Influence of a Rhamnolipid Biosurfactant on the Transport of Bacteria through a Sandy Soil

Transport of nanoscale zero-valent iron in the presence of rhamnolipid

Nanoscale zero-valent iron (nZVI) particles have gained widespread use for in-situ treatment of various chlorinated hydrocarbons. Their non-toxic nature, affordability, and minimal maintenance requirements have made them a favored material for nanoremediation. The treatment typically involves the injection of nZVI particles into contaminated sites using direct-push well injection systems. However, their small size leads to high surface energy, causing aggregation that alters their physiochemical properties, reactivity, and transport behavior. To counteract aggregation, nZVI suspension can be stabilized with different surfactants, reducing the surface energy during subsurface soil transport. This study investigates the impact of rhamnolipid, a biosurfactant produced by Pseudomonas aeruginosa during the late growth phase, on the aggregation and mobility of nZVI particles. The retardation factor of nZVI in the model media of zeolite, ZK406H, decreased from 1.66 in the absence of rhamnolipid to 1.03, 0.98, 0.93, and 0.87, corresponding to the presence of rhamnolipid at concentrations of 20, 50, 80, and 100 mg/L. The deposition coefficient also decreased from 2.39 in the absence of rhamnolipid to 0.459, 0.279, 0.217, and 0.0966, corresponding to the presence of rhamnolipid at concentrations of 20, 50, 80, and 100 mg/L. The transport parameters of nZVI in ZK406H were linked to the interactions of nZVI particles with ZK406H by the DLVO theory.

Nonionic surfactant Tween 80-facilitated bacterial transport in porous media: A nonmonotonic concentration-dependent performance, mechanism, and machine learning prediction

The surfactant-enhanced bioremediation (SEBR) of organic-contaminated soil is a promising soil remediation technology, in which surfactants not only mobilize pollutants, but also alter the mobility of bacteria. However, the bacterial response and underlying mechanisms remain unclear. In this study, the effects and mechanisms of action of a selected nonionic surfactant (Tween 80) on Pseudomonas aeruginosa transport in soil and quartz sand were investigated. The results showed that bacterial migration in both quartz sand and soil was significantly enhanced with increasing Tween 80 concentration, and the greatest migration occurred at a critical micelle concentration (CMC) of 4 for quartz sand and 30 for soil, with increases of 185.2% and 27.3%, respectively. The experimental results and theoretical analysis indicated that Tween 80-facilitated bacterial migration could be mainly attributed to competition for soil/sand surface sorption sites between Tween 80 and bacteria. The prior sorption of Tween 80 onto sand/soil could diminish the available sorption sites for P. aeruginosa, resulting in significant decreases in deposition parameters (70.8% and 33.3% decrease in KD in sand and soil systems, respectively), thereby increasing bacterial transport. In the bacterial post-sorption scenario, the subsequent injection of Tween 80 washed out 69.8% of the bacteria retained in the quartz sand owing to the competition of Tween 80 with pre-sorbed bacteria, as compared with almost no bacteria being eluted by NaCl solution. Several machine learning models have been employed to predict Tween 80-faciliated bacterial transport. The results showed that back-propagation neural network (BPNN)-based machine learning could predict the transport of P. aeruginosa through quartz sand with Tween 80 in-sample (2 CMC) and out-of-sample (10 CMC) with errors of 0.79% and 3.77%, respectively. This study sheds light on the full understanding of SEBR from the viewpoint of degrader facilitation.

A novel eco-friendly strategy for removing phenanthrene from groundwater: Synergism of nanobubbles and rhamnolipid

Nanobubbles (NBs), given their unique properties, could theoretically be paired with rhamnolipids (RL) to tackle polycyclic aromatic hydrocarbon contamination in groundwater. This approach may overcome the limitations of traditional surfactants, such as high toxicity and low efficiency. In this study, the remediation efficiency of RL, with or without NBs, was assessed through soil column experiments (soil contaminated with phenanthrene). Through the analysis of the two-site non-equilibrium diffusion model, there was a synergistic effect between NBs and RL. The introduction of NBs led to a reduction of up to 24.3 % in the total removal time of phenanthrene. The direct reason for this was that with NBs, the retardation factor of RL was reduced by 1.9 % to 15.4 %, which accelerated the solute replacement of RL. The reasons for this synergy were multifaceted. Detailed analysis reveals that NBs improve RL's colloidal stability, increase its absolute zeta potential, and reduce its soil adsorption capacity by 13.3 %-19.9 %. Furthermore, NBs and their interaction with RL substantially diminish the surface tension, contact angle, and dynamic viscosity of the leaching solution. These changes in surface thermodynamic and rheological properties significantly enhance the migration efficiency of the eluent. The research outcomes facilitate a thorough comprehension of NBs' attributes and their relevant applications, and propose an eco-friendly method to improve the efficiency of surfactant remediation.

Detection, fate and transport of the biohazardous agent Toxoplasma gondii in soil water systems: Influence of soil physicochemical properties, water chemistry and surfactant

A series of laboratory experiments were conducted to study the fate and transport of Toxoplasma gondii oocysts in soils as a function of soil physicochemical properties and soil water chemistry properties. Soil columns were homogeneously packed with loamy sand soils (Lewiston and Greenson series) and sandy loam soils (Sparta and Gilford series), and subject to hydrologic conditions characterized by the absence and presence of an anionic surfactant-Aerosol 22 in the artificial rainfall. Quantitative polymerase chain reaction (qPCR) was utilized for the detection and enumeration of oocysts in soil leachates to evaluate their breakthrough and in soil matrices to examine their spatial distribution. Differences in the rate and extent of transport of oocysts were observed as a function of physical and chemical parameters tested. The breakthrough of oocysts was observed for all the soils irrespective of the presence of surfactant. However, in the absence of surfactant, the predominant fate of oocysts in soils subject to simulated rainfall was their retention in the soil profile. The presence of surfactant induced a change in the fate of oocysts in these soils exposed to rainfall simulation as the predominant fate of oocysts was found to be in the soil leachates.

Rheological behavior of xanthan gum suspensions with Fe-based nanoparticles: the effect of nanoparticles and the mechanism

The rheological behavior of a xanthan gum (XG) suspension with Fe-based nanoparticles (Fe-NPs), e.g., nanoparticles of zerovalent iron (nZVI) and Fe3O4 (nFe3O4), needs to be understood for better injection of Fe-NPs for groundwater remediation. In this study, the rheological behavior of a XG suspension of nZVI and nFe3O4 was investigated at different particle concentrations. The Ostwald, Sisko, Williamson, and Cross models were employed to fit the rheological behavior of the suspensions for quantitatively describing the effect of the particles. The results showed that the viscosity of the XG solutions decreased with increasing particle concentrations and they maintained shear thinning properties. The Cross model was the best among the four models to describe the shear thinning behavior of the XG solution in the presence of the particles. According to Cross model analysis, increasing particle concentrations increased the degree of shear thinning behavior, as indicated by the increase of the power index (n). Also, the relaxation time (λ) decreased with increasing particle concentrations, which indicated an increase of molecule movement of XG. Compared with nFe3O4, nZVI resulted in a larger decrease in viscosity and a larger increase in the degree of shear thinning behavior. There was a good linear relation between n and λ for the suspensions (R2 = 0.85), which indicated that increasing molecule movement of XG was an important mechanism for the particles to intensify the shear thinning rheological behavior of the XG suspension of Fe-NPs. This study added insight into the knowledge of the rheological properties of the XG suspension of Fe-NPs, which is of importance for the field injection effort.

Biosurfactant-affected mobility of oxytetracycline and its variations with surface chemical heterogeneity in saturated porous media

Herein, the influences of rhamnolipid (a typical biosurfactant) on oxytetracycline (OTC) transport in the porous media and their variations with the surface heterogeneities of the media (uncoated sand, goethite (Goe)-, and humic acid (HA)-coated sands) were explored. Compared to uncoated sand, goethite and HA coatings suppressed OTC mobility by increasing deposition sites. Interestingly, rhamnolipid-affected OTC transport strongly depended on the chemical heterogeneities of aquifers and biosurfactant concentrations. Concretely, adding rhamnolipid (1-3 mg/L) inhibited OTC mobility through sand columns because of the bridging effect of biosurfactant between sand and OTC. Unexpectedly, rhamnolipid of 10 mg/L did not further improve the inhibition of OTC transport owing to the fact that the deposition capacity of rhamnolipid reached its maximum. OTC mobility in Goe-coated sand columns was inhibited by 1 mg/L rhamnolipid. However, the inhibitory effect decreased with the increasing rhamnolipid concentration (3 mg/L) and exhibited a promoted effect at 10 mg/L rhamnolipid. This surprising observation was that the increased rhamnolipid molecules gradually occupied the favorable deposition sites (i.e., the positively charged sites). In comparison, rhamnolipid facilitated OTC transport in the HA-coated sand column. The promotion effects positively correlated with rhamnolipid concentrations because of the high electrostatic repulsion and deposition site competition induced by the deposited rhamnolipid. Another interesting phenomenon was that rhamnolipid's enhanced or inhibitory effects on OTC transport declined with the increasing solution pH because of the decreased rhamnolipid deposition on porous media surfaces. These findings benefit our understanding of the environmental behaviors of antibiotics in complex soil-water systems containing biosurfactants.

Corner Flows Induced by Surfactant-Producing Bacteria Bacillus subtilis and Pseudomonas fluorescens

A mechanistic understanding of bacterial spreading in soil, which has both air and water in angular pore spaces, is critical to control pathogenic contamination of soil and to design bioremediation projects. A recent study (J. Q. Yang, J. E. Sanfilippo, N. Abbasi, Z. Gitai, et al., Proc Natl Acad Sci U S A 118:e2111060118, 2021, https://doi.org/10.1073/pnas.2111060118) shows that Pseudomonas aeruginosa can self-generate flows along sharp corners by producing rhamnolipids, a type of biosurfactants that change the hydrophobicity of solid surfaces. We hypothesize that other types of biosurfactants and biosurfactant-producing bacteria can also generate corner flows. Here, we first demonstrate that rhamnolipids and surfactin, biosurfactants with different chemical structures, can generate corner flows. We identify the critical concentrations of these two biosurfactants to generate corner flow. Second, we demonstrate that two common soil bacteria, Pseudomonas fluorescens and Bacillus subtilis (which produce rhamnolipids and surfactin, respectively), can generate corner flows along sharp corners at the speed of several millimeters per hour. We further show that a surfactin-deficient mutant of B. subtilis cannot generate corner flow. Third, we show that, similar to the finding for P. aeruginosa, the critical corner angle for P. fluorescens and B. subtilis to generate corner flows can be predicted from classic corner flow theories. Finally, we show that the height of corner flows is limited by the roundness of corners. Our results suggest that biosurfactant-induced corner flows are prevalent in soil and should be considered in the modeling and prediction of bacterial spreading in soil. The critical biosurfactant concentrations we identify and the mathematical models we propose will provide a theoretical foundation for future predictions of bacterial spreading in soil. IMPORTANCE The spread of bacteria in soil is critical in soil biogeochemical cycles, soil and groundwater contamination, and the efficiency of soil-based bioremediation projects. However, the mechanistic understanding of bacterial spreading in soil remains incomplete due to a lack of direct observations. Here, we simulate confined spaces of hydrocarbon-covered soil using a transparent material with similar hydrophobicity and visualize the spread of two common soil bacteria, Pseudomonas fluorescens and Bacillus subtilis. We show that both bacteria can generate corner flows at the velocity of several millimeters per hour by producing biosurfactants, soap-like chemicals. We provide quantitative equations to predict the critical corner angle for bacterial corner flow and the maximum distance of the corner spreading. We anticipate that bacterial corner flow is prevalent because biosurfactant-producing bacteria and angular pores are common in soil. Our results will help improve predictions of bacterial spreading in soil and facilitate the design of soil-related bioremediation projects.

Evidence for biosurfactant-induced flow in corners and bacterial spreading in unsaturated porous media

The spread of pathogenic bacteria in unsaturated porous media, where air and liquid coexist in pore spaces, is the major cause of soil contamination by pathogens, soft rot in plants, food spoilage, and many pulmonary diseases. However, visualization and fundamental understanding of bacterial transport in unsaturated porous media are currently lacking, limiting the ability to address the above contamination- and disease-related issues. Here, we demonstrate a previously unreported mechanism by which bacterial cells are transported in unsaturated porous media. We discover that surfactant-producing bacteria can generate flows along corners through surfactant production that changes the wettability of the solid surface. The corner flow velocity is on the order of several millimeters per hour, which is the same order of magnitude as bacterial swarming, one of the fastest known modes of bacterial surface translocation. We successfully predict the critical corner angle for bacterial corner flow to occur based on the biosurfactant-induced change in the contact angle of the bacterial solution on the solid surface. Furthermore, we demonstrate that bacteria can indeed spread by producing biosurfactants in a model soil, which consists of packed angular grains. In addition, we demonstrate that bacterial corner flow is controlled by quorum sensing, the cell-cell communication process that regulates biosurfactant production. Understanding this previously unappreciated bacterial transport mechanism will enable more accurate predictions of bacterial spreading in soil and other unsaturated porous media.

A novel optical biosensor for in situ and small-scale monitoring of bacterial transport in saturated columns

In situ monitoring techniques can provide new insight into bacterial transport after inoculating exogenous bacteria into contaminated soils for bioremediation. A real-time and non-destructive optical sensor (the optrode) was employed to monitor in situ transport of two fluorescently labelled bacteria - Green Fluorescent Protein (Gfp)-labelled, hydrophilic Pseudomonas putida and Tomato Fluorescent Protein (td)-labelled, hydrophobic Rhodococcus erythropolis, in a saturated sand column with and without rhamnolipid surfactant. In situ measurements were made at three sampling ports in the column with the optrode in two sets of column experiments. In Experiment 1, liquid samples were extracted for ex situ analyses (plate counts and fluorescence), while in Experiment 2 no liquid samples were extracted. Extracting liquid samples for ex situ analyses in Experiment 1 disturbed in situ measurements; in situ measured bacterial concentrations were lower, or a significant lag in breakthrough occurred relative to ex situ measurements. In Experiment 2, the optrode worked well in monitoring bacterial transport, which gave consistent transport parameters at each sampling port. Moreover, the optrode enabled the impact of bacterial hydrophobicity and rhamnolipid surfactant on bacterial transport to be observed. Specifically, hydrophilic P. putida was transported faster through the column than hydrophobic R. erythropolis; we infer from this result that fewer P. putida cells adsorb to sand particles than do R. erythropolis cells. The rhamnolipid surfactant enhanced the transport of both hydrophilic and hydrophobic bacteria. These two observations are consistent with Lifshitz-van der Waals forces and acid-base interactions between bacteria and sand.

Effect of biosurfactant derived from Vibrio natriegens MK3 against Vibrio harveyi biofilm and virulence

Vibrio harveyi is a marine luminous pathogen, which causes biofilm-mediated infections, pressures the search for an innovative alternate approach to strive against vibriosis in aquaculture. This study anticipated to explore the effect of glycolipid biosurfactant as an antipathogenic against V. harveyi to control vibriosis. In this study, 27 bacterial strains were isolated from marine soil sediments. Out of these, 11 strains exhibited surfactant activity and the strain MK3 showed high emulsification index. The potent strain was identified as Vibrio natriegens and named as V. natriegens MK3. The extracted biosurfactant was purified using high-performance liquid chromatography and it was efficient to decrease the surface tension of the growth medium up to 21 mN/m. The functional group and composition of the biosurfactant were determined by Fourier-transform infrared spectroscopy and nuclear magnetic resonance spectroscopy spectral studies and the nature of the biosurfactant was identified as glycolipid. The surfactant was capable of reducing the biofilm formation, bioluminescence, extracellular polysaccharide synthesis, and quorum sensing in marine shrimp pathogen V. harveyi. The antagonistic effect of biosurfactant was evaluated against V. harveyi-infected brine shrimp Artemia salina. This study reveals that biosurfactant can be considered for the management of biofilm-related aquatic infections.

Advances in applications of rhamnolipids biosurfactant in environmental remediation: A review

The objective of this review is to provide a comprehensive overview of the advances in the applications of rhamnolipids biosurfactants in soil and ground water remediation for removal of petroleum hydrocarbon and heavy metal contaminants. The properties of rhamnolipids associated with the contaminant removal, that is, solubilization, emulsification, dispersion, foaming, wetting, complexation, and the ability to modify bacterial cell surface properties, were reviewed in the first place. Then current remediation technologies with integration of rhamnolipid were summarized, and the effects and mechanisms for rhamnolipid to facilitate contaminant removal for these technologies were discussed. Finally rhamnolipid-based methods for remediation of the sites co-contaminated by petroleum hydrocarbons and heavy metals were presented and discussed. The review is expected to enhance our understanding on environmental aspects of rhamnolipid and provide some important information to guide the extending use of this fascinating chemical in remediation applications.

Effect of low-concentration rhamnolipid biosurfactant on Pseudomonas aeruginosa transport in natural porous media

The effect of low-concentrations of monorhamnolipid biosurfactant on transport of Pseudomonas aeruginosa ATCC 9027 in natural porous media (silica sand and a sandy soil) was studied with miscible-displacement experiments using artificial groundwater as the background solution. Transport of two types of cells was investigated, glucose- and hexadecane-grown cells with lower and higher cell surface hydrophobicity (CSH), respectively. The effect of hexadecane presence as a residual non-aqueous phase liquid (NAPLs) on transport was also examined. A clean-bed colloid deposition model was used to calculate deposition rate coefficients (k) for quantitative assessment. Significant cell retention was observed in the sand (81% and 82% for glucose- and hexadecane-grown cells, respectively). Addition of a low-concentration rhamnolipid solution enhanced cell transport, with 40 mg/L of rhamnolipid reducing retention to 50% and 60% for glucose- and hexadecane-grown cells, respectively. The k values for both glucose- and hexadecane-grown cells correlate linearly with rhamnolipid-dependent CSH represented as bacterial-adhesion-to-hydrocarbon rate of cells. Retention of cells by the soil was nearly complete (>99%). Addition of 40 mg/L rhamnolipid solution reduced retention to 95%. The presence of NAPLs in the sand increased the retention of hexadecane-grown cells with higher CSH. Transport of cells in the presence of the NAPL was enhanced by rhamnolipid at all concentrations tested, and the relative enhancement was greater than in was in the absence of NAPL. This study shows the importance of hydrophobic interaction on bacterial transport in natural porous media and the potential of using low-concentration rhamnolipid for facilitating the transport in subsurface for bioaugmentation efforts.

Effect of low-concentration rhamnolipid on transport of Pseudomonas aeruginosa ATCC 9027 in an ideal porous medium with hydrophilic or hydrophobic surfaces

The success of effective bioaugmentation processes for remediation of soil and groundwater contamination requires effective transport of the injected microorganisms in the subsurface environment. In this study, the effect of low concentrations of monorhamnolipid biosurfactant solutions on transport of Pseudomonas aeruginosa in an ideal porous medium (glass beads) with hydrophilic or hydrophobic surfaces was investigated by conducting miscible-displacement experiments. Transport behavior was examined for both glucose-grown and hexadecane-grown cells, with low and high surface hydrophobicity, respectively. A clean-bed colloid deposition model was used for determination of deposition rate coefficients. Results show that cells with high surface hydrophobicity exhibit greater retention than cells with low surface hydrophobicity. Rhamnolipid affects cell transport primarily by changing cell surface hydrophobicity, with an additional minor effect by increasing solution ionic strength. There is a good linear relation between k and rhamnolipid-regulated cell surface hydrophobicity presented as bacterial-adhesion-to-hydrocarbon (BATH) rate of cells (R(2)=0.71). The results of this study show the importance of hydrophobic interaction for transport of bacterial cells in silica-based porous media, and the potential of using low-concentration rhamnolipid solutions for facilitating bacterial transport in bioaugmentation efforts.

Impact of dissolved organic matter on bacterial tactic motility, attachment, and transport

Bacterial dispersal is a key driver of the ecology of microbial contaminant degradation in soils. This work investigated the role of dissolved organic matter (DOM) in the motility, attachment, and transport of the soil bacterium Pseudomonas putida G7 in saturated porous media. The study is based on the hypothesis that DOM quality is critical to triggering tactic motility and, consequently, affects bacterial transport and dispersal. Sunflower root exudates, humic acids (HA), and the synthetic oleophilic fertilizer S-200 were used as representatives of fresh, weathered, and artificially processed DOM with high nitrogen and phosphorus contents, respectively. We studied DOM levels of 16-130 mg L(-1), which are representative of DOM concentrations typically found in agricultural soil pore water. In contrast to its responses to HA and S-200, strain G7 exhibited a tactic behavior toward root exudates, as quantified by chemotaxis assays and single-cell motility observations. All DOM types promoted bacterial transport through sand at high concentrations (∼ 130 mg L(-1)). At low DOM concentrations (∼ 16 mg L(-1)), the enhancement occurred only in the presence of sunflower root exudates, and this enhancement did not occur with G7 bacteria devoid of flagella. Our results suggest that tactic DOM effectors strongly influence bacterial transport and the interception probability of motile bacteria by collector surfaces.

Effect of low-concentration rhamnolipid on adsorption of Pseudomonas aeruginosa ATCC 9027 on hydrophilic and hydrophobic surfaces

The effects of low-concentration monorhamnolipid (monoRL) on the adsorption of Pseudomonas aeruginosa ATCC 9027 grown on glucose or hexadecane to glass beads with hydrophobic or hydrophilic surfaces was investigated using batch adsorption experiments. Results showed that adsorption isotherms of the cells on both types of glass beads fitted the Freundlich equation better than the Langmuir equation. The Kf of the Freundlich equation for adsorption of hexadecane-grown cell to glass beads with hydrophobic surface was remarkably higher than that for adsorption of hexadecane-grown cell to glass beads with hydrophilic surface, or glucose-grown cell to glass beads with either hydrophilic or hydrophobic surface. Furthermore, it decreased with the increasing monoRL concentration. For both groups of cells, the zeta potential was close to each other and stable with the increase of monoRL concentration. The surface hydrophobicity of hexadecane-grown cells, however, was significantly higher than that of the glucose-grown cells and it decreased with the increase of monoRL concentration. The results indicate the importance of hydrophobic interaction on adsorption of bacterial cells to surfaces and monoRL plays a role in reducing the bacterial adsorption by affecting cell surface hydrophobicity.

Role of desorption kinetics in the rhamnolipid-enhanced biodegradation of polycyclic aromatic hydrocarbons

The main aim of this study was to investigate the effect of a rhamnolipid biosurfactant on biodegradation of (14)C-labeled phenanthrene and pyrene under desorption-limiting conditions. The rhamnolipid caused a significant solubilization and enhanced biodegradation of PAHs sorbed to soils. The enhancement was, however, negatively influenced by experimental conditions that caused an enrichment of slow desorption fractions. These conditions included aging, a higher organic matter content in soil, and previous extraction with Tenax to remove the labile-desorbing chemical. The decline in bioavailability caused by aging on sorbed (14)C-pyrene was partially reversed by rhamnolipids, which enhanced mineralization of the aged compound, although not so efficiently like with the unaged chemical. This loss in biosurfactant efficiency in promoting biodegradation can be explained by intra-aggregate diffusion of the pollutant during aging. We suggest that rhamnolipid can enhance biodegradation of soil-sorbed PAHs by micellar solubilization, which increase the cell exposure to the chemicals in the aqueous phase, and partitioning into soil organic matter, thus enhancing the kinetics of slow desorption. Our study show that rhamnolipid can constitute a valid alternative to chemical surfactants in promoting the biodegradation of slow desorption PAHs, which constitutes a major bottleneck in bioremediation.

Rhamnolipid surface thermodynamic properties and transport in agricultural soil

Rhamnolipid is a biosurfactant produced by several Pseudomonas species, which can wet hydrophobic soils by lowering the cohesive and/or adhesive surface tension. Because of its biodegradability, rhamnolipid applications bring minimal adverse impact on the soil and groundwater as compared with that of chemical wetting agents. Subsequently, rhamnolipid applications have more advantages when used to improve irrigation in the agricultural soil, especially under draught conditions. In the presence of rhamnolipid, water surface tension dropped linearly with the increase of rhamnolipid concentration until the rhamnolipid critical micelle concentration (CMC) of 30 mg/L was reached. Below the CMC, rhamnolipid had linear adsorption isotherms on the soil with a partition coefficient of 0.126 L/kg. Rhamnolipid transport breakthrough curves had a broad and diffuse infiltration front, indicating retention of rhamnolipid on the soil increased with time. Rhamnolipid transport was found to be well represented by the advection-dispersion equation based on a local equilibrium assumption. When applied at concentrations above the CMC, the formed rhamnolipid micelles prevented rhamnolipid adsorption (both equilibrium adsorption and kinetic adsorption) in the soil. It was discovered in this research that rhamnolipid surface thermodynamic properties played the key role in controlling rhamnolipid transport. The attractive forces between rhamnolipid molecules contributed to micelle formation and facilitated rhamnolipid transport.

Deposition and transport of Pseudomonas aeruginosa in porous media: lab-scale experiments and model analysis

In this study, the deposition and transport of Pseudomonas aeruginosa on sandy porous materials have been investigated under static and dynamic flow conditions. For the static experiments, both equilibrium and kinetic batch tests were performed at a 1:3 and 3:1 soil:solution ratio. The batch data were analysed to quantify the deposition parameters under static conditions. Column tests were performed for dynamic flow experiments with KCl solution and bacteria suspended in (1) deionized water, (2) mineral salt medium (MSM) and (3) surfactant + MSM. The equilibrium distribution coefficient (K(d)) was larger at a 1:3 (2.43 mL g(-1)) than that at a 3:1 (0.28 mL g(-1)) soil:solution ratio. Kinetic batch experiments showed that the reversible deposition rate coefficient (k(att)) and the release rate coefficient (k(det)) at a soil:solution ratio of 3:1 were larger than those at a 1:3 ratio. Column experiments showed that an increase in ionic strength resulted in a decrease in peak concentration of bacteria, mass recovery and tailing of the bacterial breakthrough curve (BTC) and that the presence of surfactant enhanced the movement of bacteria through quartz sand, giving increased mass recovery and tailing. Deposition parameters under dynamic condition were determined by fitting BTCs to four different transport models, (1) kinetic reversible, (2) two-site, (3) kinetic irreversible and (4) kinetic reversible and irreversible models. Among these models, Model 4 was more suitable than the others since it includes the irreversible sorption term directly related to the mass loss of bacteria observed in the column experiment. Applicability of the parameters obtained from the batch experiments to simulate the column breakthrough data is evaluated.

Laboratory-scale evaluation of a combined soil amendment for the enhanced biodegradation of propylene glycol-based aircraft de-icing fluids

A combined soil amendment was tested in microcosm experiments with an aim to enhance the aerobic biodegradation of propylene glycol (PG)-based aircraft de-icing fluids during and following the infiltration of contaminated snowmelt. A key objective under field conditions is to increase degradation of organic pollutants in the surface soil where higher microbial activity and plant rhizosphere effects may contribute to a more efficient biodegradation of PG, compared to subsoil ground layers, where electron acceptors and nutrients are often depleted. Microcosm experiments were set up in Petri dishes using 50 g of soil mixed with appropriate additives. The samples contained an initial de-icing fluid concentration of 10,000 mg/kg soil. A combined amendment using calcium peroxide, activated carbon and 1 x Hoagland solution resulted in significantly higher degradation rates for PG both at 4 and 22 degrees C. Most probable numbers of bacteria capable of utilizing 10,000 mg/kg de-icing fluid as a sole carbon source were about two orders of magnitude higher in the amended soil samples compared to unamended controls at both temperatures. The elevated numbers of such bacteria in surface soil may be a source of cells transported to the subsoil by snowmelt infiltration. The near-surface application of amendments tested here may enhance the growth of plants and plant roots in the contaminated area, as well as microbes to be found at greater depth, and hence increase the degradation of a contaminant plume present in the ground.

Transport and bacterial interactions of three bacterial strains in saturated column experiments

The impact of bacteria-solid and bacteria-bacteria interactions on the transport of Klebsiella oxytoca, Burkholderia cepacia G4PR1, and Pseudomonas sp. #5 was investigated in saturated sand column experiments (L = 114 mm; ø = 33 mm) under constant water velocities (∼ 5 cm · h(-1)). Bacterial strains were injected into the columns as pulses either individually, simultaneously, or successively. A one-dimensional mathematical model for advective-dispersive transport and for irreversible and reversible bacterial kinetic sorption was used to analyze the bacterial breakthrough curves. Different sorption parameters were obtained for each strain in each of the three experimental setups. In the presence of other bacteria, sorption parameters for B. cepacia G4PR1 remained similar to results from individual experiments, indicating the presence of other bacteria generally had a lesser influence on its migration than for the other bacteria. K. oxytoca is more competitive for the sorption sites when simultaneously injected with the other bacteria. Ps. sp. #5 generally yielded the greatest detachment rates and the least affinity to attach to the sand, indicative of its mobility in groundwater systems. The results of this study clearly indicate both bacteria-solid and bacteria-bacteria interactions influence the migration of bacteria. A more complete understanding of such interactions is necessary to determine potential migration in groundwater systems.

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.

Antiadhesive action of a marine microbial surfactant

The antiadhesive action of a lipopeptide biosurfactant from a marine bacterium was investigated. The effect of cultivation conditions on the adhesion property of few bacterial strains was studied. It was observed that the static cultures showed greater adhesion due to scarcity of oxygen. The biosurfactant upon surface conditioning was found to be effective in removal of the microbial adhesion at a concentration as low as 0.1 g L(-1). The percentages of inhibition of adhesion against different test bacterial strains ranged from 15 to 89% using 0.1-10 g L(-1) of purified biosurfactant. These percentages of adhesion inhibition were found to be significantly higher than the previously reported values. The antiadhesive efficacy of the biosurfactant was also evident from confocal laser scanning microscopy studies.

Adsorption of dirhamnolipid on four microorganisms and the effect on cell surface hydrophobicity

In this study, adsorption of dirhamnolipid biosurfactant on a Gram-negative Pseudomonas aeruginosa, two Gram-positive Bacillus subtilis, and a yeast, Candida lipolytica, was investigated, and the causality between the adsorption and change of cell surface hydrophobicity was discussed. The adsorption was not only specific to the microorganisms but also depended on the physiological status of the cells. Components of the biosurfactant with different rhamnosyl number or aliphatic chain length also exhibited slight difference in adsorption manner. The adsorption indeed caused the cell surface hydrophobicity to change regularly; however, the changes depended on both the concentrations of rhamnolipid solutions applied and the adsorbent physiological conditions. Orientation of rhamnolipid monomers on cell surface and micelle deposition are supposed to be the basic means of adsorption to change cell hydrophobicity at low and high rhamnolipid concentrations, respectively. This study proposed the possibility to modify cell surface hydrophobicity with biosurfactant of low concentrations, which may be of importance in in situ soil remediation.

Effects of rhamnolipid on degradation of granular organic substrate from kitchen waste by a Pseudomonas aeruginosa strain

The effect of rhamnolipid produced by a Pseudomonas aeruginosa strain on the aerobic degradation of granular organic substrate from kitchen waste by the bacterium was studied and compared with that of two synthetic surfactants, SDS and Triton X-100. The adsorption of rhamnolipid on the substrate, the surfactant-interfered adhesion of bacteria on the substrate as well as physicochemical and microbial conditions of the substrate during degradation were investigated. The adsorption isotherm of rhamnolipid on the substrate fit Freundlich law and its interactions with the substrate and bacteria weakened the adsorption of the bacteria on the substrate. The two synthetic surfactants, however, did not have such microbial effects. During degradation, rhamnolipid slowed down water evaporation in the substrate and significantly strengthened the dispersion of organic matter into the substrate water phase. The number of cells in the rhamnolipid treatment was higher than that in control and the remaining organic matter content in the substrates also had faster decreasing. SEM examination showed the on-site degradation of the substrate organic matter without rhamnolipid and the transfer of the degradation site in the presence of rhamnolipid. The results indicated that interference of rhamnolipid in the substrate matrix plays a potential role, physicochemically or microbially, on the degradation of the granular organic substrate. SDS and Triton X-100 may have the above physicochemical effects, but not so significant.

Adsorption of surfactants on a Pseudomonas aeruginosa strain and the effect on cell surface lypohydrophilic property

The adsorption behavior of five surfactants, cetyltrimethylammonium bromide (CTAB), Triton X-100, Tween 80, sodium dodecyl sulfate (SDS), and rhamnolipid, on a Pseudomonas aeruginosa strain and the effect of temperature and ionic strength (IS) on the adsorption were studied. The change of cell surface lypohydrophilic property caused by surfactant adsorption was also investigated. The results showed that the adsorption kinetics of the surfactants on the cell followed the second-order law. CTAB adsorption was the fastest one under the experimental conditions, and it took longest for SDS adsorption to equilibrate because of electric repulsion. The adsorption of Triton X-100 and Tween 80 was characterized by short equilibration time, and rhamnolipid adsorption reached equilibrium in about 90 min. The adsorption isotherms of all the surfactants on the bacterium fitted Freundlich equation well, but the adsorption capacity and mode were variations for the surfactants as indicated by k and n parameters in the equations. The adsorption mode for all the surfactants except SDS is probably hydrophilic interaction because the adsorption totally turned the cell surface to be more hydrophobic. Neither the temperature nor the IS had significant effect on CTAB adsorption, but higher IS significantly enhanced SDS adsorption and modestly strengthened adsorption of Triton X-100, Tween 80, and rhamnolipid. Higher temperature strengthened adsorption of SDS but weakened the adsorption of Triton X-100, Tween 80, and rhamnolipid.

Interference in adhesion of bacteria and yeasts isolated from explanted voice prostheses to silicone rubber by rhamnolipid biosurfactants

AIMS

The effects and extent of adhesion of four different bacterial and two yeast strains isolated from explanted voice prostheses to silicone rubber with and without an adsorbed rhamnolipid biosurfactant layer obtained from Pseudomonasaeruginosa DS10-129 was studied.

METHODS AND RESULTS

The ability of rhamnolipid biosurfactant to inhibit adhesion of micro-organisms to silicone rubber was investigated in a parallel-plate flow chamber. The anti-adhesive activity of the biosurfactant at different concentrations was significant against all the strains and depended on the micro-organism tested. The results showed an effective reduction in the initial deposition rates, and the number of bacterial cells adhering after 4 h, for all micro-organisms tested at the 4 g l(-1) undiluted rhamnolipid solution. Maximum initial reduction of adhesion rate (an average of 66%) occurred for Streptococcus salivarius GB 24/9 and Candida tropicalis GB 9/9. The number of cells adhering after 4 h on silicone rubber conditioned with biosurfactant was reduced to 48% for Staphylococcus epidermidis GB 9/6, Strep. salivarius GB 24/9, Staphylococcus aureus GB 2/1 and C. tropicalis GB 9/9 in comparison to controls. Perfusing the flow chamber with biosurfactant containing solution followed by the passage of a liquid-air interface, to investigate detachment of micro-organisms adhering to silicone rubber, produced high detachment (96%) of adhered cells for all micro-organisms studied, except for Staph. aureus GB 2/1 (67%).

SIGNIFICANCE AND IMPACT OF THE STUDY

It is concluded that biosurfactant represent suitable compounds that should be considered in developing future strategies to prevent the microbial colonization of silicone rubber voice prostheses.

Sorption kinetics of Escherichia coli and Salmonella sp on two soil layers associated with a groundwater table in Yaounde, Cameroon (Central Africa)

A laboratory study has been carried out on two soil layers (H[X] and H[Y]) located above a groundwater table in Yaounde, Cameroon (Central Africa). The main purpose of this study was to assess the retention potential or sorption kinetics of Escherichia coli and Salmonella sp. on these soil layers. For both soil layers, bacterial sorption on soil particles occurred rapidly during the first 30 minutes of incubation of bacteria and soil particles in aqueous media, and increased gradually with incubation time up to 300 min. In some cases, adsorption rates fluctuated after 30 min of incubation, probably due to bacterial cell sorption to and de-sorption from soil particles. Using Freundlich isotherms, it was noted that adsorption coefficient related to adsorption capacity varied from 19 to 4026 E. coli.mg(-1) of soil, and from 506 to 847 Salmonella sp.mg(-1) of soil. For both bacterial species, the adsorption coefficient of layer H[Y] (located in close proximity of the water table) was greater than that of H[X] (located above layer H[Y]) and seemed to positively correlate with the pH values and N/P ratios, and to negatively correlate with the values of C/N and C/P ratios. The linearity coefficient related to adsorption intensity varied from 0.5841 to 1.0023 for E. coli, and from 0.7068 to 1.5236 for Salmonella sp. The physico-chemical characteristics of soil particles seemed to influence the sorption kinetics of bacteria on soil.

Biosurfactants: potential applications in medicine

The use and potential commercial application of biosurfactants in the medical field has increased during the past decade. Their antibacterial, antifungal and antiviral activities make them relevant molecules for applications in combating many diseases and as therapeutic agents. In addition, their role as anti-adhesive agents against several pathogens indicates their utility as suitable anti-adhesive coating agents for medical insertional materials leading to a reduction in a large number of hospital infections without the use of synthetic drugs and chemicals. This review looks at medicinal and therapeutic perspectives on biosurfactant applications.

The influence of system complexity on bacterial transport in saturated porous media

A series of miscible-displacement column experiments were conducted under saturated flow conditions to systematically investigate the influence of physical and biological complexity on bacterial activity and fate in the presence and absence of a non-sorbing growth substrate, salicylate. Bacterial elution was monitored for three different systems; System I--a sterilized, inoculated, well-sorted sand, System II--a sterilized, inoculated, heterogeneous loamy sand (Hayhook), and System III--two different unsterilized loamy sands (Hayhook and Vinton) each with their associated indigenous microbial community. Results show that System I behaved ideally with respect to both cell and substrate transport, wherein: (1) growth occurred in response to substrate addition, (2) cell elution increased in response to the substrate pulse, and (3) breakthrough curves were reproducible for both substrate and cell elution. In contrast, System II showed ideal behavior with respect to substrate transport but showed variable behavior for cell transport. Further, there was no measurable growth in response to substrate addition and no increase in cell elution during the salicylate pulse. System III exhibited non-ideal behavior for both substrate and cell transport. Of particular interest is the fact that the indigenous communities of the two soils behaved differently. Specifically, for the Hayhook soil, an increased elution response was observed for the heterotrophic population while the salicylate-degrading community was preferentially retained in the column. In contrast for the Vinton soil, the substrate pulse did not elicit an elution response from either the heterotrophic or salicylate-degrading community from the culturable, indigenous Vinton microorganisms. For Systems II and III, the observed variability appears to be associated with the biological component of the system, since sterile controls were reproducible. This type of systematic study is critical for understanding cell and substrate transport behavior in complex, heterogeneous systems, and illustrates the potential uncertainty associated with measurements in such systems.

Transport of gellan gum microbeads through sand: an experimental evaluation for encapsulated cell bioaugmentation

Transport of 10-40 microm gellan gum microbeads was studied in horizontal sand columns to evaluate the feasibility of using gel-encapsulated bacteria for bioaugmentation of contaminated aquifers. Three 5.2 x 110 cm columns were packed with sand (column A: 0.5-2 mm, column B: 0.25-2 mm, and column C: 0.125-2 mm). Microbeads in artificial groundwater were injected at 0.5 l h(-1) during intermittent 12-h periods. Breakthrough of microbeads increased with injection time, varying as a descending function of travel distance. After 72 h of injection, about 75% of injected microbeads were dispersed across a 5-110 cm distance from the inlet in column A, compared to 78% across a 5-50 cm in column B, and 76% across a 5-20 cm in column C. The wider dispersion of microbeads across the length of column A, compared to those observed in columns B and C, suggests a higher potential for the formation of a uniform bioactive zone of encapsulated cells across a sandy aquifer with such grain size distribution and hydrodynamic properties.

Influence of biosurfactants produced by Candida antarctica on surface properties of microorganism and biodegradation of n-alkanes

The objective of this study was to investigate the influence of the biosurfactant BS-UC on surface properties of microbial cells and biodegradation of petroleum hydrocarbons. The biosurfactant BS-UC was produced by Candida antarctica from n-undecane as the substrate. It was found that the addition of BS-UC influenced positively the emulsification and the biodegradation of a variety of n-alkanes substrates. Besides, BS-UC also changed the hydrophobicity and zeta potential of the cell surface, and thus made the microbial cell attach to the hydrophobic substrate easily. The cell transport experiment also indicated that the addition of BS-UC changed both zeta potential of the cell and that of the porous media, and improved the retention of the cells in the media. So it will make BS-UC a promising choice for use in bioremediation of petroleum contamination.

Enhancing transport of hydrogenophaga flava ENV735 for bioaugmentation of aquifers contaminated with methyl tert-butyl ether

The gasoline oxygenate methyl tert-butyl ether (MTBE) has become a widespread contaminant in groundwater throughout the United States. Bioaugmentation of aquifers with MTBE-degrading cultures may be necessary to enhance degradation of the oxygenate in some locations. However, poor cell transport has sometimes limited bioaugmentation efforts in the past. The objective of this study was to evaluate the transport characteristics of Hydrogenophaga flava ENV735, a pure culture capable of growth on MTBE, and to improve movement of the strain through aquifer solids. The wild-type culture moved only a few centimeters in columns of aquifer sediment. An adhesion-deficient variant (H. flava ENV735:24) of the wild-type strain that moved more readily through sediments was obtained by sequential passage of cells through columns of sterile sediment. Hydrophobic and electrostatic interaction chromatography revealed that the wild-type strain is much more hydrophobic than the adhesion-deficient variant. Electrophoretic mobility assays and transmission electron microscopy showed that the wild-type bacterium contains two distinct subpopulations, whereas the adhesion-deficient strain has only a single, homogeneous population. Both the wild-type strain and adhesion-deficient variant degraded MTBE, and both were identified by 16S rRNA analysis as pure cultures of H. flava. The effectiveness of surfactants for enhancing transport of the wild-type strain was also evaluated. Many of the surfactants tested were toxic to ENV735; however, one nonionic surfactant, Tween 20, enhanced cell transport in sand columns. Improving microbial transport may lead to a more effective bioaugmentation strategy for MTBE-contaminated sites where indigenous oxygenate degraders are absent.

Effect of surfactants on the survival and sorption of viruses

There is an increasing concern about the protection of groundwater from contamination by enteric viruses and the prevention of outbreaks of waterborne diseases. Knowledge of survivability and transport of viruses from their point of origin is necessary to determine their potential effects on the neighboring groundwater systems. The distribution of virus is, in turn, dependent on the physical and chemical compositions of the surrounding soil and subsurface systems. For the present study, we have determined the effects of different surfactants (cationic, anionic, nonionic, and biological) and natural organic matter (NOM) on bacteriophages. Results indicated that surfactants and NOM adversely affect phage survival in binary systems, with surfactants being the most harmful. Studies with ternary systems also showed that the presence of surfactants reduced sorption of phages on sorbents either by occupying available sorption sites on the sorbent material or by displacing the sorbed phages from the sorbent surface. Water contact angles of the selected phages and different sorbent surfaces have been measured. Experimental data demonstrated that the sorption of hydrophobic viruses was favored by hydrophobic sorbents, while the sorption of hydrophilic viruses was favored by hydrophilic sorbents.