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

Rhamnolipid-induced alleviation of bioclogging in Managed Aquifer Recharge (MAR): Interactions with bacteria and porous media

The prevention and treatment of bioclogging is of great significance to the application of Managed Aquifer Recharge (MAR). This study investigated the alleviating effect of biosurfactant rhamnolipid (RL) on bioclogging by laboratory-scale percolation experiments. The results show that the addition of RL greatly reduced bioclogging. Compared with the group without RL, the relative hydraulic conductivity (K') of the 100 mg/L RL group increased 5 times at the end of the experiment (23 h), while the bacterial cell amount and extracellular polymeric substances (EPS) content on the sand column surface (0-2 cm) decreased by 60.8% and 85.7%, respectively. In addition, the richness and diversity of the microbial communities within the clogging matter decreased after the addition of RL. A variety of bacterial phyla were found, among which Proteobacteria were predominant in all groups. At the genus level, RL reduced the relative abundance of Acinetobacter, Bacillus, Klebsiella, and Pseudomonas. These microbes are known as strong adhesion, large size, and easy to form biofilms, therefore playing a critical role during MAR bioclogging. Moreover, RL changed the surface properties of bacteria and porous media, which results in the increase of electrostatic repulsion and decrease of hydrophobic interaction between them. Therefore, RL mediated the bacteria-porous media interaction to reduce biomass in porous media, thereby alleviating bioclogging. This study implies that RL's addition is an environmentally friendly and effective method to alleviate the bioclogging in MAR.

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.

Effect of Sodium Lauryl Sulfate on Sorption of Cells of the Electrogenic Bacterium Strain Micrococcus luteus on Carbon Cloth

Results of a study into the effect of anionic surfactant sodium lauryl sulfate on the sorption of cells of the electrogenic bacteria strain Micrococcus luteus 1-I on the surface of carbon cloth used as electrodes in microbial fuel cell (MFC) technology are presented. Investigations using spectrophotometry, microscopy and microbiology revealed an increase in the degree of sorption of microbial cells on carbon cloth under the action of sodium lauryl sulfate at concentrations of 10 and 100 mg/l. The sorption of cells did not significantly differ from the control at a surfactant content of 200, 400 and 800 mg/l. It had no negative effect on bacterial growth in the concentration range from 10 to 800 mg/l. Due to the fairly high resistance of the electrogenic strain M. luteus 1-I to sodium lauryl sulfate, a widespread component of wastewater, it may be considered as a prospective bioagent for the treatment of domestic wastewater using MFC technology.

Injection strategy for effective bacterial delivery in bioaugmentation scheme by controlling ionic strength and pore-water velocity

For the in-situ remediation of the contaminated subsurface environment, the injection of nutrients and microorganisms changes chemical and physical conditions, which control the delivery and immobilization of microorganisms. We investigated the injection strategy for effective bacterial delivery in a bioaugmentation scheme by controlling ionic strength (IS) and pore-water velocity (v). A set of bacterial transport tests was conducted using the saturated sand column to mimic the saturated subsurface environment. The effectiveness of the injection strategies was evaluated by applying solutions with different ionic strengths into the sand column with different pore-water velocities. The deposition and delivery of bacteria through the sand column were analyzed using the first-order deposition model. The deposition and delivery of bacteria injected by various strategies were numerically simulated considering the variable deposition rate. The breakthrough curves from column experiments revealed that the bacterial deposition on the sand surface was increased by an increase in the ionic strength and by a decrease in the pore-water velocities. The rates of bacterial deposition (k₁) on sand could be determined as a function of ionic strength and pore-water velocity, and it was applicable to simulate the delivery of bacteria under dynamic groundwater conditions. The numerical case study considering various injection strategies showed that the nutrient concentration controlled the bacterial delivery to the target area more significantly than the injection flow rate. Injection of bacterial solution with lower nutrient concentration could be increased the deposited bacterial concentration at the target point (S_tp) by 6.2-7.1 times higher. Short pulse injection with a high injection rate decreased S_tp by 67-78%. The efficiency of bacterial delivery (E_d) could be increased three times higher by lowering nutrient concentration in the injection solution. The process of evaluating the efficiency of bacterial delivery could be a prominent approach to determining the injection strategy for in-situ remediation considering variable conditions of a contaminated site.

Optical Tracking of Surfactant-Tuned Bacterial Adhesion: a Single-Cell Imaging Study

Probing the interfacial dynamics of single bacterial cells in complex environments is crucial for understanding the microbial biofilm formation process and developing antifouling materials, but it remains a challenge. Here, we studied single bacterial interfacial behaviors modulated by surfactants via a plasmonic imaging technique. We quantified the adhesion strength of single bacterial cells by plasmonic measurement of potential energy profiles and dissected the mechanism of surfactant-tuned single bacterial adhesion. The presence of surfactant tuned single bacterial adhesion by increasing the thickness of extracellular polymeric substances (EPS) and reducing the degree of EPS cross-linking. The adhesion kinetics and equilibrium state of bacteria attached to the surface confirmed the decrease in adhesion strength tuned by surfactants. The information obtained is valuable for understanding the interaction mechanism between a single bacterial cell and surface, developing new biofilm control strategies, and designing anticontamination materials. IMPORTANCE Studying the interfacial dynamic of single bacteria in complex environments is crucial for understanding the microbial biofilm formation process and developing antifouling materials. However, quantifying the interactions between microorganisms and surfaces in the presence of pollution at the single-cell level remains a great challenge. This paper presents the analysis of single bacterial interfacial behaviors modulated by surfactants and quantification of the adhesion strength via a plasmonic imaging technique. Our study provided insights into the mechanism of initial bacterial adhesion, facilitating our understanding of the adhesion process at the microscopic scale, and is of great value for controlling membrane fouling biofilm formation.

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.

A review of the role of biosurfactants in the biodegradation of hydrophobic organopollutants: production, mode of action, biosynthesis and applications

The increasing influence of human activity and industrialization has adversely impacted the environment via pollution with organic contaminants, which are minimally soluble in water. These hydrophobic organopollutants may be present in sediment, water or biota and have created concern due to their toxic effects in mammals. The ability of microorganisms to degrade pollutants makes their use the most effective, inexpensive and ecofriendly method for environmental remediation. Microorganisms have the ability to produce natural surfactants (biosurfactants) that increase the bioavailability of hydrophobic organopollutants, which enables their use as carbon and energy sources. Due to microbial diversity in production, and the biodegradability, nontoxicity, stability and specific activity of the surfactants, the use of microbial surfactants has the potential to overcome problems associated with contamination by hydrophobic organopollutants.This review provides an overview of the current state of knowledge regarding microbial surfactant production, mode of action in the biodegradation of hydrophobic organopollutants and biosynthetic pathways as well as their applications using emergent strategy tools to remove organopollutants from the environment. It is also specified for the first time that biosurfactants are produced either as growth-associated products or secondary metabolites, and are produced in different amounts by a wide range of microorganisms.

Crude oil biodegradation potential of biosurfactant-producing Pseudomonas aeruginosa and Meyerozyma sp

This study investigates the potential of crude oil degrading capabilities of biosurfactant-producing strains of Pseudomonas aeruginosa MF069166 and Meyerozyma sp. MF138126. P. aeruginosa produced mono-/di-rhamnolipids congeners whereas, Meyerozyma sp. produced acidic and lactonic forms of sophorolipids with crude oil. The values of critical micelle concentrations of rhamnolipids and sophorolipids were 40 mg/L and 50 mg/L with reductions in surface tension of water to 29 mN/m and 33 mN/m. Dynamic light scattering revealed that the average diameter of micellar aggregates of rhamnolipids ranged between 300 and 350 nm and the average size of sophorolipids micelles was 309 nm and 380 nm. Biosurfactants from P. aeruginosa and Meyerozyma sp. exhibited emulsification activities of 87% and 84% in crude oil. Cell surface hydrophobicity of both strains was higher in the presence of hydrophobic contaminants. The biosurfactants showed stability under varying pH, NaCl concentrations and temperatures. Gravimetric and GC-MS analyses demonstrated that P. aeruginosa degraded 91% of the petroleum hydrocarbons while Meyerozyma sp. showed 87% biodegradation efficiency. P. aeruginosa and Meyerozyma sp. have also been found to degrade halogen-containing compounds and showed excellent crude oil degradation efficiency. It is concluded that both strains have high potential of applications in the bioremediation of hydrocarbons-contaminated sites.

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.

Extreme environments: a source of biosurfactants for biotechnological applications

The surfactant industry moves billions of dollars a year and consists of chemically synthesized molecules usually derived from petroleum. Surfactant is a versatile molecule that is widely used in different industrial areas, with an emphasis on the petroleum, biomedical and detergent industries. Recently, interest in environmentally friendly surfactants that are resistant to extreme conditions has increased because of consumers' appeal for sustainable products and industrial processes that often require these characteristics. With this context, the need arises to search for surfactants produced by microorganisms coming from extreme environments and to mine their unique biotechnological potential. The production of biosurfactants is still incipient and presents challenges regarding economic viability due to the high costs of cultivation, production, recovery and purification. Advances can be made by exploring the extreme biosphere and bioinformatics tools. This review focuses on biosurfactants produced by microorganisms from different extreme environments, presenting a complete overview of what information is available in the literature, including the advances, challenges and future perspectives, as well as showing the possible applications of extreme biosurfactants.

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.

Rhamnolipid-aided biodegradation of carbendazim by Rhodococcus sp. D-1: Characteristics, products, and phytotoxicity

We successfully isolated Rhodococcus sp. D-1, an efficient carbendazim-degrading bacterium that degraded 98.20% carbendazim (200ppm) within 5days. Carbendazim was first processed into 2-aminobenzimidazole, converted to 2-hydroxybenzimidazole, and then further mineralized by subsequent processing. After genomic analysis, we hypothesized that D-1 may express a new kind of enzyme capable of hydrolyzing carbendazim. In addition, the effect of the biodegradable biosurfactant rhamnolipid on the rate and extent of carbendazim degradation was assessed in batch analyses. Notably, rhamnolipid affected carbendazim biodegradation in a concentration-dependent manner with maximum biodegradation efficiency at 50ppm (at the critical micelle concentration, CMC) (97.33% degradation within 2days), whereas 150ppm (3 CMC) rhamnolipid inhibited initial degradation (0.01%, 99.26% degradation within 2 and 5days, respectively). Both carbendazim emulsification and favorable changes in cell surface characteristics likely facilitated its direct uptake and subsequent biodegradation. Moreover, rhamnolipid facilitated carbendazim detoxification. Collectively, these results offer preliminary guidelines for the biological removal of carbendazim from the environment.

Effects of rhamnolipids on microorganism characteristics and applications in composting: A review

Biosurfactant rhmnolipids have been applied in many fields, especially in environmental bioremediation. According to previous researches, many research groups have studied the influence of rhamnolipids on microorganism characteristics and/or its application in composting. In this review, the effects of rhamnolipids on the cell surface properties of microorganisms was discussed firstly, such as cell surface hydrophobicity (CSH), electrical, surface compounds, etc. Moreover, the deeper mechanisms were also discussed, such as the effects of rhamnolipids on the structural characteristics and functional characteristics of the cell membrane, and the effects of rhamnolipids on the related enzymes and genes. Additionally, the application of rhamnolipids in composting was discussed, which is an important way for pollutant biodegradation and resource reutilization. It is believed that rhamnolipids will play more and more important role in composting.

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.

Sub-CMC solubilization of dodecane by rhamnolipid in saturated porous media

Experiments were conducted with a two-dimensional flow cell to examine the effect of monorhamnolipid surfactant at sub-CMC concentrations on solubilization of dodecane in porous media under dynamic flow conditions. Quartz sand was used as the porous medium and artificial groundwater was used as the background solution. The effectiveness of the monorhamnolipid was compared to that of SDBS, Triton X-100, and ethanol. The results demonstrated the enhancement of dodecane solubility by monorhamnolipid surfactant at concentrations lower than CMC. The concentrations (50–210 μM) are sufficiently low that they do not cause mobilization of the dodecane. Retention of rhamnolipid in the porous medium and detection of nano-size aggregates in the effluent show that the solubilization is based on a sub-CMC aggregate-formation mechanism, which is significantly stronger than the solubilization caused by the co-solvent effect. The rhamnolipid biosurfactant is more efficient for the solubilization compared to the synthetic surfactants. These results indicate a strategy of employing low concentrations of rhamnolipid for surfactant-enhanced aquifer remediation (SEAR), which may overcome the drawbacks of using surfactants at hyper-CMC concentrations.