activities induced by creating suitable environmental conditions remove biological and chemical pollutants, alleviating both problems. Bacterial predators, preying on bacterial populations in the ZDS, may affect their diversity, composition and functional redundancy, yet in-depth understanding of this phenomenon is lacking. The dynamics of populations belonging to the obligate predators Bdellovibrio and like organisms (BALOs) were analyzed in freshwater and saline ZDS over a 7-month period using QPCR targeting the Bdellovibrionaceae, and the Bacteriovorax and Bacteriolyticum genera in the Bacteriovoracaeae. Both families co-existed in ZDS compartments, constituting 0.13-1.4% of total Bacteria. Relative predator abundance varied according to the environmental conditions prevailing in different compartments, most notably salinity. Strikingly, the Bdellovibrionaceae, hitherto only retrieved from freshwater and soil, also populated the saline system. In addition to the detected BALOs, other potential predators were highly abundant, especially from the Myxococcales. Among the general bacterial population, Flavobacteria, Bacteroidetes, Fusobacteriaceae and unclassified Bacteria dominated a well mixed but seasonally fluctuating diverse community of up to 238 operational taxonomic units, as revealed by 16S rRNA gene sequencing.

Isolation and classification of Bdellovibrio and like organisms

Bdellovibrio and like organisms (BALOs) are obligate predators of Gram-negative bacteria. BALOs are isolated as plaques growing at the expense of their prey and are cultivated as two-member cultures. The growth cycle is composed of an extracellular attack phase and an intraperiplasmic elongation and replication phase. However, there are methods for obtaining host-independent (HI) mutants that grow without prey on rich media. BALOs are commonly found in the environment but generally constitute small populations; therefore, their isolation may require enrichment steps. Contamination by other bacteria during isolation necessitates efficient separation between the smaller BALO cells from the majority of larger bacteria. BALOs can also be directly detected and quantified in environmental samples using specific PCR. Synchronous cultures of both wild-type and HI derivatives can be obtained to study the different growth phases. These can be further separated by centrifugation. Classification is based on 16S rDNA analysis. Protocols relevant to these aspects of BALO detection, isolation, growth, classification, and quantitation are presented in this unit.

Bacterial predator-prey interaction at low prey density

A bacterial predator-prey interaction was studied using Bdellovibrio and bioluminescent prey bacteria. The attacking bdellovibrio causes decay of bioluminescence, which is correlated with bdellovibrio penetration into the prey. The behavior of the prey and predator populations over time was found to be well described by a Lotka-Volterra model. By using this model, the probability of bdellovibrio penetration after encountering a prey cell was found to be approximately 3.0%. The prey density required to give the bdellovibrios a 50% chance of survival was calculated to be at least 3.0 x 10 cells per ml, and the density required for population equilibria was calculated to be about 7 x 10 prey bacteria per ml. These values, not generally characteristic of natural habitats, suggest that the existence of Bdellovibrio in nature is limited to special ecological niches.

Bdellovibrio predation in the presence of decoys: Three-way bacterial interactions revealed by mathematical and experimental analyses

Bdellovibrio bacteriovorus is a small, gram-negative, motile bacterium that preys upon other gram-negative bacteria, including several known human pathogens. Its predation efficiency is usually studied in pure cultures containing solely B. bacteriovorus and a suitable prey. However, in natural environments, as well as in any possible biomedical uses as an antimicrobial, Bdellovibrio is predatory in the presence of diverse decoys, including live nonsusceptible bacteria, eukaryotic cells, and cell debris. Here we gathered and mathematically modeled data from three-member cultures containing predator, prey, and nonsusceptible bacterial decoys. Specifically, we studied the rate of predation of planktonic late-log-phase Escherichia coli S17-1 prey by B. bacteriovorus HD100, both in the presence and in the absence of Bacillus subtilis nonsporulating strain 671, which acted as a live bacterial decoy. Interestingly, we found that although addition of the live Bacillus decoy did decrease the rate of Bdellovibrio predation in liquid cultures, this addition also resulted in a partially compensatory enhancement of the availability of prey for predation. This effect resulted in a higher final yield of Bdellovibrio than would be predicted for a simple inert decoy. Our mathematical model accounts for both negative and positive effects of predator-prey-decoy interactions in the closed batch environment. In addition, it informs considerations for predator dosing in any future therapeutic applications and sheds some light on considerations for modeling the massively complex interactions of real mixed bacterial populations in nature.

Recovery of bdellovibrios from submerged surfaces and other aquatic habitats

The distribution of bdellovibrios was investigated over a wide geographical area of the Chesapeake Bay including some tributaries and subestuaries. Bdellovibrios were recovered from five aquatic habitats; water, sediment, oyster shell surface biofilm, zooplankton, and plants over a wide range of temperature and salinity measurements. Consistently, the greatest number of the predators was recovered from samples of biofilm irrespective of temperature and salinity. A decrease in the numbers and frequency of predators recovered from all habitats was observed at temperatures below 10°C. Only the shell surface biofilm samples yielded bdellovibrios 100% of the time. The organisms were recovered from 79% of water samples and 44% of sediment samples. The results reveal that bdellovibrios are surface-associated organisms and that this association appears to provide some protection for the predators at low temperatures.

Formation of Stable Bdelloplasts as a Starvation-Survival Strategy of Marine Bdellovibrios

Several wild-type isolates of marine bdellovibrios formed stable bdelloplasts when they infected gram-negative bacterial prey under certain culture conditions. Synchronous predator-prey cultures and low nutrient concentrations increased the yield of stable bdelloplasts. The bdellovibrio cells retained in the stable bdelloplasts showed a high survival capacity in nutrient-depleted saline solution (10% viable Bdellovibrio cells after 3 months at 25°C), whereas Bdellovibrio attack-phase cells kept under the same starvation conditions lost viability more quickly (1% viable cells after 48 h). The addition of yeast extract to a stable bdelloplast suspension induced lysis of the bdelloplasts and release of motile infecting attack-phase Bdellovibrio cells. Other substances, such as free amino acids, protein hydrolysates, NH 4 + , carbohydrates, and organic amines, did not induce such a release. Stable bdelloplasts were highly hydrophobic and had a lower endogenous respiration rate than attack-phase cells. In general, stable bdelloplasts were almost as sensitive to temperature changes, desiccation, sonication, tannic acid, and Triton X-100 treatment as attack-phase cells. Electron microscopy of stable bdelloplasts did not reveal any extra cell wall layer, either in the bdelloplast envelope or in the retained Bdellovibrio cells, unlike the bdellocysts of the soil bacterium Bdellovibrio sp. strain W. We propose that formation of stable bdelloplasts is a survival strategy of marine bdellovibrios which occurs in response to nutrient- and prey-poor seawater habitats.

The incidence of Bdellovibrio spp. in man-made water systems: coexistence with legionellas

Bdellovibrios have been isolated from surface waters but there are no reports of its occurrence in mains water supplies. One hundred and thirty five water samples from 81 sources were examined for the presence of Bdellovibrio bacteriovorus and Legionella spp. Bdellovibrios were isolated by a double-layer agar technique with two strains of Legionella pneumophila serogroup 1 as the host organisms. Bdellovibrio spp. were isolated from 57.8% and Legionella spp. from 9.5% of the samples. The two species occurred together in 4.4% of samples. The incidence of Bdellovibrio spp. and its occurrence with legionellas in man-made water systems is discussed.

Formate auxotroph of Methanobacterium thermoautotrophicum Marburg

A formate-requiring auxotroph of Methanobacterium thermoautotrophicum Marburg was isolated after hydroxylamine mutagenesis and bacitracin selection. The requirement for formate is unique and specific; combined pools of other volatile fatty acids, amino acids, vitamins, and nitrogen bases did not substitute for formate. Compared with those of the wild type, cell extracts of the formate auxotroph were deficient in formate dehydrogenase activity, but cells of all of the strains examined catalyzed a formate-carbon dioxide exchange activity. All of the strains examined took up a small amount (200 to 260 mumol/liter) of formate (3 mM) added to medium. The results of the study of this novel auxotroph indicate a role for formate in biosynthetic reactions in this methanogen. Moreover, because methanogenesis from H2-CO2 is not impaired in the mutant, free formate is not an intermediate in the reduction of CO2 to CH4.

Unbalanced growth as a normal feature of development of Bdellovibrio bacteriovorus

In this study we have investigated the rates and spatial patterns of chromosome replication and cell elongation during the growth phase of wild-type facultatively prey-independent mutant strains of Bdellovibrio bacteriovorus. For the facultatively prey-independent mutants, the total DNA content of synchronously growing cultures was found to increase exponentially, as the multiple chromosomes within each filamentous cell replicated simultaneously. Cell mass, measured as total cellular protein, also increased exponentially during this period, apparently by means of multiple elongation sites along the filament wall. The relative rates of DNA and protein synthesis were unbalanced during growth, however, with the cellular concentration of DNA increasing slightly faster than that of protein. The original cellular DNA: protein ratio was restored in the progeny cells by continued protein synthesis during the septation period that follows the termination of DNA replication. Because of technical problems, these experiments could not be conducted on the wild-type cells, but similar results are assumed. This unusual pattern of unbalanced growth may represent an adaptation by bdellovibrios to maximize their progeny yield from the determinate amount of substrate available within a given prey cell.

Metabolism of periplasmic membrane-derived oligosaccharides by the predatory bacterium Bdellovibrio bacteriovorus 109J

Membrane-derived oligosaccharides (MDO), a class of osmotically active carbohydrates, are the major organic solutes present in the periplasm of Escherichia coli and many other gram-negative bacteria when cells are grown in a medium of low osmolarity. Analyses of growing cells of Bdellovibrio bacteriovorus, a gram-negative predator of other bacteria, have confirmed that they also synthesize a characteristic MDO-like class of oligosaccharides. The natural growth environment of bdellovibrios is the periplasm of other gram-negative bacteria. Because of this location, prey cell MDO constitute a potential source of organic nutrients for growing bdellovibrios. Using cells of E. coli whose MDO were 3H labeled, we examined the extent to which B. bacteriovorus 109J metabolizes these prey cell components. Interestingly, there was neither significant degradation nor incorporation of prey cell MDO by bdellovibrios during the course of their intracellular growth. In fact, bdellovibrios had little capability either to degrade extracellular MDO that was made available to them or to transport glucose, the major monomeric constituent of prey cell MDO. Instead, periplasmic MDO were irreversibly lost to the extracellular environment during the period of bdellovibrio attack and penetration. Thus, although prey cell periplasmic proteins are retained, other important periplasmic components are released early in the bdellovibrio growth cycle. The loss of these MDO may aid in the destabilization of the prey cell plasma membrane, increasing the availability of cytoplasmic constituents to the periplasmic bdellovibrio.

A study of the occurrence and distribution of bdellovibrios in estuarine sediment over an annual cycle

The recovery of bdellovibrios from estuarine sediments over an annual cycle was studied. Greater numbers of the predators were recovered in sediment than in the water column. Increases in the number of bdellovibrios recovered from sediment over various periods of time suggest that multiplication of the predators occurred. Sediment was observed to be an important ecosystem for the survival of bdellovibrios in the winter months. As has been observed in water, the number of bdellovibrios in sediment fluctuated, with seasonal and temperature changes declining to very low numbers during the winter months. In the colder months, low numbers of the predators appeared to winter-over in sediment, with greater numbers of the organisms being recovered from deeper sediment. As the water temperature warmed in the spring, increases in the number of bdellovibrios occurred first in sediment and subsequently in water. This increase of bdellovibrios in sediment may have resulted in the shedding of the organisms into the water column where their numbers subsequently increased. Population fluctuations of bdellovibrios were similar in both water and sediment. Although the temperature may account for much of the observed fluctuation in the number of bdellovibrios, other factors, including salinity and the number of host bacteria, may also play a major role. The number of bdellovibrios recovered from sediment correlated positively with the water temperature, and negatively with the water salinity and the number of bacterial colony-forming units from sediment. The results of this study revealed the significance of sediment to the seasonal cycle, survival, and growth of the bdellovibrios in an estuarine environment.

Uptake of intact nucleoside monophosphates by Bdellovibrio bacteriovorus 109J

The degraded nucleic acids and ribosomes of its prey cell provide Bdellovibrio bacteriovorus 109J with a source of ribonucleoside monophosphates and deoxyribonucleoside monophosphates for biosynthesis and respiration. We demonstrate that bdellovibrios, in contrast to almost all other bacteria, take up these nucleoside monophosphates into the cell in an intact, phosphorylated form. In this way they are able to assimilate more effectively the cellular contents of their prey. Studies with UMP and dTMP demonstrate that they are transported and accumulated against a concentration gradient, achieving internal levels at least 10 times the external levels. Treatment of the bdellovibrios with azide or carbonyl cyanide m-chlorophenylhydrazone eliminates their ability to either transport or maintain accumulated UMP and suggests the presence of a freely reversible exchange mechanism. There are at least two separate classes of transport systems for nucleoside monophosphates, each exhibiting partial specificity for either ribonucleoside monophosphates or deoxyribonucleoside monophosphates. Kinetic analyses of UMP transport in different developmental stages of strain 109J indicate that each stage expresses a single, saturable uptake system with a distinct apparent substrate affinity constant (Kt) of 104 microM in attack phase cells and 35 microM in prematurely released growth phase filaments. The capacity for transport of UMP by the growth phase filaments was 2.4 times that of the attack phase cells. These data, in addition to the apparent lack of environmental control of UMP transport capacity in attack phase cells, suggest that there are two transport systems for UMP in bdellovibrios and that the high-affinity, high-capacity growth phase system is developmentally regulated.

Minimum bacterial density for bacteriophage replication: implications for significance of bacteriophages in natural ecosystems

Bacteriophage 80 alpha did not increase in number in cultures containing less than about 1.0 X 10(4) to 1.5 X 10(4) CFU of Staphylococcus aureus per ml, but bacteriophage replication did occur when the number of bacteria exceeded this density, either initially or as a result of host cell multiplication. The minimum density of an asporogenous strain of Bacillus subtilis required for an increase in the number of bacteriophage SP beta cI was about 3 X 10(4) CFU/ml. The threshold density of Escherichia coli for the multiplication of bacteriophage T4 was about 7 X 10(3) CFU/ml. In the presence of montmorillonite, bacteriophage T4 did not increase in number until the E. coli population exceeded 10(4) CFU/ml. The mineralization of glucose was not affected in E. coli cultures inoculated with a low number of bacteriophage T4, but it could not be detected in cultures inoculated with a large number of phage. The numbers of bacteriophage T4 and a bacteriophage that lyses Pseudomonas putida declined rapidly after being added to lake water or sewage. We suggest that bacteriophages do not affect the number or activity of bacteria in environments where the density of the host species is below the host cell threshold of about 10(4) CFU/ml.

Long-term nutrient starvation of continuously cultured (glucose-limited) Selenomonas ruminantium

Selenomonas ruminantium, a strictly anaerobic ruminal bacterium, was grown at various dilution rates (D = 0.05, 0.25, and 0.35 h-1) under glucose-limited continuous culture conditions. Suspensions of washed cells prepared anaerobically in mineral buffer were subjected to nutrient starvation (24 to 36 h; 39 degrees C; N2 atmosphere). Regardless of growth rate, viability declined logarithmically, and within about 2.5 h, about 50% of the populations were nonviable. After 24 h of starvation, the numbers of viable cells appeared to be inversely related to growth rate, the highest levels occurring with the slowest grown population. Cell dry weight, carbohydrate, protein, ribonucleic acid (RNA), and deoxyribonucleic acid declined logarithmically during starvation, and the decline rates of each were generally greater with cells grown at higher D values. Both cellular carbohydrate and RNA declined substantially during the first 12 h of starvation. Most of the cellular RNA that disappeared was found in the suspending buffer as low-molecular-weight, orcinol-positive materials. During growth, S. ruminantium made a variety of fermentation acids from glucose, but during starvation, acetate was the only acid made from catabolism of cellular material. Addition of glucose or vitamins to starving cell suspensions did not decrease loss of viability, whereas a starvation in the spent culture medium resulted in a slight decrease in the rate of viability loss. Overall, the data indicate that S. ruminantium strain D has very little survival capacity under the conditions tested compared with other bacterial species that have been studied.

Inhibition of the predatory activity ofBdellovibrio by various environmental pollutants

The predatory activity of bdellovibrios is affected by various environmental pollutants such as detergents, heavy metals, and pesticides. This was shown in a two-membered system ofBdellovibrio andPhotobacterium, in which the effect of the predator on the bioluminescence of the prey indicated the activity of the former. The high sensitivity of the bdellovibrios toward certain chemicals (e.g., CdCl2) indicates the possibility of using the system for biological monitoring of those chemicals.

Interaction ofBdellovibrio with Its prey in mixed microbial populations

The interaction ofBdellovibrio with its prey can be affected by the presence of other microorganisms regardless of whether they serve as a prey for the bdellovibrios. This was shown in a system in which the fate of one prey could be followed in mixed bacterial populations thanks to a specific trait, bioluminescence. The attacking bdellovibrio causes decay of bioluminescence, and the rate of light decay of the population indicates the rate at which the luminous bacteria are attacked. Using this system it was found that different bacteria affected the predatorprey interaction in different ways: some "competed" with the original prey for the predator; others enhanced the activity of the predator toward the original prey, and others inhibited it. The significance of these findings in relation to the distribution and activity ofBdellovibrio in the natural ecosystem is discussed.

Metabolism of RNA-ribose by Bdellovibrio bacteriovorus during intraperiplasmic growth on Escherichia coli

During intraperiplasmic growth of Bdellovibrio bacteriovorus 109J on Escherichia coli some 30 to 60% of the initial E. coli RNA-ribose disappeared as cell-associated orcinol-positive material. The levels of RNA-ribose in the suspending buffer after growth together with the RNA-ribose used for bdellovibrio DNA synthesis accounted for 50% or less of the missing RNA-ribose. With intraperiplasmic growth in the presence of added U-14C-labeled CMP, GMP, or UMP, radioactivity was found both in the respired CO2 and incorporated into the bdellovibrio cell components. The addition of exogenous unlabeled ribonucleotides markedly reduced the amounts of both the 14CO2 and 14C incorporated into the progeny bdellovibrios. During intraperiplasmic growth of B. bacteriovorus on [U-14C]ribose-labeled E. coli BJ565, ca. 74% and ca. 19% of the initial 14C was incorporated into the progeny bdellovibrios and respired CO2, respectively. Under similar growth conditions, the addition of glutamate substantially reduced only the 14CO2; however, added ribonucleotides reduced both the 14CO2 and the 14C incorporated into the progeny bdellovibrios. No similar effects were found with added ribose-5-phosphate. The distribution of 14C in the major cell components was similar in progeny bdellovibrios whether obtained from growth on [U-14C]ribose-labeled E. coli BJ565 or from E. coli plus added U-14C-labeled ribonucleotides. After intraperiplasmic growth of B. bacteriovorus on [5,6-3H-]uracil-[U-14C]ribose-labeled E. coli BJ565 (normal or heat treated), the whole-cell 14C/3H ratio of the progeny bdellovibrios was some 50% greater and reflected the higher 14C/3H ratios found in the cell fractions. B. bacteriovorus and E. coli cell extracts both contained 5'-nucleotidase, uridine phosphorylase, purine phosphorylase, deoxyribose-5-phosphate aldolase, transketolase, thymidine phosphorylase, phosphodeoxyribomutase, and transaldolase enzyme activities. The latter three enzyme activities were either absent or very low in cell extracts prepared from heat-treated E. coli cells. It is concluded that during intraperiplasmic growth B. bacteriovorus degrades some 20 to 40% of the ribonucleotides derived from the initial E. coli RNA into the base and ribose-1-phosphate moieties. The ribose-1-phosphate is further metabolized by B. bacteriovorus both for energy production and for biosynthesis, of non-nucleic acid cell material. In addition, the data indicate that during intraperiplasmic growth B. bacteriovorus can metabolize ribose only if this compound is available to it as the ribonucleoside monophosphate.

Penicillin-induced formation of osmotically stable spheroplasts in nongrowing Bdellovibrio bacteriovorus

Bdellovibrio peptidoglycan is of typical gram-negative composition. The molar ratios of alanine:glutamic acid:diaminopimelic acid:muramic acid:glucosamine were about 2:1:1:1:1. Nascent, nongrowing Bdellovibrio bacteriovorus 109J were converted from highly motile vibrios to highly motile spheres when shaken in dilute buffer plus penicillin, cephalothin, bacitracin, or D-cycloserine. The spherical forms contained essentially no sedimentable peptidoglycan; i.e., they were spheroplasts. Spheroplasts induced by penicillin, D-cycloserine, and lysozyme were stable in dilute buffer and did not lyse when subjected to osmotic shock. Normal Bdellovibrio suspended in buffer turned over their peptidoglycan at a rate of approximately 30% h during the initial 120 min of starvation. Chloramphenicol and sodium azide strongly inhibited Bdellovibrio peptidoglycan turnover and the induction of spheroplasts by penicillin. The data indicate that nongrowing B. bacteriovorus are sensitive to penicillin and other antibiotics affecting cell walls because of their high rate of peptidoglycan turnover. It is also concluded that an intact peptidoglycan layer is required for maintaining cell shape, but is not required for osmotic stability of B. bacteriovorus.

Chemotaxis by Bdellovibrio bacteriovorus toward prey

A chemotaxis assay system that uses a modified Boyden chamber was characterized and used for measurements of chemotaxis by Bdellovibrio bacteriovorus strain UKi2 toward several bacterial species. Bacteria tested included both susceptible and nonsusceptible cells (Escherichia coli, Pseudomonas fluorescens, Bacillus megaterium, and B. bacteriovorus strains UKi2 and D). None was attractive to bdellovibrios when present at densities below 10(7) cells per ml. Chemotaxis toward E. coli was studied most extensively; under conditions that minimized effects of osmotic shock to the cells, E. coli and exudates from E. coli at densities as high as 10(8) cells per ml failed to elicit a chemotactic response. Cell-free filtrates from mixed cultures of bdellovibrios and E. coli neither attracted nor repelled bdellovibrios. The data indicate that bdellovibrios do not use chemotaxis to locate prey cells.

Characterization of bdellocysts of Bdellovibrio sp

Bdellovibrio sp. strain W will infect and produce resting cells, termed bdellocysts, in a variety of gram-negative bacteria. Bdellocysts appeared to be produced only within susceptible prey and never in their absence. Optimum conditions for encystment included infection of stationary-phase prey cells in 0.05 M potassium phosphate buffer (pH 7.5) at concentrations of prey and bdellovibrios of 2 X 10(9) cells per ml with a multiplicity of infection of unity. Bdellocysts contained more deoxyribonucleic acid, ribonucleic acid, protein, and carbohydrate per cell than did vegetative cells. Poly-beta-hydroxybutyrate and dipicolinic acid were not detected. Bdellocysts were more resistant than vegetative cells to effects of elevated temperatures, sonic treatment, and desiccation. Bdellocysts remained viable for extended periods when incubated in the absence of prey, whereas vegetative cells lost viability rapidly under the same conditions. Their survival under starvation conditions may be due to the low rate of endogenous respiration by the bdellocysts. Bdellocysts are capable of germination in the presence or absence of prey cells in rich medium such as peptone-yeast extract.

Chemotaxis toward amino acids by Bdellovibrio bacteriovorus

Chemotaxis toward amino acids by Bdellovibrio bacteriovorous strain UKi2 was studied by the capillary technique of Adler (J. Gen. Microbiol. 74:77-91, 1973). Chemotaxis was shown to be optimal when the capillaries were incubated at between 15 and 40 degrees C for 30 min; the optimal pH was between 7.0 and 8.2. The chemotactic response was proportional to the density of the suspension of bdellovibrios up to a density of 10(8) cells/ml. B. bacteriovorus was attracted to L-asparagine, L-cysteine, L-glutamine, glycine, L-histidine, L-lysine, and L-threonine. The possible roles of chemotaxis in the life of B. bacteriovorus are discussed.

Properties of marine bdellovibrios

Marine bdellovibrio isolates from the Israeli littoral of the Mediterranean Sea were screened and characterized in terms of host range, temperature and salinity ranges, cation requirement, mutation frequency, and G + C% mole content. Ten types of isolates were distinguished on the basis of these parameters.

Ribonucleic acid destruction and synthesis during intraperiplasmic growth of Bdellovibrio bacteriovorus

During growth of Bdellovibrio bacteriovorus on (2-14C)uracil-labeled Escherichia coli approximately 50% of the radioactivity is incorporated by the bdellovibrio and most of the remainder is released as free nucleic acid bases. Kinetic studies showed that 50 and 30S ribosomal particles and 23 and 16S ribosomal ribonucleic acid (RNA) of E. coli are almost completely degraded by the first 90 min in a 210- to 240-min bdellovibrio developmental cycle. Synthesis of bdellovibrio ribosomal RNA was first detected after 90 min. The specific activity and the ratio of radioactivity in the bases of the synthesized bdellovibrio RNA was essentially the same as those of the substrate E. coli. The total radioactivity of the bdellovibrio deoxyribonucleic acid (DNA) exceeded that in the DNA of the substrate E. coli cell, and the ratio of radioactivity of cytosine to thymine residues differed. Intraperiplasmic growth of B. bacteriovorus in the presence of added nucleoside monophosphates (singly or in combination) significantly decreased the uptake of radioactivity from (2-14C)uracil-labeled E. coli; nucleosides or nucleic acid bases did not. It is concluded that the RNA of the substrate cell, in the form of nucleoside monophosphates, is the major or exclusive precursor of the bdellovirbrio RNA and also serves as a precursor for some of the bdellovibrio DNA.

Energy metabolism of Bdellovibrio bacteriovorus. I. Energy production, ATP pool, energy charge

Bdellovibrio bacteriovorus, strain Bd. 109 Sa, generates ATP mainly by oxidative phosphorylation during electron transport. During exponential growth the ATP pool is constant (9 mmoles/100 mugN) indicating that energy-producing and energy-consuming reactions are well balanced. The ratio of substrate respiration/endogenous respiration is approx. 2.5/1. Energy charge is constant both in endogenous and substrate respiration at values of 0.62 to 0.64. During endogenous respiration (starvation) the ATP pool oscillates at regular intervals. ATP over-production is started after the ATP pool has decreased to a minimum level of 6 nmoles/100 mug N. The alternating over- and under-production of ATP is interpreted as a special regulation which enables the organism to make economic use of its own cellular materials. Addition of substrate (glutamate) to starving cells does not influence the type of ATP pool oscillation as observed in endogenous respiration. The parasitic strain Bd. 109 Pa exhibits the same periodicity of ATP overproduction as does it saprophytic derivative, Bd. 109 Sa. Decrease of viability during starvation is paralleled by a decrease of the ATP pool.

Energy efficiency of intraperiplasmic growth of Bdellovibrio bacteriovorus

The Y-ATP (energy efficiency) of intraperiplasmic growth of Bdellovibrio bacteriovorus was determined from the distribution of radioactivity of the substrate organism ([U-14C]Escherichia coli) btween CO2 and bdellovibrio cells at the end of growth. A "best" Y-ATP value of 18.5 was obtained from single growth cycle experiments and an average value of 25.9 from multicycle experiments. Both values are much higher than the usual value of 10.5 for bacteria growing in rich media. The bases for the unusual energy efficiency for growth of B. bacteriovorus are discussed.

Effects of methotrexate on intraperiplasmic and axenic growth of Bdellovibrio bacteriovorus

The intraperiplasmic growth rate and cell yield of wild-type Bdellovibrio bacteriovorus 109J, growing on Escherichia coli of normal composition as the substrate, were not markedly inhibited by 10-3 M methotrexate (4-amino-N10-methylpteroylglutamic acid). In contrast, the growth rate and cell yield of the mutant 109Ja, growing axenically in 0.5% yeast extract +0.15% peptone, were strongly inhibited by 10-4 and 10-3 M methotrexate. Thymine, thymidine, and thymidine-5'-monophosphate, in increasing order of effectiveness, partially or completely reversed the inhibition. E. coli depleted of tetrahydrofolate and having an abnormally high protein/deoxyribonucleic acid (DNA) ratio was obtained by growing it in the presence of methotrexate. B. bacteriovourus grew at a normal rate on these depleted E. coli cells but with somewhat reduced cell yield. Mexthotrexate (10-3 M) inhibited intraperiplasmic growth of bdellovibrio on the depleted E. coli somewhat more than it inhibited growth on normal E. coli, but the effects were small compared with inhibition of axenic growth of the mutant. Total bdellovibrio DNA after growth on the depleted E. coli in the presence or absence of methotrexate exceeded the initial quanity of E. coli DNA present. Thymidine-5'-monophosphate (10-3 M) largely reversed the inhibition and increased the amount of net synthesis of DNA. The data are consistent with the prediction that intraperiplasmic growth of B. bacteriovorus should be insensitive to all metabolic inhibitors that act by specifically preventing synthesis of essential monomers. The data also indicate that B. bacteriovorus possesses thymidylate synthetase, thymidine phosphorylase, and thymidine kinase, and has the potential to carry out de novo DNA synthesis from non-DNA precursors during intraperiplasmic growth. The results also suggest that methionyl tRNAfMet is not required for initiation of protein synthesis by B. bacteriovorus.

Incorporation of long-chain fatty acids of the substrate organism by Bdellovibrio bacteriovorus during intraperiplasmic growth

Data are presented showing that a large proportion of the fatty acids of Bdellovibrio bacteriovorus grown intraperiplasmically are derived unaltered from the fatty acids of its substrate organism. Those fatty acids of the bdellovibrio not homologous with those of the substrate organism are derived mainly by metabolic alteration of preexisting fatty acids in the latter. De novo synthesis from acetate occurs only to a small extent. These characteristics of bdellovibrio physiology are in part responsible for its minimal energy expenditure for intraperiplasmic growth. The data presented also indicate that B. bacteriovorus is capable of hydrogenating unsaturated fatty acids, of beta-oxidation of fatty acids, and of regulating the proportion of saturated and unsaturated fatty acids in the lipids.

Chemotaxis in Bdellovibrio bacteriovorus

Chemotaxis toward yeast extract is demonstrated in obligately and facultatively parasitic strains of Bdellovibrio bacteriovorus.