Quantitative analysis of experiments on bacterial chemotaxis to naphthalene

Taxis-enhanced mineralization and co-metabolism of PAHs by bacteria in micrometer-scale environments

Polycyclic aromatic hydrocarbons (PAHs) are associated with micropores in sediments and soils. This limits the bioaccessibility of these compounds via existing bioremediation technologies, as biodegradation is strongly influenced by the ability of bacteria to access different sizes of pores. In this work, we employed naphthalene and pyrene as model contaminants to evaluate the transformation capacity of the soil bacterium Pseudomonas putida G7 (2 × 1 μm) via mineralization and co-metabolic activity, respectively. Under non-growing conditions and in the absence of hydraulic flow, we examined how the tactic behavior of this motile bacterium influenced biodegradation of these two PAHs when passing through membranes with micrometer-sized pores (3 and 5 μm). The bacteria were spontaneously retained by the membranes, which blocked the contaminants away from a passive dosing source. However, the cells were mobilized through 5 μm pores after the application plant root exudate components (γ-aminobutyric acid, citrate and fructose) as strong chemoeffectors, which enhanced the mineralization of naphthalene and co-metabolism of pyrene. The tactic-mediated biodegradation enhancement did not occur through 3 μm pores, possibly due a physical constrain to the gradient sensing mechanism. Our results suggest that bacterial transport by chemotaxis may enhance the biotransformation of poorly bioaccessible contaminants present in micro-meter scale environments.

Chemotaxis along local chemical gradients enhanced bacteria dispersion and PAH bioavailability in a heterogenous porous medium

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous, EPA-designated priority pollutants for soil and groundwater, remaining recalcitrant to bioremediation because of limited bioavailability. In this work, we used naphthalene as a model PAH and soil bacteria Pseudomonas putida G7 to investigate the potential role of chemotaxis to enhance access to PAHs in heterogenous porous media. To this aim, we conducted transport experiments and numerical simulations with chemotactic bacteria and naphthalene trapped within a non-aqueous phase liquid (NAPL) mainly in low permeable areas of a dual-permeability microfluidic device. Microscopic imaging showed higher accumulations of chemotactic bacteria, about eight times that of nonchemotactic bacteria, at the junctures between high and low permeability regions. Pore-scale simulations for fluid flow and naphthalene revealed that the junctures are stagnant areas of fluid flow, which generated strong and temporally persistent naphthalene gradients. The landscape and densities of bacterial accumulation at the junctures were strongly regulated by flow profiles and naphthalene gradients especially those transverse to flow. We conducted macroscale simulations using convective dispersion equations with an added chemotactic velocity to account for directed migration toward naphthalene. Simulated results showed good consistency with experiments and pore-scale simulation as normalized bacterial accumulation per mm of NAPL was 7.80, 7.84 and 7.71 mm^-1 for experiments, pore-scale and macroscale simulations, respectively. Macroscale simulations indicated that in the absence of grain-boundary restrictions associated with the pore structure bacterial dispersion needed to be increased by 50 % to account for the interplay between chemotactic response and naphthalene gradients at the pore-scale level. Our work details the mechanism of pore-scale chemotaxis in enhancing bioavailability of PAHs and its impact on biomass retention at the system level, which provides a potential solution toward more efficient bioremediation for contaminants such as PAHs with limited bioavailability.

Numerical Simulation of the Enrichment of Chemotactic Bacteria in Oil-Water Two-Phase Transfer Fields of Heterogeneous Porous Media

Oil pollution in soil-groundwater systems is difficult to remove, and a large amount of residual oil is trapped in the low permeable layer of the heterogeneous aquifer. Aromatic hydrocarbons in oil have high toxicity and low solubility in water, which are harmful to the ecological environment. Chemotactic degrading bacteria can perceive the concentration gradient of non-aqueous phase liquid (NAPL) pollutants in the groundwater environment, and enrich and proliferate around the pollutants, thus achieving a more efficient and thorough remediation effect. However, the existing theoretical models are relatively simple. The physical fields of oil–water two-phase flow and oil-phase solute convection and diffusion in water are not coupled, which further restricts the accuracy of studies on bacterial chemotaxis to NAPL. In this study, geometric models based on the actual microfluidic experimental study were constructed. Based on the phase field model, diffusion convection equation and chemotaxis velocity equation, the effects of heterogeneity of porous media, wall wettability and groundwater flow rate on the residual oil and the concentration distribution of chemotaxis bacteria were studied. Under all of the simulation conditions, the residual oil in the high permeable area was significantly lower than that in the low permeable area, and the wall hydrophilicity enhanced the water flooding effect. Chemotactic bacteria could react to the concentration gradient of pollutants dissolved into water in the oil phase, and enrich near the oil–water interface with high concentration of NAPL, and the density of chemotactic bacteria at the oil–water interface can be up to 1.8–2 times higher than that in the water phase at flow rates from 1.13 to 6.78 m/d.

A traveling-wave solution for bacterial chemotaxis with growth

Bacterial cells navigate their environment by directing their movement along chemical gradients. This process, known as chemotaxis, can promote the rapid expansion of bacterial populations into previously unoccupied territories. However, despite numerous experimental and theoretical studies on this classical topic, chemotaxis-driven population expansion is not understood in quantitative terms. Building on recent experimental progress, we here present a detailed analytical study that provides a quantitative understanding of how chemotaxis and cell growth lead to rapid and stable expansion of bacterial populations. We provide analytical relations that accurately describe the dependence of the expansion speed and density profile of the expanding population on important molecular, cellular, and environmental parameters. In particular, expansion speeds can be boosted by orders of magnitude when the environmental availability of chemicals relative to the cellular limits of chemical sensing is high. Analytical understanding of such complex spatiotemporal dynamic processes is rare. Our analytical results and the methods employed to attain them provide a mathematical framework for investigations of the roles of taxis in diverse ecological contexts across broad parameter regimes.

Circulating mitochondrial N-formyl peptides contribute to secondary nosocomial infection in patients with septic shock

Septic shock commonly leads to multiorgan injury both directly via tissue inflammation and secondarily via hypoperfusion, but both can result in mitochondrial N-formyl peptide (mtFP) release into the circulation. However, no studies have evaluated the role of circulating mtFPs during septic shock. We found that a relatively high plasma nicotinamide adenine dinucleotide dehydrogenase subunit-6 (the most potent human mtFP) level was independently associated with the development of secondary infection in patients with septic shock and that the increased susceptibility to secondary infection is partly attributed to the suppression of polymorphonuclear leukocyte (PMN) chemotaxis by mtFP occupancy of formyl peptide receptor-1. Incorporation of these findings into therapeutic strategies may improve clinical outcomes in septic shock patients by preventing PMN chemotactic anergy.

Secondary infections typically worsen outcomes of patients recovering from septic shock. Neutrophil [polymorphonuclear leukocytes (PMNs)] migration to secondarily inoculated sites may play a key role in inhibiting progression from local bacterial inoculation to secondary infection. Mitochondrial N-formyl peptide (mtFP) occupancy of formyl peptide receptor-1 (FPR1) has been shown to suppress PMN chemotaxis. Therefore, we studied the association between circulating mtFPs and the development of secondary infection in patients with septic shock. We collected clinical data and plasma samples from patients with septic shock admitted to the intensive care unit for longer than 72 h. Impacts of circulating nicotinamide adenine dinucleotide dehydrogenase subunit-6 (ND6) upon clinical outcomes were analyzed. Next, the role of ND6 in PMN chemotaxis was investigated using isolated human PMNs. Studying plasma samples from 97 patients with septic shock, we found that circulating ND6 levels at admission were independently and highly associated with the development of secondary infection (odds ratio = 30.317, 95% CI: 2.904 to 316.407, P = 0.004) and increased 90-d mortality (odds ratio = 1.572, 95% CI: 1.002 to 2.465, P = 0.049). In ex vivo experiments, ND6 pretreatment suppressed FPR1-mediated PMN chemotactic responses to bacterial peptides in the presence of multiple cytokines and chemokines, despite increased nondirectional PMN movements. Circulating mtFPs appear to contribute to the development of secondary infection and increased mortality in patients with septic shock who survive their early hyperinflammatory phase. The increased susceptibility to secondary infection is probably partly mediated by the suppression of FPR1-mediated PMN chemotaxis to secondary infected sites.

Chemotaxis Toward Crude Oil by an Oil-Degrading Pseudomonas aeruginosa 6-1B Strain

The chemotactic properties of an oil-degrading Pseudomonas aeruginosa strain 6-1B, isolated from Daqing Oilfield, China, have been investigated. The strain 6-1B could grow well in crude oil with a specific rhamnolipid biosurfactant production. Furthermore, it exhibits chemotaxis toward various substrates, including glycine, glycerol, glucose, and sucrose. Compared with another oil-degrading strain, T7-2, the strain 6-1B presented a better chemotactic response towards crude oil and its vital component, n-alkenes. Based on the observed distribution of the strain 6-1B cells around the oil droplet in the chemotactic assays, the potential chemotaxis process of bacteria toward crude oil could be summarized in the following steps: searching, moving and consuming.

Microbial Degradation of Naphthalene and Substituted Naphthalenes: Metabolic Diversity and Genomic Insight for Bioremediation

Low molecular weight polycyclic aromatic hydrocarbons (PAHs) like naphthalene and substituted naphthalenes (methylnaphthalene, naphthoic acids, 1-naphthyl N-methylcarbamate, etc.) are used in various industries and exhibit genotoxic, mutagenic, and/or carcinogenic effects on living organisms. These synthetic organic compounds (SOCs) or xenobiotics are considered as priority pollutants that pose a critical environmental and public health concern worldwide. The extent of anthropogenic activities like emissions from coal gasification, petroleum refining, motor vehicle exhaust, and agricultural applications determine the concentration, fate, and transport of these ubiquitous and recalcitrant compounds. Besides physicochemical methods for cleanup/removal, a green and eco-friendly technology like bioremediation, using microbes with the ability to degrade SOCs completely or convert to non-toxic by-products, has been a safe, cost-effective, and promising alternative. Various bacterial species from soil flora belonging to Proteobacteria (Pseudomonas, Pseudoxanthomonas, Comamonas, Burkholderia, and Novosphingobium), Firmicutes (Bacillus and Paenibacillus), and Actinobacteria (Rhodococcus and Arthrobacter) displayed the ability to degrade various SOCs. Metabolic studies, genomic and metagenomics analyses have aided our understanding of the catabolic complexity and diversity present in these simple life forms which can be further applied for efficient biodegradation. The prolonged persistence of PAHs has led to the evolution of new degradative phenotypes through horizontal gene transfer using genetic elements like plasmids, transposons, phages, genomic islands, and integrative conjugative elements. Systems biology and genetic engineering of either specific isolates or mock community (consortia) might achieve complete, rapid, and efficient bioremediation of these PAHs through synergistic actions. In this review, we highlight various metabolic routes and diversity, genetic makeup and diversity, and cellular responses/adaptations by naphthalene and substituted naphthalene-degrading bacteria. This will provide insights into the ecological aspects of field application and strain optimization for efficient bioremediation.

Bacterial chemotaxis: a way forward to aromatic compounds biodegradation

Worldwide industrial development has released hazardous polycyclic aromatic compounds into the environment. These pollutants need to be removed to improve the quality of the environment. Chemotaxis mechanism has increased the bioavailability of these hydrophobic compounds to microorganisms. The mechanism, however, is poorly understood at the ligand and chemoreceptor interface. Literature is unable to furnish a compiled review of already published data on up-to-date research on molecular aspects of chemotaxis mechanism, ligand and receptor-binding mechanism, and downstream signaling machinery. Moreover, chemotaxis-linked biodegradation of aromatic compounds is required to understand the chemotaxis role in biodegradation better. To fill this knowledge gap, the current review is an attempt to cover PAHs occurrence, chemical composition, and potential posed risks to humankind. The review will cover the aspects of microbial signaling mechanism, the structural diversity of methyl-accepting chemotaxis proteins at the molecular level, discuss chemotaxis mechanism role in biodegradation of aromatic compounds in model bacterial genera, and finally conclude with the potential of bacterial chemotaxis for aromatics biodegradation.

Modeling Transport of Chemotactic Bacteria in Granular Media with Distributed Contaminant Sources

Chemotaxis has the potential to improve bioremediation strategies by enhancing the transport of pollutant-degrading bacteria to the source of contamination, leading to increased pollutant accessibility and biodegradation. This computational study extends work reported previously in the literature to include predictions of chemotactic bacterial migration in response to multiple localized contaminant sources within porous media. An advection-dispersion model, in which chemotaxis was represented explicitly as an additional advection-like term, was employed to simulate the transport of bacteria within a sand-packed column containing a distribution of chemoattractant sources. Simulation results provided insight into attractant and bacterial distributions within the column. In particular, it was found that chemotactic bacteria exhibited a distinct biased migration toward contaminant sources that resulted in a 30% decrease in cell recovery, and concomitantly an enhanced retention within the sand column, compared to the nonchemotactic control. Model results were consistent with experimental observations. Parametric studies were conducted to provide insight into the influence of chemotaxis parameters on bacterial migration and cell percent recovery. The model results provide a better understanding of the effect of chemotaxis on bacterial transport in response to distributed contaminant sources.

An alternative smooth particle hydrodynamics formulation to simulate chemotaxis in porous media

Chemotaxis, the microorganisms autonomous motility along or against the concentration gradients of a chemical species, is an important, yet often neglected factor controlling the transport of bacteria through saturated porous media. For example, chemotactic bacteria could enhance bioremediation by directing their own motion to residual contaminants trapped in low hydraulic conductive zones of contaminated aquifers. The aim of the present work is to develop an accurate numerical scheme to model chemotaxis in saturated porous media and other advective dominating flow systems. We propose to model chemotaxis by using a new class of meshless Lagrangian particle methods we recently developed for applications in fluid mechanics. The method is based on the Smooth Particle Hydrodynamics (SPH) formulation of (Ben Moussa et al., Int Ser Numer Math, 13(1):29-62, 2006), combined with a new Weighted Essentially Non-Oscillatory (WENO) reconstruction technique on moving point clouds in multiple space dimensions. The purpose of this new numerical scheme is to fully exploit the advantages of SPH among traditional mesh-based and mesh-free schemes and to overcome drawbacks related to the use of standard SPH for modeling chemotaxis in porous media. First, we test the new scheme against analytical reference solutions. Then, under the assumption of complete mixing at the Darcy scale, we perform two-dimensional conservative solute transport simulations under steady-state flow conditions, to show the capability of the proposed new scheme to model chemotaxis.

Video processing analysis for the determination and evaluation of the chemotactic response in bacterial populations

The aim of the present work was to design a methodology based on video processing to obtain indicators of bacterial population motility that allow the quantitative and qualitative analysis and comparison of the chemotactic phenomenon with different attractants in the agarose-in plug bridge method. Video image sequences were processed applying Shannon's entropy to the intensity time series of each pixel, which conducted to a final pseudo colored image resembling a map of the dynamic bacterial clusters. Processed images could discriminate perfectly between positive and negative attractant responses at different periods of time from the beginning of the assay. An index of spatial and temporal motility was proposed to quantify the bacterial response. With this index, this video processing method allowed obtaining quantitative information of the dynamic changes in space and time from a traditional qualitative assay. We conclude that this computational technique, applied to the traditional agarose-in plug assay, has demonstrated good sensitivity for identifying chemotactic regions with a broad range of motility.

Pseudomonas chemotaxis

Pseudomonads sense changes in the concentration of chemicals in their environment and exhibit a behavioral response mediated by flagella or pili coupled with a chemosensory system. The two known chemotaxis pathways, a flagella-mediated pathway and a putative pili-mediated system, are described in this review. Pseudomonas shows chemotaxis response toward a wide range of chemicals, and this review includes a summary of them organized by chemical structure. The assays used to measure positive and negative chemotaxis swimming and twitching Pseudomonas as well as improvements to those assays and new assays are also described. This review demonstrates that there is ample research and intellectual space for future investigators to elucidate the role of chemotaxis in important processes such as pathogenesis, bioremediation, and the bioprotection of plants and animals.

A multiple-relaxation-time lattice-boltzmann model for bacterial chemotaxis: effects of initial concentration, diffusion, and hydrodynamic dispersion on traveling bacterial bands

Bacterial chemotaxis can enhance the bioremediation of contaminants in aqueous and subsurface environments if the contaminant is a chemoattractant that the bacteria degrade. The process can be promoted by traveling bands of chemotactic bacteria that form due to metabolism-generated gradients in chemoattractant concentration. We developed a multiple-relaxation-time (MRT) lattice-Boltzmann method (LBM) to model chemotaxis, because LBMs are well suited to model reactive transport in the complex geometries that are typical for subsurface porous media. This MRT-LBM can attain a better numerical stability than its corresponding single-relaxation-time LBM. We performed simulations to investigate the effects of substrate diffusion, initial bacterial concentration, and hydrodynamic dispersion on the formation, shape, and propagation of bacterial bands. Band formation requires a sufficiently high initial number of bacteria and a small substrate diffusion coefficient. Uniform flow does not affect the bands while shear flow does. Bacterial bands can move both upstream and downstream when the flow velocity is small. However, the bands disappear once the velocity becomes too large due to hydrodynamic dispersion. Generally bands can only be observed if the dimensionless ratio between the chemotactic sensitivity coefficient and the effective diffusion coefficient of the bacteria exceeds a critical value, that is, when the biased movement due to chemotaxis overcomes the diffusion-like movement due to the random motility and hydrodynamic dispersion.

Coupled effects of chemotaxis and growth on traveling bacterial waves

Traveling bacterial waves are capable of improving contaminant remediation in the subsurface. It is fairly well understood how bacterial chemotaxis and growth separately affect the formation and propagation of such waves. However, their interaction is not well understood. We therefore perform a modeling study to investigate the coupled effects of chemotaxis and growth on bacterial migration, and examine their effects on contaminant remediation. We study the waves by using different initial electron acceptor concentrations for different bacteria and substrate systems. Three types of traveling waves can occur: a chemotactic wave due to the biased movement of chemotactic bacteria resulting from metabolism-generated substrate concentration gradients; a growth/decay/motility wave due to a dynamic equilibrium between bacterial growth, decay and random motility; and an integrated wave due to the interaction between bacterial chemotaxis and growth. Chemotaxis hardly enhances the bacterial propagation if it is too weak to form a chemotactic wave or its wave speed is less than half of the growth/decay/motility wave speed. However, chemotaxis significantly accelerates bacterial propagation once its wave speed exceeds the growth/decay/motility wave speed. When convection occurs, it speeds up the growth/decay/motility wave but slows down or even eliminates the chemotactic wave due to the dispersion. Bacterial survival proves particularly important for bacterial propagation. Therefore we develop a conceptual model to estimate the speed of growth/decay/motility waves.

Tactic responses to pollutants and their potential to increase biodegradation efficiency

A significant number of bacterial strains are able to use toxic aromatic hydrocarbons as carbon and energy sources. In a number of cases, the evolution of the corresponding degradation pathway was accompanied by the evolution of tactic behaviours either towards or away from these toxic carbon sources. Reports are reviewed which show that a chemoattraction to heterogeneously distributed aromatic pollutants increases the bioavailability of these compounds and their biodegradation efficiency. An extreme form of chemoattraction towards aromatic pollutants, termed 'hyperchemotaxis', was described for Pseudomonas putida DOT-T1E, which is based on the action of the plasmid-encoded McpT chemoreceptor. Cells with this phenotype were found of being able to approach and of establishing contact with undiluted crude oil samples. Although close McpT homologues are found on other degradation plasmids, the sequence of their ligand-binding domains does not share significant similarity with that of NahY, the other characterized chemoreceptor for aromatic hydrocarbons. This may suggest the existence of at least two families of chemoreceptors for aromatic pollutants. The use of receptor chimers comprising the ligand-binding region of McpT for biosensing purposes is discussed.

Transverse chemotactic migration of bacteria from high to low permeability regions in a dual permeability microfluidic device

Low permeability regions such as clay lenses are difficult to remediate using conventional treatment methods. Bacterial chemotaxis (directed migration toward a contaminant source) may be helpful in enhancing bioremediation of such contaminated sites. This study experimentally simulates a two-dimensional dual-permeability groundwater contamination scenario using a microfluidic device (MFD) and evaluates transverse chemotactic migration of bacteria from high to low permeability regions under various flow velocities. Chemotaxis of Escherichia coli (E. coli) HCB33 to the chemoattractant dl-aspartic acid was quantified in terms of change in total bacterial counts in pore throats in low permeability regions containing attractant. An increase in total bacterial counts, ranging from 1.09 to 1.74 times, was observed in low permeability regions in experiments under chemotactic conditions. Experiments with no attractant showed no increase in total bacterial counts in low permeability regions. A large increase in bacterial counts in the pore throats just outside the low permeability region was also observed in chemotaxis experiments. The bacterial chemotactic response was observed to decrease linearly with increase in flow velocity, with no observed response at the highest flow velocity (Darcy velocity = 0.22 mm/s), where chemotaxis was offset by advective flow.

Bacterial chemotaxis along vapor-phase gradients of naphthalene

The role of bacterial growth and translocation for the bioremediation of organic contaminants in the vadose zone is poorly understood. Whereas air-filled pores restrict the mobility of bacteria, diffusion of volatile organic compounds in air is more efficient than in water. Past research, however, has focused on chemotactic swimming of bacteria along gradients of water-dissolved chemicals. In this study we tested if and to what extent Pseudomonas putida PpG7 (NAH7) chemotactically reacts to vapor-phase gradients forming above their swimming medium by the volatilization from a spot source of solid naphthalene. The development of an aqueous naphthalene gradient by air-water partitioning was largely suppressed by means of activated carbon in the agar. Surprisingly, strain PpG7 was repelled by vapor-phase naphthalene although the steady state gaseous concentrations were 50-100 times lower than the aqueous concentrations that result in positive chemotaxis of the same strain. It is thus assumed that the efficient gas-phase diffusion resulting in a steady, and possibly toxic, naphthalene flux to the cells controlled the chemotactic reaction rather than the concentration to which the cells were exposed. To our knowledge this is the first demonstration of apparent chemotactic behavior of bacteria in response to vapor-phase effector gradients.

Quantitative analysis of transverse bacterial migration induced by chemotaxis in a packed column with structured physical heterogeneity

A two-dimensional mathematical model was developed to simulate transport phenomena of chemotactic bacteria in a sand-packed column designed with structured physical heterogeneity in the presence of a localized chemical source. In contrast to mathematical models in previous research work, in which bacteria were typically treated as immobile colloids, this model incorporated a convective-like chemotaxis term to represent chemotactic migration. Consistency between experimental observation and model prediction supported the assertions that (1) dispersion-induced microbial transfer between adjacent conductive zones occurred at the interface and had little influence on bacterial transport in the bulk flow of the permeable layers and (2) the enhanced transverse bacterial migration in chemotactic experiments relative to nonchemotactic controls was mainly due to directed migration toward the chemical source zone. On the basis of parameter sensitivity analysis, chemotactic parameters determined in bulk aqueous fluid were adequate to predict the microbial transport in our intermediate-scale porous media system. Additionally, the analysis of adsorption coefficient values supported the observation of a previous study that microbial deposition to the surface of porous media might be decreased under the effect of chemoattractant gradients. By quantitatively describing bacterial transport and distribution in a heterogeneous system, this mathematical model serves to advance our understanding of chemotaxis and motility effects in granular media systems and provides insights for modeling microbial transport in in situ microbial processes.

Kinetics of trichloroethylene and toluene toxicity to Pseudomonas putida F1

The goal of the present study was to elucidate the distribution of viable bacteria in chemical gradients and to evaluate the toxic effect of high concentrations of contaminants on contaminant-degrading bacteria under prolonged exposure. Accumulations of viable Pseudomonas putida F1 (P. putida F1) cells were observed surrounding trichloroethylene (TCE)-containing plugs. Results from this work indicate that P. putida F1 immediately adjacent to a TCE source become nonviable, whereas cells accumulating farther away use chemotaxis to migrate toward regions with optimal chemical concentrations in the form of concentrated bacterial bands. A method was developed to test the toxicity of model contaminant stressors, TCE and toluene, to P. putida F1; data obtained from toxicity experiments were fit to linear and exponential bacterial viability-decay models. Toxicity of TCE to P. putida F1 was best described with an exponential viability-decay model, with a viability-decay constant k(TCE) = 0.025 h(-4.95) (r(2) = 0.965). Toluene toxicity showed a marginally better fit to the linear viability-decay model (r(2) = 0.976), with a viability-decay constant k(toluene) = 0.208 h(-1). Best-fit model parameters obtained for both TCE and toluene were used to predict bacterial viability in toxicity experiments with higher contaminant concentrations and matched well with experimental data. Results from the present study can be used to predict bacterial accumulation and viability near nonaqueous-phase liquid (NAPL) sources in groundwater and may be helpful in designing bioremediation strategies for sites contaminated with residual NAPLs.

Transverse bacterial migration induced by chemotaxis in a packed column with structured physical heterogeneity

The significance of chemotaxis in directing bacterial migration toward contaminants in natural porous media was investigated under groundwater flow conditions. A laboratory-scale column, with a coarse-grained sand core surrounded by a fine-grained annulus, was used to simulate natural aquifers with strata of different hydraulic conductivities. A chemoattractant source was placed along the central axis of the column to model contaminants trapped in the heterogeneous subsurface. Chemotactic bacterial strains, Escherichia coli HCB1 and Pseudomonas putida F1, introduced into the column by a pulse injection, were found to alter their transport behaviors under the influence of the attractant chemical emanating from the central source. For E. coil HCB1, approximately 18% more of the total population relative to the control without attractant exited the column from the coarse sand layer due to the chemotactic effects of alpha-methylaspartate under an average fluid velocity of 5.1 m/d. Although P. putida F1 demonstrated no observable changes in migration pathways with the model contaminant acetate under the same flow rate, when the flow rate was reduced to 1.9 m/d, approximately 6-10% of the population relative to the control migrated from the fine sand layer toward attractant into the coarse sand layer. Microbial transport properties were further quantified by a mathematical model to examine the significance of bacterial motility and chemotaxis under different hydrodynamic conditions, which suggested important considerations for strain selection and practical operation of bioremediation schemes.

Chemoeffectors decrease the deposition of chemotactic bacteria during transport in porous media

Bacterial chemotaxis enables motile cells to move along chemical gradients and to swim toward optimal places for biodegradation. However, its potentially positive effects on subsurface remediation rely on the efficiency of bacterial movement in porous media, which is often restricted by high deposition rates and adhesion to soil surfaces. In well-controlled column systems, we assessed the influence of the chemo-effectors naphthalene, salicylate, fumarate, and acetate on deposition of chemotactic, naphthalene-degrading Pseudomonas putida G7 in selected porous environments (sand, forest soil, and clay aggregates). Our data showed that the presence of naphthalene in the pore water decreased deposition of strain 67 (but not of a derivative strain, P. putida 67.C1 (pHG100), nonchemotactic to naphthalene) by 50% in sand-filled columns, as calculated by the relative adhesion efficiency (at). Similar effects were observed with P. putida G7 strain for the other chemoeffectors. Deposition, however, depended on the chemoeffector's chemical structure, its interaction with the column packing material, and concomitantly its pore-water concentration. As the presence of the chemoeffectors had no influence on the physicochemical surface properties of the bacteria, we suggest that chemotactic sensing, combined with changed swimming modes, is likely to influence the deposition of bacteria in the subsurface, provided that the chemoeffector is dissolved at sufficient concentration in the pore water.

The effect of oxygen on chemotaxis to naphthalene byPseudomonas putida G7

Chemotactic bacteria can be attracted to electron donors they consume. In systems where donor is heterogeneously distributed, chemotaxis can lead to enhanced removal of donor relative to that achieved in the absence of chemotaxis. However, simultaneous consumption of an electron acceptor may result in the formation of an acceptor gradient to which the bacteria also respond, thus diminishing the positive effect of chemotaxis. Depletion of an electron acceptor can also reduce the rate of electron donor consumption in addition to its effect on chemotaxis. In this study, we examined the effect of oxygen on chemotaxis to naphthalene and on naphthalene consumption by Pseudomonas putida G7. The organism was able to move up an oxygen gradient when there was a naphthalene gradient in the opposite direction. In the absence of an oxygen gradient, low levels of oxygen attenuated chemotaxis to naphthalene but did not affect random motility. The rate of naphthalene consumption decreased at dissolved oxygen concentrations similar to those at which chemotaxis was attenuated. These results suggest that low dissolved oxygen concentrations can reduce naphthalene removal by P. putida G7 in systems where naphthalene is heterogeneously distributed by simultaneously attenuating chemotactic motion toward naphthalene and decreasing the rate of naphthalene degradation.

Denitrification and chemotaxis of Pseudomonas stutzeri KC in porous media

Chemotaxis is an important mechanism by which microorganisms are dispersed in porous media. A vigorous chemotactic response to concentration gradients formed by microbial consumption of chemoattractants can accelerate transport of bacteria to highly contaminated regions of soils and sediments, enhancing the efficiency of in situ bioremediation operations. Although chemotaxis plays a key role in establishment of biodegradation zones in the subsurface, the effects of physical heterogeneity on bacterial motility are poorly understood. To investigate the influence of porous media heterogeneity on microbial chemotaxis, swarm plate migration experiments were conducted using Pseudomonas stutzeri strain KC, a denitrifying bacterium used for in situ biodegradation of carbon tetrachloride in groundwater. Swarm plate measurements indicate that strain KC is strongly chemotactic toward both acetate and nitrate. A three-component mathematical model was developed to describe the migration of strain KC. Estimates of chemotactic sensitivity were obtained in the homogeneous (agar) phase and in a heterogeneous medium of aquifer solids extracted from the Schoolcraft bioremediation field site in western Michigan. Interestingly, the motility of strain KC is significantly larger in the porous medium than in the aqueous phase. We hypothesize that chemotactic response is enhanced within the heterogeneous medium because chemoattractant gradients formed by nitrate consumption are larger in the confined spaces of the porous medium than in unconfined agar solution.

Continuous-flow capillary assay for measuring bacterial chemotaxis

Bacterial chemotaxis may have a significant impact on the structure and function of bacterial communities. Quantification of chemotactic motion is necessary to identify chemoeffectors and to determine the bacterial transport parameters used in predictive models of chemotaxis. When the chemotactic bacteria consume the chemoeffector, the chemoeffector gradient to which the bacteria respond may be significantly perturbed by the consumption. Therefore, consumption of the chemoeffector can confound chemotaxis measurements if it is not accounted for. Current methods of quantifying chemotaxis use bacterial concentrations that are too high to preclude chemoeffector consumption or involve ill-defined conditions that make quantifying chemotaxis difficult. We developed a method of quantifying bacterial chemotaxis at low cell concentrations ( approximately 10(5) CFU/ml), so metabolism of the chemoeffector is minimized. The method facilitates quantification of bacterial-transport parameters by providing well-defined boundary conditions and can be used with volatile and semivolatile chemoeffectors.

Lattice-Boltzmann model for bacterial chemotaxis

We present a new numerical approach for modeling bacterial chemotaxis and the fate and transport of a chemoattractant in bulk liquids. This Lattice-Boltzmann method represents the microorganisms and the chemoattractant by quasi-particles that move, collide, and react with each other on a two-dimensional numerical lattice. We use the model to simulate traveling bands of bacteria along self-generated gradients in substrate concentration in bulk liquids. Particularly, we simulate Pseudomonas putida that respond chemotactically to naphthalene dissolved in water. We find that only a fraction of a bacterial slug injected into a domain containing the chemoattractant at constant concentration forms a traveling band as the slug length exceeds a critical value. An expanding bacterial ring forms as one injects a droplet of bacteria into a two-dimensional domain.

Quantification of bacterial chemotaxis in porous media using magnetic resonance imaging

Bacterial chemotaxis has the potential to enhance biodegradation of organic contaminants in polluted groundwater systems. However, studies of bacterial chemotaxis in porous media are scarce. In this study we use magnetic resonance imaging (MRI) for the noninvasive measurement of changes in bacterial-density distributions in a packed column at a spatial resolution of 330 microm as a function of time. We analyze both the diffusive and the chemotactic behavior of Pseudomonas putida F1 in the presence of the chemical stimulus trichloroethylene (TCE). The migration of motile bacteria in experiments without TCE was described using an effective motility coefficient, whereas the presence of TCE required addition of a nonzero chemotactic sensitivity coefficient, indicating a significant response to TCE. The need for a chemotactic sensitivity term was justified by a test for statistical significance. This study represents the first quantification of bacterial chemotactic parameters within a packed column. For conditions under which chemotaxis occurs in porous media, it may potentially be exploited to significantly improve rates of in situ pollutant biodegradation in the subsurface environment, particularlyfor pollutants dissolved in water trapped in low-permeability formations or lenses.

Bacterial chemotaxis to naphthalene desorbing from a nonaqueous liquid

Bacterial chemotaxis has the potential to increase the rate of degradation of chemoattractants, but its influence on degradation of hydrophobic attractants initially dissolved in a non-aqueous-phase liquid (NAPL) has not been examined. We studied the effect of chemotaxis by Pseudomonas putida G7 on naphthalene mass transfer and degradation in a system in which the naphthalene was dissolved in a model NAPL. Chemotaxis by wild-type P. putida G7 increased the rates of naphthalene desorption and degradation relative to rates observed with nonchemotactic and nonmotile mutant strains. While biodegradation alone influenced the rate of substrate desorption by increasing the concentration gradient against which desorption occurred, chemotaxis created an even steeper gradient as the cells accumulated near the NAPL source. The extent to which chemotaxis affected naphthalene desorption and degradation depended on the initial bacterial and naphthalene concentrations, reflecting the influences of these variables on concentration gradients and on the relative rates of mass transfer and biodegradation. The results of this study suggest that chemotaxis can substantially increase the rates of mass transfer and degradation of NAPL-associated hydrophobic pollutants.