The dispersal of an initial concentration of motile bacteria.

The dispersal of an initial concentration of identical Brownian particles is accurately described by the solution of the conventional diffusion equation, and a diffusion coefficient can be assigned to the assembly of particles. However, the dispersal of an initial concentration of motile bacteria is not well described by the same solution, in spite of the similarity between the random motion of a bacterium and a Brownian particle. Reasons for the failure of the Gaussian solution of the diffusion equation to describe the dispersal of Escherichia coli are discussed. An equation is formulated which gives the concentration of dispersing organisms as a function of space and time if the speed distribution function of the assembly of organism is known and reproduction is suppressed. For three assumed speed distributions the results are compared with concentrations measured by previous authors.     

Gains of Bacterial Flagellar Motility in a Fungal World

The maintenance of energetically costly flagella by bacteria in non-water-saturated media, such as soil, still presents an evolutionary conundrum. Potential explanations have focused on rare flooding events allowing dispersal. Such scenarios, however, overlook bacterial dispersal along mycelia as a possible transport mechanism in soils. The hypothesis tested in this study is that dispersal along fungal hyphae may lead to an increase in the fitness of flagellated bacteria and thus offer an alternative explanation for the maintenance of flagella even in unsaturated soils. Dispersal along fungal hyphae was shown for a diverse array of motile bacteria. To measure the fitness effect of dispersal, additional experiments were conducted in a model system mimicking limited dispersal, using Pseudomonas putida KT2440 and its nonflagellated (ΔfliM) isogenic mutant in the absence or presence of Morchella crassipes mycelia. In the absence of the fungus, flagellar motility was beneficial solely under conditions of water saturation allowing dispersal, while under conditions limiting dispersal, the nonflagellated mutant exhibited a higher level of fitness than the wild-type strain. In contrast, in the presence of a mycelial network under conditions limiting dispersal, the flagellated strain was able to disperse using the mycelial network and had a higher level of fitness than the mutant. On the basis of these results, we propose that the benefit of mycelium-associated dispersal helps explain the persistence of flagellar motility in non-water-saturated environments.     

Motility and chemotaxis in Agrobacterium tumefaciens surface attachment and biofilm formation

Bacterial motility mechanisms, including swimming, swarming, and twitching, are known to have important roles in biofilm formation, including colonization and the subsequent expansion into mature structured surface communities. Directed motility requires chemotaxis functions that are conserved among many bacterial species. The biofilm-forming plant pathogen Agrobacterium tumefaciens drives swimming motility by utilizing a small group of flagella localized to a single pole or the subpolar region of the cell. There is no evidence for twitching or swarming motility in A. tumefaciens. Site-specific deletion mutations that resulted in either aflagellate, flagellated but nonmotile, or flagellated but nonchemotactic A. tumefaciens derivatives were examined for biofilm formation under static and flowing conditions. Nonmotile mutants were significantly deficient in biofilm formation under static conditions. Under flowing conditions, however, the aflagellate mutant rapidly formed aberrantly dense, tall biofilms. In contrast, a nonmotile mutant with unpowered flagella was clearly debilitated for biofilm formation relative to the wild type. A nontumbling chemotaxis mutant was only weakly affected with regard to biofilm formation under nonflowing conditions but was notably compromised in flow, generating less adherent biomass than the wild type, with a more dispersed cellular arrangement. Extragenic suppressor mutants of the chemotaxis-impaired, straight-swimming phenotype were readily isolated from motility agar plates. These mutants regained tumbling at a frequency similar to that of the wild type. Despite this phenotype, biofilm formation by the suppressor mutants in static cultures was significantly deficient. Under flowing conditions, a representative suppressor mutant manifested a phenotype similar to yet distinct from that of its nonchemotactic parent.     

Dispersal dynamics of groundwater bacteria

Dispersal of bacteria in saturated, porous soils can be characterized by the partitioning of cells between the aqueous and solid phases, as a result of the physical and chemical nature of the soil and water and cell surface modifications. The purpose of this work is to understand variations in partitioning as a consequence of the nutrient conditions and to use this information in mathematical models to predict the dispersal rate of bacteria in aquifer material. Two different models were used to describe dispersal: an advective-dispersive-sorptive model with a first order kinetic sink term to account for irreversible cell reactions, such as death and sorption; and a two-site reaction model, in which the retardation was assumed to be determined by two types of sites, one characterized by instantaneous equilibrium sorption reactions and the other by kinetic nonequilibrium reactions. Water-saturated sand columns were used as continuous-flow groundwater microcosms to test the models under different nutrient regimes. Two strains of indigenous groundwater bacteria were isolated from aquifer material and labelled with³H-alanine,¹⁴C-pyruvic acid,³H-glucose, and³H-adenosine for different measurements of sorption and dispersal, which were estimated from breakthrough curves. Both experimental data and model variables showed that dispersal of bacteria was a dynamic nonequilibrium process, possibly shaped by two subpopulations, one strongly, even irreversibly, adsorbing to the solid particles, and one with very slow adsorption kinetics. The cell surfaces were modified in response to the growth conditions, which was demonstrated by hydrophobic and electrostatic interaction chromatography. Cell surface hydrophobicity was about eight times higher in groundwater than in eutrophic lake water. The partition coefficient varied between 12.6 in the groundwater and 6.4 in the lake water, indicating the prime importance of hydrophobic binding for attachment in low nutrient conditions. The partitioning was also sensitive to the hydrodynamics of the system and the oxygen supply, as demonstrated by comparison of sorption in agitated test tubes, gently shaken vials, and air-flushed bottles. Sorption kinetics were demonstrated in a continuous flow cell. About 45% of a population was associated with sand particles with a continuous flow of pure groundwater and as little as 20% in lake water. However, more than 50% of the bacteria in the aqueous phase were associated with suspended material of less than 60 μm in diameter. This association may enhance dispersal for example, by size exclusion of the colloidal material in the interstitial pores.     

Bacterial motility confers fitness advantage in the presence of phages

Dispersal provides the opportunity to escape harm and colonize new patches, enabling populations to expand and persist. However, the benefits of dispersal associated with escaping harm will be dependent on the structure of the environment and the likelihood of escape. Here, we empirically investigate how the spatial distribution of a parasite influences the evolution of host dispersal. Bacteriophages are a strong and common threat for bacteria in natural environments and offer a good system with which to explore parasite‐mediated selection on host dispersal. We used two transposon mutants of the opportunistic bacteria, Pseudomonas aeruginosa, which varied in their motility (a disperser and a nondisperser), and the lytic bacteriophage ФKZ. The phage was distributed either in the central point of colony inoculation only, thus offering an escape route for the dispersing bacteria; or, present throughout the agar, where benefits of dispersal might be lost. Surprisingly, we found dispersal to be equally advantageous under both phage conditions relative to when phages were absent. A general explanation is that dispersal decreased the spatial structuring of host population, reducing opportunities for parasite transmission, but other more idiosyncratic mechanisms may also have contributed. This study highlights the crucial role the parasites can play on the evolution of dispersal and, more specifically, that bacteriophages, which are ubiquitous, are likely to select for bacterial motility.     

Bacterial chemoreceptor signaling probed by flash photorelease of a caged serine.

A caged serine, a photolabile compound that liberates serine upon photolysis, has been synthesized. Smooth-swimming responses of the bacterium Escherichia coli to caged serine photorelease were videotaped. The mean latency was measured from the videorecords using computerized motion analysis. This time was approximately 0.2 s. Caged photorelease of a photolabile but nonchemotactic serine analogue had no effect on the swimming behavior of the bacteria. A tumbly mutant strain lacking tsr, the serine chemoreceptor, did not respond to caged serine photorelease.     

Identification of specific chemoattractants and genetic complementation of a Borrelia burgdorferi chemotaxis mutant: flow cytometry-based capillary tube chemotaxis assay

Measuring the chemotactic response of Borrelia burgdorferi, the bacterial species that causes Lyme disease, is relatively more difficult than measuring that of other bacteria. Because these spirochetes have long generation times, enumerating cells that swim up a capillary tube containing an attractant by using colony counts is impractical. Furthermore, direct counts with a Petroff-Hausser chamber is problematic, as this method has a low throughput and necessitates a high cell density; the latter can lead to misinterpretation of results when assaying for specific attractants. Only rabbit serum and tick saliva have been reported to be chemoattractants for B. burgdorferi. These complex biological mixtures are limited in their utility for studying chemotaxis on a molecular level. Here we present a modified capillary tube chemotaxis assay for B. burgdorferi that enumerates cells by flow cytometry. Initial studies identified N-acetylglucosamine as a chemoattractant. The assay was then optimized with respect to cell concentration, incubation time, motility buffer composition, and growth phase. Besides N-acetylglucosamine, glucosamine, glucosamine dimers (chitosan), glutamate, and glucose also elicited significant chemoattractant responses, although the response obtained with glucose was weak and variable. Serine and glycine were nonchemotactic. To further validate and to exploit the use of this assay, a previously described nonchemotactic cheA2 mutant was shown to be nonchemotactic by this assay; it also regained the wild-type phenotype when complemented in trans. This is the first report that identifies specific chemical attractants for B. burgdorferi and the use of flow cytometry for spirochete enumeration. The method should also be useful for assaying chemotaxis for other slow-growing prokaryotic species and in specific environments in nature.     

The MIGCLIM R package – seamless integration of dispersal constraints into projections of species distribution models

The MIGCLIM R package is a function library for the open source R software that enables the implementation of species‐specific dispersal constraints into projections of species distribution models under environmental change and/or landscape fragmentation scenarios. The model is based on a cellular automaton and the basic modeling unit is a cell that is inhabited or not. Model parameters include dispersal distance and kernel, long distance dispersal, barriers to dispersal, propagule production potential and habitat invasibility. The MIGCLIM R package has been designed to be highly flexible in the parameter values it accepts, and to offer good compatibility with existing species distribution modeling software. Possible applications include the projection of future species distributions under environmental change conditions and modeling the spread of invasive species.     

On several conjectures from evolution of dispersal

We address several conjectures raised in Cantrell et al. [Evolution of dispersal and ideal free distribution, Math. Biosci. Eng. 7 (2010), pp. 17-36 [ 9 ]] concerning the dynamics of a diffusion-advection-competition model for two competing species. A conditional dispersal strategy, which results in the ideal free distribution of a single population at equilibrium, was found in Cantrell et al. [ 9 ]. It was shown in [ 9 ] that this special dispersal strategy is a local evolutionarily stable strategy (ESS) when the random diffusion rates of the two species are equal, and here we show that it is a global ESS for arbitrary random diffusion rates. The conditions in [ 9 ] for the coexistence of two species are substantially improved. Finally, we show that this special dispersal strategy is not globally convergent stable for certain resource functions, in contrast with the result from [ 9 ], which roughly says that this dispersal strategy is globally convergent stable for any monotone resource function.     

Random-walk models of cell dispersal included in mechanobiological simulations of tissue differentiation

Computational models have shown that biophysical stimuli can be correlated with observed patterns of tissue differentiation, and simulations have been performed that predict the time course of tissue differentiation in, for example, long bone fracture healing. Some simulations have used a diffusion model to simulate the migration and proliferation of cells with the differentiating tissue. However, despite the convenience of the diffusion model, diffusion is not the mechanism of cell dispersal: cells disperse by crawling or proliferation, or are transported in a moving fluid. In this paper, a random-walk model (i.e., a stochastic model), with and without a preferred direction, is studied as an approach to simulate cell proliferation/migration in differentiating tissues and it is compared with the diffusion model. A simulation of tissue differentiation of gap tissue in a two-dimensional model of a bone/implant interface was performed to demonstrate the differences between diffusion vs. random walk with a preferred direction. Results of diffusion and random-walk models are similar with respect to the change in the stiffness of the gap tissue but rather different results are obtained regarding tissue patterning in the differentiating tissues; the diffusion approach predicted continuous patterns of tissue differentiation whereas the random-walk model showed a more discontinuous pattern-histological results are not available that can unequivocally establish which is most similar to experimental observation. Comparing isotropic to anisotropic random walk (preferred direction of proliferation and cell migration), a more rapid reduction of the relative displacement between implant and bone is predicted. In conclusion, we have shown how random-walk models of cell dispersal and proliferation can be implemented, and shown where differences between them exist. Further study of the random-walk model is warranted, given the importance of cell seeding and cell dispersal/proliferation in many mechanobiological problems.     

Application of glutaraldehyde for the staining of esterase-active cells with carboxyfluorescein diacetate

Staining of esterase-active bacteria with carboxyfluorescein diacetate (CFDA) has been used to evaluate the viability of various types of cell. However, the outer membrane of Gram-negative bacteria prevents CFDA from permeating into the cell. Although EDTA can increase the permeability of the outer membrane allowing CFDA to enter the cells, it was experimentally confirmed that there is still considerable difficulty in visualizing viable cells due to passive diffusion of carboxyfluorescein (CF), a hydrolyzed product of CFDA, out of the cells. We found that glutaraldehyde enhances the discriminative recognition of esterase-active Gram-negative bacteria under microscopic observation by improving the efficacy of staining. We believe the successful staining in the presence of glutaraldehyde is due to two separate effects: an increase in the permeability of CFDA into the cell and prevention of leakage of CF out of the cell.     

The Dispersal System of a Butterfly: A Test of Source-Sink Theory Suggests the Intermediate-Scale Hypothesis

Theory predicts source-sink dynamics can occur in species with the ideal preemptive distribution but not with the ideal free distribution. Source-sink dynamics can also occur in species with passive dispersal, in which a fixed fraction of the population disperses each generation. However, in nature, dispersal often approximates random diffusion rather than ideal choices or fixed probabilities. Here, I ask which dispersal system occurred in a butterfly (Euphydryas editha) known to have source-sink dynamics. The study used 13 experimental sites, where vacant and occupied habitat patches were juxtaposed. I estimated movement during the flight season and tested hypotheses about the type of dispersal system. Ideal free and ideal preemptive models were rejected because per capita movement rates were density independent. Passive dispersal was rejected because per capita rates were related to patch area and habitat preference. The diffusion model best explained the data because it predicted both the area relationship and an odd feature of the habitat preference: immigration was not higher in preferred habitat; rather, emigration was lower. The diffusion model implied that source-sink dynamics were driven by diffusion from areas of high to low population density. Existing source-sink theory assumes fine-scale patchiness, in which animals have perfect knowledge and ease of mobility. The results from the butterfly suggest that source-sink dynamics arise at coarser spatial scales, where diffusion models apply.     

Heterogeneous Diffusion of a Membrane-Bound pHLIP Peptide

Lateral diffusion of cell membrane constituents is a prerequisite for many biological functions. However, the diffusivity (or mobility) of a membrane-bound species can be influenced by many factors. To provide a better understanding of how the conformation and location of a membrane-bound biological molecule affect its mobility, herein we study the diffusion properties of a pH low insertion peptide (pHLIP) in model membranes using fluorescence correlation spectroscopy. It is found that when the pHLIP peptide is located on the membrane surface, its lateral diffusion is characterized by a distribution of diffusion times, the characteristic of which depends on the peptide/lipid ratio. Whereas, under conditions where pHLIP adopts a well-defined transmembrane α-helical conformation the peptide still exhibits heterogeneous diffusion, the distribution of diffusion times is found to be independent of the peptide/lipid ratio. Taken together, these results indicate that the mobility of a membrane-bound species is sensitive to its conformation and location and that diffusion measurement could provide useful information regarding the conformational distribution of membrane-bound peptides. Furthermore, the observation that the mobility of a membrane-bound species depends on its concentration may have important implications for diffusion-controlled reactions taking place in membranes.     

Identification of Specific Chemoattractants and Genetic Complementation of a Borrelia burgdorferi Chemotaxis Mutant: Flow Cytometry-Based Capillary Tube Chemotaxis Assay

Measuring the chemotactic response of Borrelia burgdorferi, the bacterial species that causes Lyme disease, is relatively more difficult than measuring that of other bacteria. Because these spirochetes have long generation times, enumerating cells that swim up a capillary tube containing an attractant by using colony counts is impractical. Furthermore, direct counts with a Petroff-Hausser chamber is problematic, as this method has a low throughput and necessitates a high cell density; the latter can lead to misinterpretation of results when assaying for specific attractants. Only rabbit serum and tick saliva have been reported to be chemoattractants for B. burgdorferi. These complex biological mixtures are limited in their utility for studying chemotaxis on a molecular level. Here we present a modified capillary tube chemotaxis assay for B. burgdorferi that enumerates cells by flow cytometry. Initial studies identified N-acetylglucosamine as a chemoattractant. The assay was then optimized with respect to cell concentration, incubation time, motility buffer composition, and growth phase. Besides N-acetylglucosamine, glucosamine, glucosamine dimers (chitosan), glutamate, and glucose also elicited significant chemoattractant responses, although the response obtained with glucose was weak and variable. Serine and glycine were nonchemotactic. To further validate and to exploit the use of this assay, a previously described nonchemotactic cheA2 mutant was shown to be nonchemotactic by this assay; it also regained the wild-type phenotype when complemented in trans. This is the first report that identifies specific chemical attractants for B. burgdorferi and the use of flow cytometry for spirochete enumeration. The method should also be useful for assaying chemotaxis for other slow-growing prokaryotic species and in specific environments in nature.     

Model to predict aerial dispersal of bacteria during environmental release

Risk assessment for genetically engineered bacteria sprayed onto crops includes determination of off-site dispersal and deposition. The ability to predict microbial dispersal patterns is essential to characterize the uncertainty (risk) associated with environmental release of recombinant organisms. Toward this end, a particle dispersal model was developed to predict recovery of bacteria on fallout plates at various distances and directions about a test site. The microcomputer simulation incorporates particle size distribution, wind speed and direction, turbulence, evaporation, sedimentation, and mortality, with a time step of 0.5 s. The model was tested against data reported from three field applications of nonrecombinant bacteria and two applications of recombinant bacteria. Simulated dispersal of 10(5) particles was compared with reported deposition measurements. The model may be useful in defining appropriate populations of organisms for release, methods of release or application, characteristics of a release site that influence containment or dispersal, and in developing an appropriate sampling methodology for monitoring the dispersal of organisms such as genetically engineered bacteria.     

Model to predict aerial dispersal of bacteria during environmental release.

Risk assessment for genetically engineered bacteria sprayed onto crops includes determination of off-site dispersal and deposition. The ability to predict microbial dispersal patterns is essential to characterize the uncertainty (risk) associated with environmental release of recombinant organisms. Toward this end, a particle dispersal model was developed to predict recovery of bacteria on fallout plates at various distances and directions about a test site. The microcomputer simulation incorporates particle size distribution, wind speed and direction, turbulence, evaporation, sedimentation, and mortality, with a time step of 0.5 s. The model was tested against data reported from three field applications of nonrecombinant bacteria and two applications of recombinant bacteria. Simulated dispersal of 10(5) particles was compared with reported deposition measurements. The model may be useful in defining appropriate populations of organisms for release, methods of release or application, characteristics of a release site that influence containment or dispersal, and in developing an appropriate sampling methodology for monitoring the dispersal of organisms such as genetically engineered bacteria.     

Wind dispersal of alpine plant species: A comparison with lowland species

Question: The prominent role of wind dispersal in alpine habitats has been recognized early but has rarely been quantified. The aim of this study is to compare wind dispersal under alpine and lowland conditions and to analyse whether differences are caused by species traits, e.g. terminal velocity of seeds (V_term) or weather conditions. Location and Methods: We characterized wind dispersal potential of > 1100 Central European species using measured V_term To quantify the habitat effect on wind dispersal, we measured meteorological key‐parameters and simulated dispersal distance spectra of nine selected species under typical alpine conditions (foreland of the Scaletta‐glacier, Switzerland) and typical lowland conditions (grassland in Bad Lippspringe, Germany). Results: Lowland species had higher V_term compared to alpine species. However, this difference is absent when only species of species of open habitats are concerned. The meteorological measurements showed that the alpine habitat was mainly characterized by higher frequency and strength of updrafts. The simulations showed that under alpine conditions long distance dispersal occurred much more frequent. Conclusions: More than 50 % of the alpine species have a fair chance to be dispersed by wind over long distances, while this proportion is less than 25 % for species from open habitats in the lowland. The more prominent role of wind dispersal in alpine habitats is mainly a result of differences in environmental conditions, namely more intense vertical turbulence in the alpine habitat, and does not result from prominent differences in plant traits, namely V_term, between alpine and lowland species.     

Transport and survival of alginate-encapsulated and free lux-lac marked Pseudomonas aeruginosa UG2Lr cells in soil

Transport and survival of alginate-encapsulated and unencapsulated Pseudomonas aeruginosa UG2Lr through soil microcosms was examined. Bacterial cells encapsulated in alginate beads or mixed with soil were introduced into soil microcosms. Microbial cell survival and cell transport were monitored by destructive sampling and selective plating of the microcosms over a 9-week period. Survival rates were greatest when using encapsulated P. aeruginosa UG2Lr cells. Water flow increased microbial cell dispersal from the site of inoculation. After 3 weeks, encapsulated and free cells showed similar distribution patterns. However, after 9 weeks microbial cell distribution was more extensive throughout the soil in the encapsulated treatments under all conditions. Therefore, alginate encapsulation is a suitable method to enhance survival and distribution of microbial inocula in the soil environment.     

Influence of the Gas-Water Interface on Transport of Microorganisms through Unsaturated Porous Media

In this article, a new mechanism influencing the transport of microorganisms through unsaturated porous media is examined, and a new method for directly visualizing bacterial behavior within a porous medium under controlled chemical and flow conditions is introduced. Resting cells of hydrophilic and relatively hydrophobic bacterial strains isolated from groundwater were used as model microorganisms. The degree of hydrophobicity was determined by contact-angle measurements. Glass micromodels allowed the direct observation of bacterial behavior on a pore scale, and three types of sand columns with different gas saturations provided quantitative measurements of the observed phenomena on a porous medium scale. The reproducibility of each break-through curve was established in three to five repeated experiments. The data collected from the column experiments can be explained by phenomena directly observed in the micromodel experiments. The retention rate of bacteria is proportional to the gas saturation in porous media because of the preferential sorption of bacteria onto the gas-water interface over the solid-water interface. The degree of sorption is controlled mainly by cell surface hydrophobicity under the simulated groundwater conditions because of hydrophobic forces between the organisms and the interfaces. The sorption onto the gas-water interface is essentially irreversible because of capillary forces. This preferential and irreversible sorption at the gas-water interface strongly influences the movement and spatial distribution of microorganisms.