Bacteria cell properties and grain size impact on bacteria transport and deposition in porous media

Modeling bacterial transport and fate: Insight into the cascading consequences of soil water repellency and contrasting hydraulic conditions

The mechanisms governing bacteria transport and fate rely on their hydrophobicity and the wettability of porous media across a wide range of soil moisture conditions, extending from extreme dryness to highly saturated states. However, it largely remains unknown how transport, retention, and release mechanisms change in natural soil systems in such conditions. We thus optimized our previously published unique transport data for hydrophilic Escherichia coli (E. coli) and hydrophobic Rhodococcus erythropolis (R. erythropolis) bacteria, and bromide (Br-) in two distinct wettable and water-repellent soils at column scale. The soils were initially dry, followed by injecting influents in two pulses followed by a flushing step under saturated flow conditions for six pore volumes. We conducted simulations for each pulse separately and simultaneously for soils. There were differences in hydraulic properties of the soils due to their contrasting wetting characteristic in separate and simultaneously modeling of each pulse affecting Br- and bacteria transport fate. Bacteria attachment was the dominant retention mechanism in both soils in these conditions. Notably, the 82.4 min-1 attachment rate in wettable soil was almost 10× greater than in the water-repellent soil and it governed optimization of bacteria die-off. Physicochemical detachment and physical release unraveled the effect of bacteria size and hydrophobicity interacting with soil wettability. The smaller and hydrophobic R. erythropolis detached more easily while hydrophilic E. coli released; the rates were enhanced by soil water repellency. Further research is needed to reveal the effects of surface wettability properties on bacteria survival especially at the nanoscale.

Bacterial accumulation dynamics in runoff from extreme precipitation

Extreme precipitation can significantly influence the water quality of surface waters. However, the total amount of bacteria carried by rainfall runoff is poorly understood. Here, thirty rainfall scenarios were simulated by artificial rainfall simulators, with designed rainfall intensity ranging from 19.3 to 250 mm/h. The instantaneous concentration ranges of R2A, nutrient agar (NA) culturable bacteria, and viable bacteria in runoff depended on the types of underlying surfaces. The instantaneous bacterial concentrations in runoff generated by forest lands, grasslands and bare soil were: R2A culturable bacteria = 104.5-6.3, 104.5-6.1, 104.0-5.3 colony-forming units (CFU)/mL, NA culturable bacteria = 104.0-6.0, 103.9-5.8, 103.2-4.9 CFU/mL, and viable bacteria = 106.4-8.0, 107.0-8.9, 106.4-7.6 cells/mL. Based on the measured bacterial instantaneous concentration in runoff, cumulative dynamic models were established, and the maximum amount of culturable bacteria and viable bacteria entering water sources were estimated to be 109.38-11.31 CFU/m2 and 1011.84-13.25 cells/m2, respectively. The model fitting and the bacterial accumulation dynamics were influenced by the rainfall types (p < 0.01). Surface runoff from the underlying surface of forest lands and grasslands had a high microbial risk that persisted even during the "Drought-to-Deluge Transition". Bacterial accumulation models provide valuable insight for predicting microbial risks in catchments during precipitation and can serve as theoretical support for further ensuring the safety of drinking water under the challenge of climate change.

Bacterial surface properties and transport behavior actively respond to an extracellular polymeric substance gradient in saturated porous media

The transport and retention of bacteria in porous media, such as aquifer, are governed by the solid-liquid interface characteristics and bacterial mobility. The secretion of extracellular polymeric substance (EPS) by bacteria modifies their surface property, and thereby has effects on their adhesion to surface. The role of EPS in bacterial mobility within saturated quartz sand media is uncertain, as both promoting and inhibitory effects have been reported, and underlying mechanisms remain unclear. In this study, the effects of EPS on bacterial transport behavior and possible underlying mechanism were investigated at 4 concentrations (0 mg L-1, 50 mg L-1, 200 mg L-1 and 1000 mg L-1) using laboratory simulation experiments in conjunction with Extend Derjaguin-Landau-Verweu-Overbeek (XDLVO) modeling. The results showed that EPS facilitated bacterial mobility at all tested concentrations. It could be partially explained by the increased energy barrier between bacterial cells and quartz sand surface in the presence of EPS. The XDLVO sphere-plate model predicted that EPS induced a higher electrostatic double layer (EDL) repulsive force, Lewis acid-base (AB) and steric stabilization (ST), as well as a lower Lifshitz-van der Waals (LW) attractive force. However, at the highest EPS concentration (1000 mg L-1), the promotion of EPS on bacterial mobility weakened as a result of lower repulsive interactions between cells, which was supported by observed enhanced bacterial aggregation. Consequently, the increased aggregation led to greater bio-colloidal straining and ripening in the sand column, weakening the positive impact of EPS on bacterial transport. These findings suggested that EPS exhibited concentration-dependent effects on bacterial surface properties and transport behavior and revealed non-intuitive dual effects of EPS on those processes.

Enhancing slow sand filtration for safe drinking water production: interdisciplinary insights into Schmutzdecke characteristics and filtration performance in mini-scale filters

The demand for safe drinking water is constantly challenged by increasing biohazards. One widely used solution is implementing indoor-operated slow sand filtration (SSF) as one of the final barriers in water production. SSF has gained popularity due to its low energy consumption and efficient removal of biohazards, especially microorganisms, without using chemicals. SSF involves both physical-chemical and biological removal, particularly in the "Schmutzdecke", which is a biofilm-like layer on the sand bed surface. To achieve the optimal performance of SSF, a systematic understanding of the influence of SSF operating parameters on the Schmutzdecke development and filter filtration performance is required. Our study focused on three operational parameters, i.e., sand material, sand size, and the addition of an inoculum (suspension of matured Schmutzdecke), on the mini-scale filters. The effects of these parameters on the Schmutzdecke development and SSF removal performance were studied by biochemical analyses and 16S amplicon sequencing, together with spiking experiments with Escherichia coli (E. coli) in the mini-scale filters. Our results indicate that the mini-scale filters successfully developed Schmutzdeckes and generated bacterial breakthrough curves efficiently. The sand size and material were found to have an impact on Schmutzdecke's development. The addition of inoculum to new filters did not induce significant changes in the microbial community composition of the Schmutzdecke, but we observed positive effects of faster Schmutzdecke development and better removal performance in some inoculated filters. Our study highlights the value of mini-scale filters for SSF studies, which provide insights into Schmutzdecke microbial ecology and bacterial removal with significantly reduced requirements of materials and effort as compared to larger-scale filters. We found that operational parameters have a greater impact on the Schmutzdecke biochemical characteristics and removal performances than on the microbial community composition. This suggests that Schmutzdecke characteristics may provide more reliable predictors of SSF removal performance, which could help to improve safe drinking water production.

Plasmid size determines adsorption to clay and breakthrough in a saturated sand column

Horizontal gene transfer (HGT) is a major factor in the spread of antibiotic resistant genes (ARG). Transformation, one mode of HGT, involves the acquisition and expression of extracellular DNA (eDNA). eDNA in soils is degraded rapidly by extracellular nucleases. However, if bound to a clay particle, eDNA can persist for long periods of time without losing its transformation ability. To better understand the mechanism of eDNA persistence in soil, this experiment assessed the effects of 1) clay mineralogy, 2) mixed salt solution, 3) plasmid size on DNA adsorption to clay and 4) breakthrough behavior of three differently sized plasmids in an environmentally relevant solution. Batch test methods were used to determine adsorption trends of three differently sized DNA plasmids, pUC19, pBR322, and pTYB21, to several pure clay minerals, goethite (α-FeOOH), illite, and kaolinite, and one environmental soil sample. Results show not all sorbents have equal adsorption capacity based on surface area with adsorption capacities decreasing from goethite > illite = kaolinite > bulk soil, and low ionic strength solutions will likely not significantly alter sorption trends. Additionally, plasmid DNA size (i.e., length) was shown to be a significant predictor of adsorption efficiency and that size affects DNA breakthrough, with breakthroughs occurring later with larger plasmids. Given that DNA persistence is linked to its adsorption to soil constituents and breakthrough, eDNA size is likely an important contributor to the spread of ARG within natural microbial communities.

A critical review of the mechanisms, factors, and performance of pervious concrete to remove contaminants from stormwater runoff

Stormwater can carry pollutants accumulated on impervious surfaces in urban areas into natural water bodies in absence of stormwater quality improvement devices. Pervious concrete (PC) pavement is one of the low-impact development practices introduced for urban flooding prevention and stormwater pollution reduction. PC removes various types of water contaminants. Mechanisms contributing to the water pollution removal capacity of PC can be categorized into three groups: physical, chemical, and biological. Properties of PC such as permeability, porosity, thickness, and adsorption capacity influence removal of all contaminants, although their impact might differ depending on the pollutant properties. Chemical mechanisms include precipitation, co-precipitation, ion and ligand exchange, complexation, diffusion, and sorption. Bulk organics and nutrients are removed primarily by biodegradation. Physical filtration is the primary mechanism to retain suspended solids, although biological activities may have a minor contribution. Release of calcium (Ca2+) and hydroxide (OH-) from hardened cement elevates the effluent pH, which is an environmental concern. However, the pH elevation is also the prime contributor to heavy metals and nutrients removal through precipitation. Specific cementitious materials (e.g., Pozzolans and nanoparticles) and carbonation curing approach are recommended to control effluent pH elevation. Complexation, diffusion, ion solubility, and stability constants are other mechanisms and parameters that influence heavy metal removal. Organic matter availability, electrostatic attraction, temperature, pH, contact time, specific surface area, and roughness of PC pores contribute to the pathogen removal process. Although PC has been found promising in removing various water pollutants, limited salinity removal can be achieved due to the inherent release of Ca2+and OH- from PC.

Fate and transport modelling for evaluating antibiotic resistance in aquatic environments: Current knowledge and research priorities

Antibiotics have revolutionised medicine in the last century and enabled the prevention of bacterial infections that were previously deemed untreatable. However, in parallel, bacteria have increasingly developed resistance to antibiotics through various mechanisms. When resistant bacteria find their way into terrestrial and aquatic environments, animal and human exposures increase, e.g., via polluted soil, food, and water, and health risks multiply. Understanding the fate and transport of antibiotic resistant bacteria (ARB) and the transfer mechanisms of antibiotic resistance genes (ARGs) in aquatic environments is critical for evaluating and mitigating the risks of resistant-induced infections. The conceptual understanding of sources and pathways of antibiotics, ARB, and ARGs from society to the water environments is essential for setting the scene and developing an appropriate framework for modelling. Various factors and processes associated with hydrology, ecology, and climate change can significantly affect the fate and transport of ARB and ARGs in natural environments. This article reviews current knowledge, research gaps, and priorities for developing water quality models to assess the fate and transport of ARB and ARGs. The paper also provides inputs on future research needs, especially the need for new predictive models to guide risk assessment on AR transmission and spread in aquatic environments.

In Situ Measurements of Dynamic Bacteria Transport and Attachment in Heterogeneous Sand-Packed Columns

Prevention, mitigation, and regulation of bacterial contaminants in groundwater require a fundamental understanding of the mechanisms of transport and attachment in complex geological materials. Discrepancies in bacterial transport behaviors observed between field studies and laboratory experiments indicate an incomplete understanding of dynamic bacterial transport and immobilization processes in realistic heterogeneous geologic systems. Here, we develop a new experimental approach for in situ quantification of dynamic bacterial transport and attachment distribution in geologic media that relies on radiolabelingEscherichia coliwith positron-emitting radioisotopes and quantifying transport with three-dimensional (3D) positron emission tomography (PET) imaging. Our results indicate that the highest bacterial attachment occurred at the interfaces between sand layers oriented orthogonal to the direction of flow. The predicted bacterial attachment from a 3D numerical model matched the experimental PET results, highlighting that the experimentally observed bacterial transport behavior can be accurately captured with a distribution of a first-order irreversible attachment model. This is the first demonstration of the direct measurement of attachment coefficient distributions from bacterial transport experiments in geologic media and provides a transformational approach to better understand bacterial transport mechanisms, improve model parametrization, and accurately predict how local geologic conditions can influence bacterial fate and transport in groundwater.

Transport and Retention of Fecal Indicator Bacteria in Unsaturated Porous Media: Effect of Transient Water Flow

For production of clean drinking water, the processes governing bacterial remobilization in the unsaturated zone at transient water flow are critical. Although managed aquifer recharge is an effective way to dispose of pathogens, there are concerns about recontamination after heavy precipitation. To better understand how bacteria that were initially retained in porous media can be released to groundwater due to transient water content, transport experiments and modeling for Escherichia coli and Enterococcus moraviensis were conducted at the soil column scale. After inoculating dune sand columns with a bacteria suspension for 4 h, three rainfall events were performed at 24-h intervals. The effluent from sand columns was collected to analyze bacteria breakthrough curves (BTCs). After the rainfall experiments, the bacteria distribution in the sand column was determined. The collected BTCs and profile retentions were modeled with HYDRUS-1D, using different model concepts, including one-site kinetic attachment/detachment (M1), Langmuirian (M2), Langmuirian and blocking (M3), and two-site attachment/detachment (M4). After inoculation, almost 99% of the bacteria remained in the soil. The M1 and M2 bacteria models had a high agreement between observed and modeled concentrations, and attachment and detachment were two significant mechanisms for regulating bacteria movement in a porous medium with fluctuations in water flow. At the end of the experiment, the majority of bacteria were still found within the depth range of 5 cm to 15 cm. Our experiments show that E. coli is more mobile in sandy soils than E. moraviensis. The results of this study also suggest that the unsaturated zone is an important barrier between microbial contamination at the soil surface and groundwater. Follow-up studies are needed to completely understand the variables that regulate bacteria remobilization in the unsaturated zone of dune sands. IMPORTANCE At managed artificial recharge sites in the Netherlands, recontamination of infiltrated water with fecal indicator bacteria has been observed. The results of this study suggest that the unsaturated zone is an important barrier between microbial contamination at the soil surface and groundwater. Bacteria that accumulate in the unsaturated zone, on the other hand, can multiply to such an extent that they can be released into the saturated zone when saturation increases due to major rain events or a rise in groundwater level.

Contrasting transport and fate of hydrophilic and hydrophobic bacteria in wettable and water-repellent porous media: Straining or attachment?

Bacterial transport and retention likely depend on bacterial and soil surface properties, especially hydrophobicity. We used a controlled experimental setup to explore hydrophilic Escherichia coli (E. coli) and hydrophobic Rhodococcus erythropolis (PTCC1767) (R. erythropolis) transport through dry (- 15,000 cm water potential) and water saturated (0 cm water potential) wettable and water-repellent sand columns. A pulse of bacteria (1 × 10⁸ CFU mL^-1) and bromide (10 mmol L^-1) moved through the columns under saturated flow (0 cm) for four pore volumes. A second bacteria and bromide pulse was then poured on the column surfaces and leaching was extended six more pore volumes. In dry wettable sand attachment dominated E. coli retention, whereas R. erythropolis was dominated by straining. Once wetted, the dominant retention mechanisms flipped between these bacteria. Attachment by either bacteria decreased markedly in water-repellent sand, so straining was the main retention mechanism. We explain this from capillary potential energy, which enhanced straining under the formation of water films at very early times (i.e., imbibing) and film thinning at much later times (i.e., draining). The interaction between the hydrophobicity of bacteria and soil on transport, retention and release mechanisms needs greater consideration in predictions.

Migration risk of Escherichia coli O157:H7 in unsaturated porous media in response to different colloid types and compositions

The vadose zone is a critical zone for microbial entry into the subsurface environment, and various types of inorganic and organic colloids can affect the migration of pathogenic bacteria. In the study, we explored the migration behavior of Escherichia coli O157:H7 with humic acids (HA), iron oxides (Fe₂O₃) or their mixture, uncovering their migration mechanisms in the vadose zone. The effect of complex colloids on the physiological properties of E. coli O157:H7 was analyzed based on the measured particle size, zeta potential and contact angle. HA colloids significantly promoted the migration of E. coli O157:H7, where Fe₂O₃ was opposite. The migration mechanism of E. coli O157:H7 with HA and Fe₂O₃ is obviously different. Multiple colloids dominated by organic colloid will further highlight its promoting effect on E. coli O157:H7 under the guidance of electrostatic repulsion due to the influence of colloidal stability. Multiple colloids dominated by metallic colloid will inhibit the migration of E. coli O157:H7 under the control of capillary force due to the restriction of contact angle. The risk of secondary release of E. coli O157:H7 can be effectively reduced when the ratio of HA/Fe₂O₃ is ≥ 1. Combining this conclusion with the distribution characteristics of soil in China, an attempt was made to analyse the migration risk of E. coli O157:H7 on a national scale. In China, from north to south, the migration capacity of E. coli O157:H7 gradually decreased, and the risk of secondary release gradually increased. These results provide ideas for the subsequent study of the effect of other factors on the migration of pathogenic bacteria on a national scale and provide risk information about soil colloids for the construction of pathogen risk assessment model under comprehensive conditions in the future.

Influence of flagella and their property on the initial attachment behaviors of bacteria onto plastics

Flagella and their property would influence the initial attachment of bacteria onto plastics, yet their impacts have not been investigated. In present study, four types of E. coli with or without flagella as well as with normal or sticky flagella were utilized to investigate the effects of flagella and their property on the initial attachment behaviors of bacteria onto six types of plastics in freshwater systems. We found that E. coli with flagella exhibited better initial attachment performance onto all six types of plastics than strain without flagella. Flagella could help bacteria swim near to plastics, pierce the energy barrier, and subsequently attach onto plastics. With stronger adhesive force, sticky flagella could further facilitate bacterial attachment onto plastics. Moreover, flagella especially sticky flagella could help bacteria form more rigid attachment layer on plastics. Even with humic acid in suspensions or in river water, flagellar E. coli showed greater attachment onto plastics than E. coli without flagella. Humic acid might adsorb onto sticky flagella and thus decreased the attachment of bacteria with sticky flagella onto plastics. Obviously, flagella as well as their property would impact the initial attachment of bacteria onto plastics and the subsequent formation of plastisphere in freshwater.

Quantifying bacterial concentration in water and sand media during flow-through experiments using a non-invasive, real-time, and efficient method

Monitoring the dynamics of bacteria in porous media is of great significance to understand the bacterial transport and the interplay between bacteria and environmental factors. In this study, we reported a non-invasive, real-time, and efficient method to quantify bioluminescent bacterial concentration in water and sand media during flow-through experiments. First, 27 column experiments were conducted, and the bacterial transport was monitored using a real-time bioluminescent imaging system. Next, we quantified the bacterial concentration in water and sand media using two methods-viable count and bioluminescent count. The principle of the bioluminescent count in sand media was, for a given bioluminescence image, the total number of bacteria was proportionally allocated to each segment according to its bioluminescence intensity. We then compared the bacterial concentration for the two methods and found a good linear correlation between the bioluminescent count and viable count. Finally, the effects of porous media surface coating, pore water velocity, and ionic strength on the bioluminescent count in sand media were investigated, and the results showed that the bioluminescence counting accuracy was most affected by surface coating, followed by ionic strength, and was hardly affected by pore water velocity. Overall, the study proved that the bioluminescent count was a reliable method to quantify bacterial concentration in water (10⁶ to 2 × 10⁸ cell mL^-1) or sand media (5 × 10⁶-5 × 10⁸ cell cm^-3). This approach also offers a new way of thinking for in situ bacterial enumeration in two-dimensional devices such as 2D flow cells, microfluidic devices, and rhizoboxes.

Polycyclic aromatic hydrocarbons remobilization from contaminated porous media by (bio)surfactants washing

Biosurfactants, surface-active agents produced by microorganisms, are increasingly studied for their potential use in soil remediation processes because they are more environmentally friendly than their chemically produced homologues. In this work, we report on the use of a crude biosurfactant produced by a bacterial consortium isolated from a PAHs-contaminated soil, compared with other (bio)surfactants (Tween80, Sodium dodecyl sulfate - SDS, rhamnolipids mix), to wash PAHs from a contaminated porous media. Assays were done using columns filled with sand or sand-clay mixtures (95:5) spiked with four model PAHs. The crude biosurfactant showed less adsorption to the [sand] and the [sand + clay] columns compared to Tween 80, SDS and the rhamnolipid mix. The biosurfactant showed the second best capacity to remove PAHs from the columns (as dissolved and particulate phases), both from [sand] and [sand + clay], after SDS when applied at lower concentrations than the other sufactants. The effluent concentrations of phenanthrene (PHE), pyrene (PYR) and benzo[a]pyrene (BAP) increased in the presence of the crude biosurfactant. Compared to the control experiment using only water, the global PAHs washed mass (amount of PAHs removed from the columns) increased between 9 and 1000 times for PHE and BAP in the [sand] column, and between 55 and 6000 times respectively for PHE and BAP in the [sand + clay] columns. Moreover, in the [sand + clay] columns, leaching of a part of the clays was observed in the SDS and the biosurfactant injections assays. This clay leaching resulted in higher PAHs removal, due not to desorption but rather to particulate transport. In the context of washing PAH-contaminated soils in biopiles or subsurface remediation, our results could help in sizing the remediation approach using an environmental friendly biosurfactant, before a pump-and-treat process.

Flagellar Phenotypes Impact on Bacterial Transport and Deposition Behavior in Porous Media: Case of Salmonella enterica Serovar Typhimurium

Bacterial contamination of groundwater has always been an ecological problem worthy of attention. In this study, Salmonella enterica serovar Typhimurium with different flagellar phenotypes mainly characterized during host-pathogen interaction were analyzed for their transport and deposition behavior in porous media. Column transport experiments and a modified mobile-immobile model were applicated on different strains with flagellar motility (wild-type) or without motility (ΔmotAB), without flagella (ΔflgKL), methylated and unmethylated flagellin (ΔfliB), and different flagella phases (fliC^ON, fljB^ON). Results showed that flagella motility could promote bacterial transport and deposition due to their biological advantages of moving and attaching to surfaces. We also found that the presence of non-motile flagella improved bacterial adhesion according to a higher retention rate of the ΔmotAB strain compared to the ΔflgKL strain. This indicated that bacteria flagella and motility both had promoting effects on bacterial deposition in sandy porous media. Flagella phases influenced the bacterial movement; the fliC^ON strain went faster through the column than the fljB^ON strain. Moreover, flagella methylation was found to favor bacterial transport and deposition. Overall, flagellar modifications affect Salmonella enterica serovar Typhimurium transport and deposition behavior in different ways in environmental conditions.

Physiological characteristics, geochemical properties and hydrological variables influencing pathogen migration in subsurface system: What we know or not?

The global outbreak of coronavirus infectious disease-2019 (COVID-19) draws attentions in the transport and spread of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in aerosols, wastewater, surface water and solid wastes. As pathogens eventually enter the subsurface system, e.g., soils in the vadose zone and groundwater in the aquifers, they might survive for a prolonged period of time owing to the uniqueness of subsurface environment. In addition, pathogens can transport in groundwater and contaminate surrounding drinking water sources, possessing long-term and concealed risks to human society. This work critically reviews the influential factors of pathogen migration, unravelling the impacts of pathogenic characteristics, vadose zone physiochemical properties and hydrological variables on the migration of typical pathogens in subsurface system. An assessment algorithm and two rating/weighting schemes are proposed to evaluate the migration abilities and risks of pathogens in subsurface environment. As there is still no evidence about the presence and distribution of SARS-CoV-2 in the vadose zones and aquifers, this study also discusses the migration potential and behavior of SARS-CoV-2 viruses in subsurface environment, offering prospective clues and suggestions for its potential risks in drinking water and effective prevention and control from hydrogeological points of view.

Host-Endosymbiont Relationship Impacts the Retention of Bacteria-Containing Amoeba Spores in Porous Media

Amoebae are protists that are commonly found in water, soil, and other habitats around the world and have complex interactions with other microorganisms. In this work, we investigated how host-endosymbiont interactions between amoebae and bacteria impacted the retention behavior of amoeba spores in porous media. A model amoeba species, Dictyostelium discoideum, and a representative bacterium, Burkholderia agricolaris B1qs70, were used to prepare amoeba spores that carried bacteria. After interacting with B. agricolaris, the retention of D. discoideum spores was enhanced compared to noninfected spores. Diverse proteins, especially proteins contributing to the looser exosporium structure and cell adhesion functionality, are secreted in higher quantities on the exosporium surface of infected spores compared to that of noninfected ones. Comprehensive examinations using a quartz crystal microbalance with dissipation (QCM-D), a parallel plate chamber, and a single-cell force microscope present coherent evidence that changes in the exosporium of D. discoideum spores due to infection by B. agricolaris enhance the connections between spores in the suspension and the spores that were previously deposited on the collector surface, thus resulting in more retention compared to the uninfected ones in porous media. This work provides novel insight into the retention of amoeba spores after bacterial infection in porous media and suggests that the host-endosymbiont relationship regulates the fate of biocolloids in drinking water systems, groundwater, and other porous environments.

Freeze-thaw cycles induce diverse bacteria release behaviors from quartz sand columns with different water saturations

Bacteria present in natural environment especially those in cold regions would experience freeze-thaw (FT) process during day-night and season turns. However, knowledge about the influence of FT on bacteria release behaviors in porous media was limited. In present study, the bacteria release behaviors from quartz sand columns without and with 1 and 3 FT treatment cycles under three water saturations (θ=100%, 90%, and 60%) were investigated. We found that for all three water saturated columns without FT treatment, negligible bacteria released from columns via background salt solution elution, while the subsequent release of bacteria from sand columns via low ionic strength (IS) solution elution decreased with decreasing column water saturations. More importantly, we found unlike the negligible bacteria release in columns without FT treatment, for columns with high saturations (θ=100% and 90%), FT treatment could promote bacteria release with background salt solution elution. Moreover, for high saturated columns, FT treatment would decrease subsequent bacteria release with low IS solution elution. This phenomenon was more obvious with increasing FT treatment cycles. In contrast, FT treatment had negligible influence on bacteria release from columns with lower saturation (θ=60%). The decreased bacterial sizes, the loss of bacterial flagella, as well as the change of local configuration of porous media (via changing water into ice and ice back into water) during the FT processes contributed to increased bacteria release via background salt solution elution from high saturated sand columns. While, the reduced amount of bacteria being retained at secondary energy minima drove to the subsequently decreased bacteria release via low IS solution elution. The results of this study clearly showed that for porous media with high saturations, FT cycles would increase the risk of bacteria detaching from porous media with flushing by the background solution.

Measuring and modeling the influence of salinity change on the transport behaviour of Escherichia coli through quartz sand

Pathogenic bacteria can be discharged in the environment through natural as well as anthropogenic activities. Once in the environment, they may contaminate soil and sediments and migrate towards water bodies. Transient chemical conditions may occur in soil/sediments and favor mobilization of bacteria, e.g., upon the reduction of salinity (or ionic strength). However, the magnitude of this phenomenon and its relationship with particle size is not well understood, yet. In this work, we investigated the transport of Escherichia coli under variable salinity conditions (between 1 and 20 part per thousand, ppt) and for different soil grain sizes (between 150 and 710 μm). A model developed in our group was applied in this work. It couples bacteria and salinity transport equations in order to account for transient water composition in the description of bacteria migration. The model was calibrated and validated with laboratory experiments. The tests were monitored continuously with UV-Vis spectroscopy, which allowed to record highly resolved concentration fronts. The results show that salinity increases the retardation of the bacteria. Upon salinity drop, a release of bacteria occurs forming a peak whose magnitude increases with salinity change. This effect becomes more important as the grain size decreases. Simulations suggest that the dominant retention mechanism is attachment for coarse sand and straining for fine sand. The retention can be reversed as the salinity is reduced causing a sudden bacteria mobilization. Such a behaviour may have important implications on microbial contamination of water bodies when soil/sediments undergo transient chemical conditions.

Transport and retention of bacteria through a filtration system consisting of sands and geotextiles

Water-saturated column experiments were conducted to study the effect of nonwoven geotextiles on bacteria transport and deposition through two sandy porous media with grain sizes 1.05 and 3.25 mm. The breakthrough curves (BTCs) of tracer for the all porous media exhibited an asymmetrical shape with a substantial tailing, indicating that non-equilibrium and dispersive flow patterns in these porous media. The mass recovery of the bacteria from the effluent (M_eff) increased with grain size. The retention profiles (RPs) exhibited hyper-exponential behavior, especially in the finer sand. The presence of the geotextiles increased bacteria retention rate. For a given geotextile, greater retention was observed in the surrounding region close to the geotextile. Moreover, the retention of bacteria became more significant in the geotextile with a lower porosity. Results demonstrated that model simulations of bacteria transport and fate need to accurately account for both observed BTC and RP behaviors and also the geotextile placement can impact mechanisms of retention.

Comparison of the behavior of negative electrically charged E. coli and E. faecalis bacteria under electric field effect

The negative electrical charge of Escherichia coli and Enterococcus faecalis bacteria is an indication that they can be affected by an electric field. To show that the movements of electrically charged bacteria can be controlled, impedance spectroscopy method was used on a porous silicon (PS) structure with 60 % porosity and 7-12 μm pore size. The main purpose of this study is to use the electric charge of these two bacterial species to bring bacteria closer to the sensors with the help of an electric field, and to compare the behavior of these bacterial species in the process. The effect of bacterial contact on porous silicon surface impedance spectra was studied under electrical fields between 0 and 5 kV/cm at a constant bacterial concentration. It was observed that both bacteria can be approximated to the PS surface by the electric field effect. However, the shape and dimensional differences of these two bacterial species caused differences both in their movements in the electric field and in their settlement on the PS surface, and these differences were interpreted. In addition, similar experiments were repeated for dead bacteria and it was determined that the electric field control was not the same as for living bacteria.

Modeling the vertical transport of antibiotic resistance genes in agricultural soils following manure application

Antibiotic resistance genes (ARGs) may be introduced to agricultural soil through the land application of cattle manure. During a rainfall event, manure-borne ARGs may infiltrate into subsurface soil and leach into groundwater. The objective of this study was to characterize and model the vertical transport of manure-borne ARGs through soil following the land application of beef cattle manure on soil surface. In this study, soil column experiments were conducted to evaluate the influence of manure application on subsurface transport of four ARGs: erm(C), erm(F), tet(O) and tet(Q). An attachment-detachment model with the decay of ARGs in the soil was used to simulate the breakthrough of ARGs in leachates from the control column (without manure) and treatment (with manure) soil columns. Results showed that the first-order attachment coefficient (k_a) was five to six orders of magnitude higher in the treatment column than in the control column. Conversely, the first-order detachment and decay coefficients (k_d and μ_s) were not significantly changed due to manure application. These findings suggest that in areas where manure is land-applied, some manure-borne bacteria-associated ARGs will be attached to the soil, instead of leaching to groundwater in near terms.

Extracellular polymeric substances induced cell-surface interactions facilitate bacteria transport in saturated porous media

Bacteria often respond to dynamic soil environment through the secretion of extracellular polymeric substances (EPS). The EPS modifies cell surface properties and soil pore-scale hydration status, which in turn, influences bacteria transport in soil. However, the effect of soil particle size and EPS-mediated surface properties on bacterial transport in the soil is not well understood. In this study, the simultaneous impacts of EPS and collector size on Escherichia coli (E. coli) transport and deposition in a sand column were investigated. E. coli transport experiments were carried out under steady-state flow in saturated columns packed with quartz sand with different size ranges, including 0.300-0.425 mm (sand-I), 0.212-0.300 mm (sand-II), 0.106-0.150 mm (sand-III) and 0.075-0.106 mm (sand-IV). Bacterial retention increased with decreasing sand collector size, suggesting that straining played an important role in fine-textured media. Both experiment and simulation results showed a clear drop in the retention rate of the bacterial population with the presence of additional EPS (200 mg L-1) (EPS+). The inhibited retention of cells in sand columns under EPS+ scenario was likely attributed to enhanced bacteria hydrophilicity and electrostatic repulsion between cells and sand particles as well as reduced straining. Calculations of the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) interactions energies revealed that high repulsive energy barrier existed between bacterial cells and sand particles in EPS+ environment, primarily due to high repulsive electrostatic force and Lewis acid-base force, as well as low attractive Lifshitz-van der Waals force, which retarded bacterial population deposition. Steric stabilization of EPS would also prevent the approaching of cells close to the quartz surface and thereby hinder cell attachment. This study was the first to show that EPS reduced bacterial straining in saturated porous media. These findings provide new insight into the functional effects of extrinsic EPS on bacterial transport behavior in the saturated soil environment, e.g., aquifers.

Flagella and Their Properties Affect the Transport and Deposition Behaviors of Escherichia coli in Quartz Sand

The effects of flagella and their properties on bacterial transport and deposition behaviors were examined by using four types of Escherichia coli (E. coli) with or without flagella, as well as with normal or sticky flagella. Packed column, quartz crystal microbalance with dissipation, visible parallel-plate flow chamber system, and visible flow chamber packed with porous media system were employed to investigate the deposition mechanisms of bacteria with different properties of flagella. We found that the presence of flagella favored E. coli deposition onto quartz sand/silica surfaces. Moreover, by changing the porous media porosity and directly observing the bacterial deposition process, local sites with high roughness, narrow flow channels, and grain-to-grain contacts were found to be the major sites for bacterial deposition. Particularly, flagella could help bacteria swim near and then deposit at these sites. In addition, we found that due to the stronger adhesive forces, sticky flagella could further enhance bacterial deposition onto quartz sand/silica surfaces. Elution experiments indicated that flagella could help bacteria attach onto sand surfaces more irreversibly. Clearly, flagella and their properties would have obvious impacts on the transport/deposition behaviors of bacteria in porous media.

Bacteria transport and deposition in an unsaturated aggregated porous medium with dual porosity

Bacterial transport and deposition play an important role in the assessment and prediction of subsurface pollution risks. Bacteria transport experiments were performed under unsaturated flow conditions in an aggregated porous medium at the laboratory column scale, to investigate how the inter- and intra-aggregated pore space of this medium could affect transport and deposition under unsaturated flow conditions, where inter- and intra-pore spaces are not fully activated. The results obtained through experimental observations and numerical simulations showed that some intra- and inter-pore space of this medium was excluded from bacteria transport and retention, as confirmed by the non-uniform transport of bacteria pathways in the aggregated porous media under unsaturated flow conditions. Capillary energy was higher the than other forces acting at bacteria air-water-solid interfaces. If this energy should contribute in increasing bacteria deposition under unsaturated conditions, similar to what has been reported for sandy media, similar overall retention of E. coli and R. rhodochrous was obtained under unsaturated flow conditions, suggesting that capillary energy was not the driving force for bacteria deposition.

Spatial pattern and surface-specificity of particle and microorganism deposition and attachment: Modeling, analytic solution and experimental test

HYPOTHESIS

Understanding microparticle and living cell deposition and attachment on surfaces from a flow is a long-standing surface-science problem, pivotal to developing antifouling strategies. Recent studies indicate a complex non-conservative and surface-specific nature of adhesion and mechanical contact forces that determine attachment kinetics. This requires new models and kinetic data, however, observed deposition rates (e.g., in parallel-plate flow chamber, PPFC) represent a superposition of attachment and bulk transport. Here, we propose to deduce attachment rates (as an appropriate rate constant) from spatial deposition profiles along PPFC and develop an analytical solution for the full problem, suitable for deposition data analysis and parameter fitting.

EXPERIMENTS

Analytical solution, validated by numerical simulations, reveals relation between the deposition profile along PPFC and key model parameter B, the ratio of sedimentation and attachment rates. Its use is demonstrated on experimental data obtained in a PPFC for particles and bacteria on various surfaces.

FINDINGS

Fitted B values highlight correlation with the particle/substrate nature and consistently explain the observed trends along PPFC, both decreasing and increasing. Thus derived attachment rates will serve as basis for future microscopic modelling that would relate attachment to appropriate surface and contact-mechanical characteristics of particles and substrate and flow conditions.

The attachment process and physiological properties of Escherichia coli O157:H7 on quartz

Background

Manure application and sewage irrigation release many intestinal pathogens into the soil. After being introduced into the soil matrix, pathogens are commonly found to attach to soil minerals. Although the survival of mineral-associated Escherichia coli O157:H7 has been studied, a comprehensive understanding of the attachment process and physiological properties after attachment is still lacking.

Results

In this study, planktonic and attached Escherichia coli O157:H7 cells on quartz were investigated using RNA sequencing (RNA-seq) and the isobaric tagging for relative and absolute quantitation (iTRAQ) proteomic method. Based on the transcriptomic and proteomic analyses and gene knockouts, functional two-component system pathways were required for efficient attachment; chemotaxis and the Rcs system were identified to play determinant roles in E. coli O157:H7 attachment on quartz. After attachment, the pyruvate catabolic pathway shifted from the tricarboxylic acid (TCA) cycle toward the fermentative route. The survival rate of attached E. coli O157:H7 increased more than 10-fold under penicillin and vancomycin stress and doubled under alkaline pH and ferric iron stress.

Conclusions

These results contribute to the understanding of the roles of chemotaxis and the Rcs system in the attachment process of pathogens and indicate that the attachment of pathogens to minerals significantly elevates their resistance to antibiotics and environmental stress, which may pose a potential threat to public health.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-020-02043-8.

Influence of hydraulic gradient and temperature on the migration of E. coli in saturated porous media during bank filtration: a case study at the Second Songhua River, Songyuan, Northeastern China

River bank filtration can effectively reduce the number of pathogenic microorganisms infiltrating into groundwater from surface water. Groundwater seepage velocity and temperature are considered to be important factors affecting the process, but the magnitude and mechanism of their impacts have not been clear for a long time. Based on the actual monitoring data of the Escherichia coli concentrations and soil samples of Second Songhua riverside source area, the migration of E. coli in saturated porous media under different velocities and different temperatures was studied using saturated soil column transport experiments. Concurrently, the migration characteristics of E. coli in the riverside source area were replicated by mathematical simulation. According to the field monitoring results, the concentration of E. coli decreased in the riverbank infiltration zone, and the removal rate was greater than 96%. The column experimental results showed that the lower the flow velocity was and the higher the temperature was, the greater the removal rate of E. coli was. And the flow velocity was the main factor affecting the removal of E. Coli. The mathematical simulation results showed that under the conditions of the largest hydraulic gradient (20%) and the highest concentration of E. coli (2500 MPN/100 mL) in river water, the safe exploitation distance of groundwater that did not cause a risk of E. coli pollution was more than 7 m away from the river bank. These findings are expected to provide a scientific basis for the design of water intake schemes and the optimization of mining technology.

Bioluminescent Imaging and Tracking of Bacterial Transport in Soils

Bioimaging instrumentation can be used to observe environmental phenomena such as the transport, retention, and distribution of bacteria in soils in situ in a real-time, nondestructive manner. Bacteria designed to emit bioluminescence light signals are injected into a transparent column packed with soils, and then the column is placed into a bioimaging instrument, such as a PerkinElmer IVIS Spectrum, while it is connected through thin teflon tubes to other parts of the column system located outside of the imaging chamber, including a fraction collector for collecting effluent solution and a pump for introducing bacterial suspension or experimental solution. After self-correction of soil autofluorescence and bioluminescence and setup of required imaging parameters, the transport experiment is initiated by introducing the bacterial suspension to the soil column while the spatiotemporal distribution of bioluminescent bacteria in the entire soil column is imaged. Finally, the images are processed to analyze bacterial migration in the soil under various environmental conditions in comparison with the breakthrough and elution curves of the bacteria obtained by analyzing the effluent samples.

Transport and retention of Shewanella oneidensis strain MR1 in water-saturated porous media with different grain-surface properties

Bacterium Shewanella oneidensis strain (MR1), a facultative microorganism that plays critical role in contaminant transformation and degradation, was used as an example to decipher the transport and retention of microorganisms in water-saturated porous media with different grain-surface properties and under different ionic compositions (i.e. Na⁺, Ca²⁺, and Mg²⁺). Dolomite and quartz sands, which contained different surface charge properties, were used as the representative minerals. Dolomite was selected because its surface charges are significantly affected by solution composition. The mobility of MR1 in the dolomite column was lower than that in the quartz column, because the lower energy barrier between MR1 and dolomite than that between MR1 and quartz, resulting in the larger retention of MR1 in the dolomite column. The breakthrough curves were well simulated by the two sites kinetic model with HYDRUS-1D. The maximum concentration of attached bacteria (S_max) were positively correlated to the ionic strength regardless of mineral types. The values of S_max were about 1.1-4.0 times larger in the MR1-dolomite system than that in the MR1-quartz system under different ionic strength conditions. The retention of the MR1 on dolomite surfaces in the presence of divalent cations Ca²⁺ is significantly higher than that on quartz surfaces primarily due to the larger electrostatic attraction energy between the MR1 and dolomite grains. The findings demonstrate that the porous media with the lower negative charge has the higher capacity for the retention and deposition of MR1, potentially affecting the transport of MR1 and other bacteria in the subsurface.

Adhesion mediated transport of bacterial pathogens in saturated sands coated by phyllosilicates and Al-oxides

The current knowledge of bacterial migration is mainly derived from work using bare or Fe-coated quartz sands as porous media. However, mineral coatings on quartz by phyllosilicates and Al-oxides prevail in natural soils, and their effect on bacterial transport remains unknown. Herein, we systematically explored the transport of two bacterial pathogens (Escherichia coli and Staphylococcus aureus) through saturated bare quartz and those coated by kaolinite (KaoQuartz), montmorillonite (MontQuartz) or Al-oxides (AlQuartz) under various solution ionic strength (IS) and pH levels. Elevating IS or decreasing pH discouraged bacterial mobility in all cases, with one exception for the migration of S. aureus through AlQuartz at various IS levels. E. coli showed a higher mobility than S. aureus in all cases. All the three coatings, especially the Al-oxides inhibited bacterial transport through quartz. Overall, the two phyllosilicates-coated sands showed transport behaviors (mobility trends with IS, pH, and cell type) similar to those for the bare quartz which could be explained by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Nevertheless, for transport within AlQuartz, there were deviations between the observations and the DLVO predictions, probably because of the existence of non-DLVO forces such as hydrophobic and chemical interactions. More importantly, the bacterial retention was found to correlate well with the adhesion regardless of the solution condition and the bacteria and media type, thereby revealing a central role of adhesion in mediating migration through mineral-coated sands. These findings highlight the significance of mineral coating and adhesion in pathogen dissemination in natural soils.

Online assessment of sand filter performance for bacterial removal in a full-scale drinking water treatment plant

Microbiological risks associated with drinking water can be minimized by providing enhanced integrity monitoring of bacterial removal by water treatment processes. This study aimed to evaluate the efficacy of real-time bacteriological counters for continuously assessing the performance of a full-scale sand filter to remove bacteria. Over the course of an 8-day evaluation, online counting of bacteria was successfully performed, providing continuous bacterial counts in the sand filter influent and effluent over approximate ranges from 17 × 10⁴ to 94 × 10⁴ and from 0.2 × 10⁴ to 1.3 × 10⁴ counts/mL, respectively. Periodic variations were observed with online bacterial counts in the sand filter influent because of the changes in the performance of flocculation and sedimentation processes. Overall, online removal rates of bacteria determined during the full-scale test were 95.2-99.3% (i.e., 1.3-2.2-log), indicating that online bacterial counting can continuously demonstrate over 1.3-log removal in the sand filter. Real-time bacteriological counting technology can be a useful tool for assessing variability and detecting bacterial breakthrough. It can be integrated with other online water quality measurements to evaluate underlying trends and the performance of sand filters for bacterial removal, which can enhance the safety of drinking water.

Influence of Nano- and Microplastic Particles on the Transport and Deposition Behaviors of Bacteria in Quartz Sand

Plastic particles are widely present in the natural environment and are highly likely to interact with bacteria (the ubiquitous microbes in the natural environment), which might affect the transport and deposition of bacteria in porous media. In this study, the significance of plastic particles from nanoscale to micrometer-scale (0.02-2 μm) on the transport and deposition behaviors of bacteria ( Escherichia coli) in quartz sand was examined under environmentally relevant conditions in both NaCl and CaCl₂ solutions at pH 6. The results showed that the presence of different-sized plastic particles did not affect bacterial transport behaviors at low ionic strength (10 mM NaCl and 1 mM CaCl₂), whereas, at high ionic strength conditions (50 mM NaCl and 5 mM in CaCl₂), plastic particles increased bacterial transport in quartz sand. At low ionic strength conditions, the mobility of both plastic particles and bacteria was high, which might drive the negligible effects of plastic particles on bacterial transport behaviors. The mechanisms driving the enhanced cell transport at high ionic strength were different for different-sized plastic particles. Specifically, for 0.02 μm nanoplastic particles, the adsorption of plastic particles onto cell surfaces and the repel effect induced by suspended plastic particles contributed to the increased cell transport. As for 0.2 μm microplastics (MPs), the suspended plastic particles induced  repel effect contributed to the increased cell transport, whereas, for 2 μm MPs, the competition deposition sites by the plastic particles were the contributor to the increased cell transport.