Bacterial transport experiments in fractured crystalline bedrock

Factors Influencing Microbial Contamination of Groundwater: A Systematic Review of Field-Scale Studies

Pathogenic microorganisms released onto the soil from point or diffuse sources represent a public health concern. They can be transported by rainwater that infiltrates into subsoil and reach the groundwater where they can survive for a long time and contaminate drinking water sources. As part of the SCA.Re.S. (Evaluation of Health Risk Related to the Discharge of Wastewater on the Soil) project, we reviewed a selection of field-scale studies that investigated the factors that influenced the fate of microorganisms that were transported from the ground surface to the groundwater. A total of 24 studies published between 2003 and 2022 were included in the review. These studies were selected from the PubMed and Web of Science databases. Microbial contamination of groundwater depends on complex interactions between human activities responsible for the release of contaminants onto the soil, and a range of environmental and biological factors, including the geological, hydraulic, and moisture characteristics of the media traversed by the water, and the characteristics and the viability of the microorganisms, which in turn depend on the environmental conditions and presence of predatory species. Enterococci appeared to be more resistant in the underground environment than thermotolerant coliforms and were suggested as a better indicator for detecting microbial contamination of groundwater.

Sustenance of Himalayan springs in an emerging water crisis

Springs are a significant source of high quality and perennial freshwater supply for remote communities and sustain rich biodiversity and ecosystems in the Himalayas. About 60-70% of the Himalayan population directly depends on springs to meet their domestic and livelihood needs. Despite that, decline in approximately 60% of low discharge springs have been reported in the last couple of decades. In addition, nitrates and faecal coliform contamination linked to septic tanks, open defecation, and fertiliser application have been reported. A high degree of urbanization with 500 growing townships and 8-10 large cities has further threatened the sustenance of these vital resources, causing a severe water crisis in the Himalayas. Spring rejuvenation can enhance water access and livelihoods and help achieve several sustainable development goals (SDGs). However, multiple challenges hinder the success of such initiatives. A fundamental limitation is the poor understanding of complex groundwater (spring) systems and their interactions with human societies. This review identified crucial knowledge gaps by synthesizing available knowledge on springs and revival efforts from peer-reviewed journals and reports by practitioners and governing bodies. The review also highlights the limitations of spring revival approaches and recommends future management options. There is a critical lack of comprehensive data as a large research on the Himalayan spring systems results from small-scale spring centric studies focussing primarily on hydrology. In contrast, the impacts of hydrogeology, ecology, socio-economics and developmental activities on springs are less explored. Lack of scientific inputs on the hydrogeological regime and limited support by the state is a barrier to scaling spring rejuvenation programs. Long term monitoring, location-specific mapping of local hydrogeological and socio-economic settings at aquifer scale and collaborations among different stakeholders are essential to facilitate holistic knowledge development on spring systems and successful spring revival. The authors recommend ensuring sustenance by recognizing the value of springs in the mainstream programs and policies and develop appropriate management framework for the management of spring systems.

Favorable and unfavorable attachment of colloids in a discrete sandstone fracture

The transport of cationic amine-modified latex (AML) and anionic carboxylate-modified latex (CML) microspheres through a discrete sandstone fracture with mineralogical heterogeneity and roughness was studied. Two microsphere sizes (200 nm and 1000 nm), two ionic strengths (5 mM and 10 mM), and two specific discharges (0.35 mm.s-1 and 0.70 mm.s-1) were tested to observe the impact on transport under favorable and unfavorable conditions. The difference in retention between AML (net favorable) and CML (net unfavorable) microsphere attachment was 25% for the 200 nm microspheres and 13% for the 1000 nm microspheres. Less than 50% of the AML microspheres were retained in the fracture, postulated to be due to the effects of mineralogical heterogeneity and fracture surface roughness. The effect of an increase in ionic strength in increasing retention was significant for unfavorable attachment, but was not significant for favorable attachment conditions. The effect of specific discharge was minor for all but the 200 nm CML microspheres at 10 mM ionic strength. When flushing the fracture first with cationic microspheres, then with anionic microspheres, the recovery of anionic microspheres resembled favorable attachment presumably due to interaction with cationic microspheres that remained attached to the sandstone surface. Colloid breakthrough curves could be fit well with a two site attachment model, with reversible and irreversible sites.

Use of bacteria community analysis to distinguish groundwater recharge sources to shallow wells

In this study, bacteria community analysis was performed to supplement a preexisting evaluation of nitrate contamination in drinking water wells at a coastal site in Old Lyme, CT. Given well usage and coastal hydrogeologic conditions, the source(s) of nitrate contamination in domestic wells could not be discerned between local septic systems or a nearby farm where organic fertilizers were used. Groundwater bacteria communities are known to be sensitive to a variety of environmental conditions. As such, they are potentially useful in distinguishing groundwater recharge sources. Groundwater samples collected from wells were analyzed using polymerase chain reactions (PCR) and 16S rRNA sequencing to determine the bacteria distributions in each well. The biostatistical analysis of the data using Bray-Curtis nonmetric multidimensional scaling and permutational multivariate analysis of variance revealed three distinct bacteria community distributions that coincided with three different areas on the site. Additionally, principal component analysis (PCA) of the water quality data revealed that wells with similar bacteria shared similar water quality, all of which was indicative of local recharge. These findings suggested that the domestic well nitrate contamination was derived from local septic systems rather than the farm. Septic indicator analysis using ultra-performance liquid chromatography-tandem mass spectrometry determined the presence of caffeine in domestic wells, which was consistent with the conclusions from the bacteria analysis, PCA, and the known hydrogeologic conditions. The low cost, ease of sample collection, and growing availability of bioinformatics laboratory services and software are conducive to the application of microbial community analysis as a supplemental tool for groundwater investigations.

New Methods for Microbiological Monitoring at Riverbank Filtration Sites

Water suppliers aim to achieve microbiological stability throughout their supply system by regular monitoring of water quality. Monitoring temporal biomass dynamics at high frequency is time consuming due to the labor-intensive nature and limitations of conventional, cultivation-based detection methods. The goal of this study was to assess the value of new rapid monitoring methods for quantifying and characterizing dynamic fluctuations in bacterial biomass. Using flow cytometry and two precise enzymatic detection methods, bacterial biomass-related parameters were monitored at three riverbank filtration sites. Additionally, the treatment capacity of an ultrafiltration pilot plant was researched using online flow-cytometry. The results provide insights into microbiological quality of treated water and emphasize the value of rapid, easy and sensitive alternatives to traditional bacterial monitoring techniques.

Differential Transport of Escherichia coli Isolates Compared to Abiotic Tracers in a Karst Aquifer

Lack of filtration and rapid transport of groundwater and particulate matter make karst aquifers susceptible to bacterial contamination. This study utilized quantitative polymerase chain reaction (qPCR) to examine the transport and attenuation of two nonvirulent isolates of Escherichia coli (E. coli) in relation to traditional groundwater tracers (rhodamine WT dye and 1-µm diameter latex microspheres) in a karst-conduit aquifer in central Kentucky. Bacterial isolates were labeled with stable isotopes (¹⁵ N and ¹³ C). All tracers were detected more than 6 km downstream from the injection site and demonstrated overlapping breakthrough curves, with differential transport observed between the two bacterial strains. The E. coli isolate containing the kps gene (low attachment) arrived at sampling sites 1.25 to 36 h prior to the bacterial isolate containing the iha gene (high attachment) and was detected in samples collected following storm events in which the iha isolate was not detected. The storage potential of contaminants within karst systems was demonstrated by the remobilization of all tracers during storm events more than 1 month after injection. Bacteria-sized microspheres were more easily remobilized during periods of increased discharge compared to other tracers. The study demonstrated that molecular biology techniques such as qPCR can be utilized as a sensitive analysis of bacterial tracers in karst aquifers and may prove to be a more sensitive analytical technique than stable isotope analysis for field-scale traces.

Use of Molecular Markers to Compare Escherichia coli Transport with Traditional Groundwater Tracers in Epikarst

Bacterial contamination of karst aquifers is a global concern as water quality deteriorates in the face of decreasing water security. Traditional abiotic groundwater tracers, which do not exhibit surface properties similar to bacteria, may not be good proxies for risk assessment of bacterial transport in karst environments. This study examined the transport and attenuation of two isolates of in relation to traditional groundwater tracers (rhodamine WT dye and 1-μm-diam. latex microspheres) through ∼30 m of epikarst in western Kentucky. Differential movement of the four tracers was observed, with tracer behavior dependent on flow conditions. Dye arrived at the sampling site prior to particulates. Molecular biology techniques successfully detected bacteria in the cave and showed attenuation was greater for a bacterial isolate with high attachment efficiency compared with an isolate known to have low attachment efficiency. Microspheres were first detected simultaneously with the low-attachment isolate but attained maximum concentrations during increases in discharge >11 d post-injection. Bacteria were remobilized by storm events >60 d after injection, illustrating the storage capacity of epikarst with regard to potential contaminants. The two bacterial strains were not transported at the same rate within the epikarst, showing breakthroughs during differing storm events and illustrating the importance of cell surface chemistry in the prediction of microorganism movement. Moreover, this study has shown that molecular analysis can be successfully used to target, quantify, and track introduced microbial tracers in karst terrains.

Colloid retention mechanisms in single, saturated, variable-aperture fractures

The characterization of fractured aquifers is commonly limited to the methodologies developed for unconsolidated porous media aquifers, which results in many uncertainties. Recent work indicates that fractured rocks remove more particulates than they are conventionally credited for. This research was designed to quantify the number of Escherichia coli RS2-GFP retained in single, saturated, variable-aperture fractures extracted from the natural environment. Conservative solute and E. coli RS2-GFP tracer experiments were used to elucidate the relationships between dominant retention mechanisms, aperture field characteristics, and flow rate. A non-destructive method of determining a surrogate measure of a coefficient of variation (COV(S)) for each fracture was used to better understand the transport behaviour of E. coli RS2-GFP. The results from this research all point to the importance of aperture field characterization in understanding the fate and transport of contaminants in fractured aquifers. The mean aperture was a very important characteristic in determining particulate recovery, so were matrix properties, COV(s), and flow rate. It was also determined that attachment is a much more significant retention mechanism than straining under the conditions employed in this research. Finally, it was demonstrated that the dominant retention mechanism in a fracture varies depending on the specific discharge. An improved understanding of the mechanisms that influence the fate and transport of contaminants through fractures will lead to the development of better tools and methodologies for the characterization of fractured aquifers, as well as the ability to manipulate the relevant mechanisms to increase or decrease retention, depending on the application.

Thermodynamic and kinetic controls on cotransport of Pantoea agglomerans cells and Zn through clean and iron oxide coated sand columns

Recent observations that subsurface bacteria quickly adsorb metal contaminants raise concerns that they may enhance metal transport, given the high mobility of bacteria themselves. However, metal adsorption to bacteria is also reversible, suggesting that mobility within porous medium will depend on the interplay between adsorption-desorption kinetics and thermodynamic driving forces for adsorption. Till now there has been no systematic investigation of these important interactions. This study investigates the thermodynamic and kinetic controls of cotransport of Pantoea agglomerans cells and Zn in quartz and iron-oxide coated sand (IOCS) packed columns. Batch kinetic studies show that significant Zn sorption on IOCS takes place within two hours. Adsorption onto P. agglomerans surfaces reaches equilibrium within 30 min. Experiments in flow through quartz sand systems demonstrate that bacteria have negligible effect on zinc mobility, regardless of ionic strength and pH conditions. Zinc transport exhibits significant retardation in IOCS columns at high pH in the absence of cells. Yet, when mobile bacteria (non attached) are passed through simultaneously with zinc, no facilitated transport is observed. Adsorption onto cells becomes significant and plays a role in mobile metal speciation only once the IOCS is saturated with zinc. This suggests that IOCS exhibits stronger affinity for Zn than cell surfaces. However, when bacteria and Zn are preassociated on entering the column, zinc transport is initially facilitated. Subsequently, zinc partly desorbs from the cells and redistributes onto the IOCS as a result of the higher thermodynamic affinity for IOCS.

Colloid transport in dolomite rock fractures: effects of fracture characteristics, specific discharge, and ionic strength

The effects of fracture characteristics, specific discharge, and ionic strength on microsphere transport in variable-aperture dolomite rock fractures were studied in a laboratory-scale system. Fractures with different aperture distributions and mineral compositions were artificially created in two dolomite rock blocks. Transport tests were conducted with bromide and carboxylate-modified latex microspheres (20, 200, and 500 nm diameter). Under overall unfavorable attachment conditions, there was significant retention of the 20 nm microsphere and minimal retention of the 500 nm microsphere for all conditions examined. Aperture variability produced significant spatial variation in colloid transport. Flushing with low ionic strength solution (1 mM) following microsphere transport at 12 mM ionic strength solution produced a spike in effluent microsphere concentrations, consistent with retention of colloids in secondary energy minima. Surface roughness and charge heterogeneity effects may have also contributed to the effect of microsphere size on retention. Matrix diffusion influenced bromide transport but was not a dominant factor in transport for any microsphere size. Calibrated one-dimensional, two-site kinetic model parameters for colloid transport in fractured dolomite were sensitive to the physical and chemical properties of both the fractured dolomite and the colloids, indicating the need for mechanistic modeling for accurate prediction.

Effects on groundwater microbial communities of an engineered 30-day in situ exposure to the antibiotic sulfamethoxazole

Effects upon microbial communities from environmental exposure to concentrations of antibiotics in the μg L(-1) range remain poorly understood. Microbial communities from an oligotrophic aquifer (estimated doubling rates of only once per week) that were previously acclimated (AC) or unacclimated (UAC) to historical sulfamethoxazole (SMX) contamination, and a laboratory-grown Pseudomonas stutzeri strain, were exposed to 240-520 μg L(-1) SMX for 30 days in situ using filter chambers allowing exposure to ambient groundwater, but not to ambient microorganisms. SMX-exposed UAC bacterial communities displayed the greatest mortality and impairment (viable stain assays), the greatest change in sensitivity to SMX (dose-response assays), and the greatest change in community composition (Terminal Restriction Fragment Length Polymorphism; T-RFLP). The sul1 gene, encoding resistance to SMX at clinically relevant levels, and an element of Class I integrons, was not detected in any community. Changes in microbial community structure and SMX resistance over a short experimental period in previously nonexposed, slow-growing aquifer communities suggest concentrations of antibiotics 2-3 orders of magnitude less than those used in clinical applications may influence ecological function through changes in community composition, and could promote antibiotic resistance through selection of naturally resistant bacteria.

Chemical effectors cause different motile behavior and deposition of bacteria in porous media

We tested the hypothesis whether chemically induced motility patterns of bacteria may affect their transport in porous media. Naphthalene-degrading Pseudomonas putida G7 cells were exposed to glucose, salicylate, and silver nanoparticles (AgNPs) and their motility was assessed by computer-assisted, quantitative swimming and capillary-based taxis determinations. Exposure to salicylate induced smooth movement with few acceleration events and positive taxis, whereas cells exposed to AgNPs exhibited tortuous movement and a repellent response. Although metabolized by strain G7, glucose did not cause attraction and induced a hyper-motile mode of swimming, characterized by a high frequency of acceleration events, high swimming speed (>60 μm s(-1)), and a high tortuosity in the trajectories. Chemically induced motility behavior correlated with distinct modes of attachment to sand in batch assays and breakthrough curves in percolation column experiments. Salicylate significantly reduced deposition of G7 cells in column experiments whereas glucose and AgNPs enhanced attachment and caused filter blocking that resulted in a progressive decrease in deposition. These findings are relevant for bioremediation scenarios that require an optimized outreach of introduced inoculants and in other environmental technologies, such as water disinfection and microbially enhanced oil recovery.

Impact of fluorochrome stains used to study bacterial transport in shallow aquifers on motility and chemotaxis of Pseudomonas species

One of the most common methods of tracking movement of bacteria in groundwater environments involves a priori fluorescent staining. A major concern in using these stains to label bacteria in subsurface injection-and-recovery studies is the effect they may have on the bacterium's transport properties. Previous studies investigated the impact of fluorophores on bacterial surface properties (e.g. zeta potential). However, no previous study has looked at the impact of fluorescent staining on swimming speed and chemotaxis. It was found that DAPI lowered the mean population swimming speed of Pseudomonas putida F1 by 46% and Pseudomonas stutzeri by 55%. DAPI also inhibited the chemotaxis in both strains. The swimming speeds of P. putida F1 and P. stutzeri were diminished slightly by CFDA/SE, but not to a statistically significant extent. CFDA/SE had no effect on chemotaxis of either strain to acetate. SYBR(®) Gold had no effect on swimming speed or the chemotactic response to acetate for either strain. This research indicates that although DAPI may not affect sorption to grain surfaces, it adversely affects other potentially important transport properties such as swimming and chemotaxis. Consequently, bacterial transport studies conducted using DAPI are biased to nonchemotactic conditions and do not appear to be suitable for monitoring the effect of chemotaxis on bacterial transport in shallow aquifers.

Motility is critical for effective distribution and accumulation of bacteria in tumor tissue

Motile bacteria can overcome the penetration limitations of cancer chemotherapeutics because they can actively migrate into solid tumors. Although several genera of bacteria have been shown to accumulate preferentially in tumors, the spatiotemporal dynamics of bacterial tumor colonization and their dependence on bacterial motility are not clear. For effective tumor regression, bacteria must penetrate and distribute uniformly throughout tumors. To measure these dynamics, we used an in vitro model of continuously perfused tumor tissue to mimic the delivery and systemic clearance of Salmonella typhimurium strains SL1344 and VNP20009, and Escherichia coli strains K12 and DH5α. Tissues were treated for 1 hour with 10(5) or 10(7) CFU ml(-1) suspensions of each strain and the location and extent of bacterial accumulation were observed for 30 hours. Salmonella had 14.5 times greater average swimming speed than E. coli and colonized tissues at 100 times lower doses than E. coli. Bacterial motility strongly correlated (R(2) = 99.3%) with the extent of tissue accumulation. When inoculated at 10(5) CFU ml(-1), motile Salmonella formed colonies denser than 10(10) CFU/(g-tissue) and less motile E. coli showed no detectable colonization. Based on spatiotemporal profiles and a mathematical model of motility and growth, bacterial dispersion was found to be necessary for deep penetration into tissue. Bacterial colonization caused apoptosis in tumors and apoptosis levels correlated (R(2) = 98.6%) with colonization density. These results show that motility is critical for effective distribution of bacteria in tumors and is essential for designing cancer therapies that can overcome the barrier of limited tumor penetration.

Idling time of motile bacteria contributes to retardation and dispersion in sand porous medium

The motility of microorganisms affects their transport in natural systems by altering their interactions with the solid phase of the soil matrix. To assess the effect of these interactions on transport parameters, a series of breakthrough curves (BTCs) for motile and nonmotile bacteria, including E. coli and P. putida species, were measured from a homogeneously packed sand column under three different interstitial velocities of 1 m/d, 5 m/d, and 10 m/d. BTCs for the nonmotile bacteria were nearly identical for all three flow rates, except that the recovery percentage at 1 m/d was reduced by 5% compared to the higher flow rates. In contrast, for the motile bacteria, the recovery percentages were not affected by flow rate, but their BTCs exhibited a higher degree of retardation and dispersion as the flow velocity decreased, which was consistent with increased idling times of the motile strains. The smooth-swimming mutant E. coli HCB437, which is unable to change its swimming direction after encountering the solid surfaces and thus has the largest idling time, also exhibited the greatest degree of retardation and dispersion. All of the experimental observations were compared to results from an advection-dispersion transport model with three fitting parameters: retardation factor (R), longitudinal dispersivity (α(L)), and attachment rate coefficient (k(att)). In addition, the single-collector efficiency (η₀) and collision efficiency (α) were calculated according to the colloid filtration theory (CFT), and confirmed that motile bacteria had lower collision efficiencies than nonmotile bacteria. This is consistent with previously reported observations that motile bacteria can avoid attachment to a solid surface by their active swimming capabilities. By quantifying the effect of bacterial motility on various transport parameters, more robust fate and transport models can be developed for decision-making related to environmental remediation strategies and risk assessment.

Differential effects of dissolved organic carbon upon re-entrainment and surface properties of groundwater bacteria and bacteria-sized microspheres during transport through a contaminated, sandy aquifer

Injection-and-recovery studies involving a contaminated, sandy aquifer (Cape Cod, Massachusetts) were conducted to assess the relative susceptibility for in situ re-entrainment of attached groundwater bacteria (Pseudomonas stuzeri ML2, and uncultured, native bacteria) and carboxylate-modified microspheres (0.2 and 1.0 μm diameters). Different patterns of re-entrainment were evident for the two colloids in response to subsequent injections of groundwater (hydrodynamic perturbation), deionized water (ionic strength alteration), 77 μM linear alkylbenzene sulfonates (LAS, anionic surfactant), and 76 μM Tween 80 (polyoxyethylene sorbitan monooleate, a very hydrophobic nonionic surfactant). An injection of deionized water was more effective in causing detachment of micrsopheres than were either of the surfactants, consistent with the more electrostatic nature of microsphere's attachment, their extreme hydrophilicity (hydrophilicity index, HI, of 0.99), and negative charge (zeta potentials, ζ, of -44 to -49 mv). In contrast, Tween 80 was considerably more effective in re-entraining the more-hydrophobic native bacteria. Both the hydrophilicities and zeta potentials of the native bacteria were highly sensitive to and linearly correlated with levels of groundwater dissolved organic carbon (DOC), which varied modestly from 0.6 to 1.3 mg L(-1). The most hydrophilic (0.52 HI) and negatively charged (ζ -38.1 mv) indigenous bacteria were associated with the lowest DOC. FTIR spectra indicated the latter community had the highest average density of surface carboxyl groups. In contrast, differences in groundwater (DOC) had no measurable effect on hydrophilicity of the bacteria-sized microspheres and only a minor effect on their ζ. These findings suggest that microspheres may not be very good surrogates for bacteria in field-scale transport studies and that adaptive (biological) changes in bacterial surface characteristics may need to be considered where there is longer-term exposure to contaminant DOC.

Contribution of extracellular polymeric substances on representative gram negative and gram positive bacterial deposition in porous media

The significance of extracellular polymeric substances (EPS) on cell transport and retained bacteria profiles in packed porous media (quartz sand) was examined by direct comparison of the overall deposition kinetics and retained profiles of untreated bacteria (with EPS) versus those of treated cells (without EPS) from the same cell type. Four representative cell types, Pseudomonas sp. QG6 (gram-negative, motile), mutant Escherichia coli BL21 (gram-negative, nonmotile), Bacillus subtilis (gram-positive, motile), and Rhodococcus sp. QL2 (gram-positive, nonmotile), were employed to systematically determine the influence of EPS on cell transport and deposition behavior. Packed column experiments were conducted for the untreated and treated cells in both NaCl (four ionic strength ranging from 2.5 mM to 20 mM) and CaCl(2) (5 mM) solutions at pH 6.0. The breakthrough plateaus of untreated bacteria were lower than those of treated bacteria for all four cell types under all examined conditions (in both NaCl and CaCl(2) solutions), indicating that the presence of EPS on cell surfaces enhanced cell deposition in porous media regardless of cell type and motility. Retained profiles of both untreated and treated cells for all four cell types deviated from classic filtration theory (log-linear decreases). However, the degree of deviation was greater for all four untreated cells, indicating that the presence of EPS on cell surfaces increased the deviation of retained profiles from classic filtration theory. Elution experiments demonstrated that neither untreated nor treated cells preferentially deposited in secondary energy minima. Furthermore, the release of previously deposited cells in the secondary energy minima did not change the shape of retained cell profiles, indicating that deposition in secondary energy minima did not produce the observed deviations of retained profiles from classic filtration theory.

The utility of microspheres as surrogates for the transport of E. coli RS2g in partially saturated agricultural soil

Polystyrene latex microspheres are widely used as surrogates for biocolloid transport in porous media; however, relatively few studies directly compare microsphere transport with that of the microorganism it is intended to represent, particularly at the field scale. Here, we compared the transport behaviour of a bacterium (Escherichia coli RS2g; 1.2 microm in diameter) and three different sized microspheres (1.1, 3.9, and 4.8 microm in diameter) within undisturbed agricultural field soil following infiltration under partially saturated conditions. The soil contained significant macroporosity. A tension infiltrometer was used to control the application of a transport solution containing Brilliant Blue FCF dye to two plots. A >2 log reduction in the concentration of all colloids was observed from the soil surface to 5 cm depth in both plots. The concentration of colloids in the soil was generally proportional to the intensity of soil dye staining; however, both the E. coli RS2g bacterium and the 1.1 microm microspheres appeared to be transported deeper than the other colloids and the visible dye along root holes at the bottom of the profile in both plots. The similarities in size and zeta potential of the 1.1 microm microspheres and the E. coli RS2g likely contributed to that outcome. Colloid concentrations in dyed soil by depth were similar between the two plots, despite differences in soil properties and infiltration patterns. The properties of the colloids and macropore density were the most important factors affecting colloid transport. These results suggest that microspheres with size and surface properties similar to the microbe of interest are useful surrogates to trace potential pathways of transport in the subsurface.

Protistan predation affects trichloroethene biodegradation in a bedrock aquifer

Despite extensive research on the bottom-up force of resource availability (e.g., electron donors and acceptors), slow biodegradation rates and stalling at cis-dichloroethene (cDCE) and vinyl chloride continue to be observed in aquifers contaminated with trichloroethene (TCE). The objective of this research was to gauge the impact of the top-down force of protistan predation on TCE biodegradation in laboratory microcosms. When indigenous bacteria from an electron donor-limited TCE-contaminated bedrock aquifer were present, the indigenous protists inhibited reductive dechlorination altogether. The presence of protists during organic carbon-amended conditions caused the bacteria to elongate (length:width, > or =10:1), but reductive dechlorination was still inhibited. When a commercially available dechlorinating bacterial culture and an organic carbon amendment were added in he presence of protists, the elongated bacteria predominated and reductive dechlorination stalled at cDCE. When protists were removed under organic carbon-amended conditions, reductive dechlorination stalled at cDCE, whereas in the presence organic carbon and bacterial amendments, the total chlorinated ethene concentration decreased, indicating TCE was converted to ethene and/or CO2. The data suggested that indigenous protists grazed dechlorinators to extremely low levels, inhibiting dechlorination altogether. Hence, in situ bioremediation/bioaugmentation may not be successful in mineralizing TCE unless the top-down force of protistan predation is inhibited.

The implications of groundwater velocity variations on microbial transport and wellhead protection - review of field evidence

Current strategies to protect groundwater sources from microbial contamination (e.g., wellhead protection areas) rely upon natural attenuation of microorganisms between wells or springs and potential sources of contamination and are determined using average (macroscopic) groundwater flow velocities defined by Darcy's Law. However, field studies of sewage contamination and microbial transport using deliberately applied tracers provide evidence of groundwater flow paths that permit the transport of microorganisms by rapid, statistically extreme velocities. These paths can be detected because of (i) the high concentrations of bacteria and viruses that enter near-surface environments in sewage or are deliberately applied as tracers (e.g., bacteriophage); and (ii) low detection limits of these microorganisms in water. Such paths must comprise linked microscopic pathways (sub-paths) that are biased toward high groundwater velocities. In media where microorganisms may be excluded from the matrix (pores and fissures), the disparity between the average linear velocity of groundwater flow and flow velocities transporting released or applied microorganisms is intensified. It is critical to recognise the limited protection afforded by source protection measures that disregard rapid, statistically extreme groundwater velocities transporting pathogenic microorganisms, particularly in areas dependent upon untreated groundwater supplies.

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.

Surface association of motile bacteria at granular porous media interfaces

Bacterial populations were observed using dark-field light scattering at porous media interfaces comprised of a dilute solution containing the polymer additives methylcellulose and a transparent particulate suspension composed of mechanically agitated Gelrite gellan gum. Population-scale experiments with a nonchemotactic smooth-swimming mutant, Escherichia col HCB 437, yielded a variety of distinct and reproducible bacterial distributions that included highly concentrated bands of bacteria near the interface. While no physical attachment was observed between the bacteria and granular Gelrite media, the population exhibited surface associations characterized by reversible physical obstructions of the motile bacteria at the solid granular surfaces. These interactions decreased translational motion, which reduced bacterial migration and concentrated bacterial populations near the interface. Results from glass bead experiments indicated similar surface associations in high-surface area glass bead environments. Experimental results were semiquantitatively analyzed using a one-dimensional population-scale transport model. Theoretical profiles were generated using a single set of parameters and simultaneously compared with averaged bacterial distributions from multiple interface configurations. Parameter estimates were consistent with expected values. The agreement between the theoretical and experimental data suggests a quantifiable approach for modeling bacterial migration within high-surface area granular media environments.

Influence of surface porosity and pH on bacterial adherence to hydroxyapatite and biphasic calcium phosphate bioceramics

Hydroxyapatite (HA) and biphasic calcium phosphate (BCP) ceramic materials are widely employed as bone substitutes due to their porous and osteoconductive structure. Their porosity and the lowering of surrounding pH as a result of surgical trauma may, however, predispose these materials to bacterial infections. For this reason, the influence of porosity and pH on the adherence of common Gram-positive bacteria to the surfaces of these materials requires investigation. Mercury intrusion porosimetry measurements revealed that the pore size distribution of both bioceramics had, on a logarithmic scale, a sinusoidal frequency distribution ranging from 50 to 300 nm, with a mean pore diameter of 200 nm. Moreover, total porosity was 20 % for HA and 50 % for BCP. Adherence of Staphylococcus aureus and Staphylococcus epidermidis was studied at a physiological pH of 7.4 and at a pH simulating bone infection of 6.8. Moreover, the effect of pH on the zeta potential of HA, BCP and of both staphylococci was evaluated. Results showed that when pH decreased from 7.4 to 6.8, the adherence of both staphylococci to HA and BCP surfaces decreased significantly, although at the same time the negative zeta-potential values of the ceramic surfaces and both bacteria diminished. At both pH values, the number of S. aureus adhered to the HA surface appeared to be lower than that for BCP. A decrease in pH to 6.8 reduced the adherence of both bacterial species (mean 57 %). This study provides evidence that HA and BCP ceramics do not have pores sufficiently large to allow the internalization of staphylococci. Their anti-adherent properties seemed to improve when pH value decreased, suggesting that HA and BCP bioceramics are not compromised upon orthopaedic use.

Statistical assessment of the accuracy and precision of bacteria- and virus-sized microsphere enumerations by epifluorescence microscopy

Fluorescent microspheres are increasingly used in environmental studies to evaluate threats of viral and bacterial pathogens in drinking water and to investigate colloid-facilitated contaminant transport. A commonly accepted technique for the enumeration of viruses, bacteria, and virus- and bacteria-sized particles by microscopy involves a field-of-view (field) approach to estimate concentration. Few studies have focused on those factors that are most important in ensuring precise and accurate measures of concentration. Microsphere counts in suspensions of artificial groundwater and deionized water were contrasted in this study to gain a greater understanding of the effect of ionic strength and the presence of precipitates in groundwater matrices that can bias microsphere enumerations. To investigate microsphere enumeration with minimal bias from other factors, a commonly used standard method was used to prepare slides and enumerate microspheres, with particular care to randomly select fields for counting. A factorial experiment evaluated two factors, (1) the density of microspheres in each field and (2) the number of counts in an enumeration. Two parameters, relative standard deviation and percent error, were used to assess methodological precision and accuracy. Visual observations of the slides indicated that some biases, such as undulation in the filter membrane or bubble entrained in the mounting medium, create biases in microsphere enumeration. Additional biases were introduced by the presence of precipitates that form in artificial groundwater saturated with calcite. Microsphere density was found to be critical for ensuring methodological precision, whereas the total number of microspheres counted was essential to ensuring methodological accuracy. The results suggested that to minimize variability using the field approach, the enumeration of at least 350 microspheres and 25-40 microspheresfield (-1) is necessary.

The role of starvation on Escherichia coli adhesion and transport in saturated porous media

The influence of bacterial starvation on cell transport has been examined using two Escherichia coli isolates: one from human (HU) and one from dairy cattle (DC). To better understand the fate of starved bacteria, experiments were conducted in a packed bed column using cells that had been incubating at room temperature without nutrients for 6, 12, and 18h, as well as cells, which had not been starved (referred to as time zero). Complimentary cell characterization techniques were conducted to evaluate the hydrophobicity, mobility, size, and surface charge density of the cells at the conditions considered. It was observed that non-starved HU cells were more adhesive than starved HU cells. This behavior is attributed to the relatively high hydrophobicity of the starved cells, which resulted from greater extracellular polymeric substance (EPS) presence. Non-starved DC cells were also the most adhesive whereas 18h starved cells were the least adhesive, although cell characterization results did not correlate to transport trends like HU cells. For both isolates, the cells after 6h of starvation showed high levels of sugar relative to protein in the EPS. Additionally, following 6h of starvation, the cells did not follow expected transport trends as anticipated from the cellular characterization. Our results suggest transport behavior of environmental E. coli isolates differs in terms of isolate host and starvation conditions. Possible mechanisms responsible for this are changes in key cell surface characteristics and synthesis of starvation-induced proteins. This study highlights the importance of consistency in bacterial preparation for experimental studies and has considerable implications for future evaluation and prediction of E. coli fate in subsurface environments.

A wall of funnels concentrates swimming bacteria

Randomly moving but self-propelled agents, such as Escherichia coli bacteria, are expected to fill a volume homogeneously. However, we show that when a population of bacteria is exposed to a microfabricated wall of funnel-shaped openings, the random motion of bacteria through the openings is rectified by tracking (trapping) of the swimming bacteria along the funnel wall. This leads to a buildup of the concentration of swimming cells on the narrow opening side of the funnel wall but no concentration of nonswimming cells. Similarly, we show that a series of such funnel walls functions as a multistage pump and can increase the concentration of motile bacteria exponentially with the number of walls. The funnel wall can be arranged along arbitrary shapes and cause the bacteria to form well-defined patterns. The funnel effect may also have implications on the transport and distribution of motile microorganisms in irregular confined environments, such as porous media, wet soil, or biological tissue, or act as a selection pressure in evolution experiments.

Comparison of release and transport of manure-borne Escherichia coli and enterococci under grass buffer conditions

AIM

To test the hypothesis that Escherichia coli and enterococci bacteria have similar release rates and transport characteristics after being released from land-applied manure.

METHODS AND RESULTS

Turfgrass soil sod was placed into 200 cm long boxes that had the top two 25 cm sections separated to monitor the release and infiltration of bacteria, which affected bacteria transport in the rest of the box. Dairy manure with added KBr was broadcast on the top two sections. Boxes with either live or dead grass stand were placed under a rainfall simulator for 90 min. Runoff and infiltration samples were collected and analysed for Br, E. coli, enterococci and turbidity. Significant differences in release kinetics of E. coli and enterococci were found. A change from first-order release kinetics to zero-order kinetics after 1 h of rainfall simulation was observed.

CONCLUSION

Differences in release rates but not in the subsequent transport were observed for E. coli and enterococci.

SIGNIFICANCE AND IMPACT OF THE STUDY

Because both E. coli and enterococci are currently used as indicator organisms for manure-borne pathogens, the differences in their release rates may affect the efficiency of using these organisms as indicators.

The selective value of bacterial shape

Why do bacteria have shape? Is morphology valuable or just a trivial secondary characteristic? Why should bacteria have one shape instead of another? Three broad considerations suggest that bacterial shapes are not accidental but are biologically important: cells adopt uniform morphologies from among a wide variety of possibilities, some cells modify their shape as conditions demand, and morphology can be tracked through evolutionary lineages. All of these imply that shape is a selectable feature that aids survival. The aim of this review is to spell out the physical, environmental, and biological forces that favor different bacterial morphologies and which, therefore, contribute to natural selection. Specifically, cell shape is driven by eight general considerations: nutrient access, cell division and segregation, attachment to surfaces, passive dispersal, active motility, polar differentiation, the need to escape predators, and the advantages of cellular differentiation. Bacteria respond to these forces by performing a type of calculus, integrating over a number of environmental and behavioral factors to produce a size and shape that are optimal for the circumstances in which they live. Just as we are beginning to answer how bacteria create their shapes, it seems reasonable and essential that we expand our efforts to understand why they do so.

Analysis of column tortuosity for MnCl2 and bacterial diffusion using magnetic resonance imaging

Subsurface bacteria often have to travel significant distances through tortuous porous media for purposes of groundwater remediation. In modeling such processes, motile bacteria are often represented as suspended colloids, ignoring their individual swimming or diffusive properties. In fact, bacterial migration is much more profoundly affected by the presence of porous media than is that of a chemical contaminant. In this study, we use magnetic resonance imaging (MRI) to perform noninvasive measurements of changes in bacterial concentration distributions across a packed column at a spatial resolution of 330 microm as a function of time. We analyze the diffusive behavior of Pseudomonas putida F1 under static conditions and compare that behavior to the diffusion of a chemical solute and of Escherichia coli NR50. Results indicate that P. putida cells experience a column tortuosity 50 times higher than that predicted from solute diffusion experiments. E. coli cells, which display shorter swimming run lengths in bulk solution than P. putida, seem to be less affected by the constricted pore space. Knudsen diffusion, or reductions in run length because of interactions between the diffusing bacteria and the porous media, may help to explain some of this discrepancy.

Influence of precipitation and soil on transport of fecal enterococci in fractured limestone aquifers

Limestone aquifers provide the main drinking water resources of southern Italy. The groundwater is often contaminated by fecal bacteria because of the interaction between rocks having high permeability and microbial pollutants introduced into the environment by grazing and/or manure spreading. The microbial contamination of springwater in picnic areas located in high mountains can cause gastrointestinal illness. This study was carried out in order to analyze the interaction between Enterococcus faecalis and the soil of a limestone aquifer and to verify the influence of this interaction on the time dependence of groundwater contamination. E. faecalis was chosen because, in the study area involved, it represents a better indicator than Escherichia coli. The research was carried out through field (springwater monitoring) and laboratory experiments (column tests with intact soil blocks). The transport of bacterial cells through soil samples was analyzed by simulating an infiltration event that was monitored in the study area. Comparison of laboratory results with data acquired in the field showed that discontinuous precipitation caused an intermittent migration of microorganisms through the soil and produced, together with dispersion in the fractured medium (unsaturated and saturated zones), an articulated breakthrough at the spring. The short distances of bacterial transport in the study area produced a significant daily variability of bacterial contamination at the field scale.

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.

Effect of cell physicochemical characteristics and motility on bacterial transport in groundwater

The influence of physicochemical characteristics and motility on bacterial transport in groundwater were examined in flow-through columns. Four strains of bacteria isolated from a crystalline rock groundwater system were investigated, with carboxylate-modified and amidine-modified latex microspheres and bromide as reference tracers. The bacterial isolates included a gram-positive rod (ML1), a gram-negative motile rod (ML2), a nonmotile mutant of ML2 (ML2m), and a gram-positive coccoid (ML3). Experiments were repeated at two flow velocities, in a glass column packed with glass beads, and in another packed with iron-oxyhydroxide coated glass beads. Bacteria breakthrough curves were interpreted using a transport equation that incorporates a sorption model from microscopic observation of bacterial deposition in flow-cell experiments. The model predicts that bacterial desorption rate will decrease exponentially with the amount of time the cell is attached to the solid surface. Desorption kinetics appeared to influence transport at the lower flow rate, but were not discernable at the higher flow rate. Iron-oxyhydroxide coatings had a lower-than-expected effect on bacterial breakthrough and no effect on the microsphere recovery in the column experiments. Cell wall type and shape also had minor effects on breakthrough. Motility tended to increase the adsorption rate, and decrease the desorption rate. The transport model predicts that at field scale, desorption rate kinetics may be important to the prediction of bacteria transport rates.