Comparative studies on the structural composition, surface/interface activity and application potential of rhamnolipids produced by Pseudomonas aeruginosa using hydrophobic or hydrophilic substrates

A Rare Mono-Rhamnolipid Congener Efficiently Produced by Recombinant Pseudomonas aeruginosa YM4 via the Expression of Global Transcriptional Regulator irrE

Rhamnolipids (RLs) are widely used biosurfactants produced mainly by Pseudomonas aeruginosa and Burkholderia spp. in the form of mixtures of diverse congeners. The global transcriptional regulator gene irrE from radiation-tolerant extremophiles has been widely used as a stress-resistant element to construct robust producer strains and improve their production performance. A PrhlA-irrE cassette was constructed to express irrE genes in the Pseudomonas aeruginosa YM4 of the rhamnolipids producer strain. We found that the expression of irrE of Deinococcus radiodurans in the YM4 strain not only enhanced rhamnolipid production and the strain's tolerance to environmental stresses, but also changed the composition of the rhamnolipid products. The synthesized rhamnolipids reached a maximum titer of 26 g/L, about 17.9% higher than the original, at 48 h. The rhamnolipid production of the recombinant strain was determined to be mono-rhamnolipids congener Rha-C10-C12, accounting for 94.1% of total products. The critical micelle concentration (CMC) value of the Rha-C10-C12 products was 62.5 mg/L and the air-water surface tension decreased to 25.5 mN/m. The Rha-C10-C12 products showed better emulsifying activity on diesel oil than the original products. This is the first report on the efficient production of the rare mono-rhamnolipids congener Rha-C10-C12 and the first report that the global regulator irrE can change the components of rhamnolipid products in Pseudomonas aeruginosa.

Microbe cultivation guidelines to optimize rhamnolipid applications

In the growing landscape of interest in natural surfactants, selecting the appropriate one for specific applications remains challenging. The extensive, yet often unsystematized, knowledge of microbial surfactants, predominantly represented by rhamnolipids (RLs), typically does not translate beyond the conditions presented in scientific publications. This limitation stems from the numerous variables and their interdependencies that characterize microbial surfactant production. We hypothesized that a computational recipe for biosynthesizing RLs with targeted applicational properties could be developed from existing literature and experimental data. We amassed literature data on RL biosynthesis and micellar solubilization and augmented it with our experimental results on the solubilization of triglycerides (TGs), a topic underrepresented in current literature. Utilizing this data, we constructed mathematical models that can predict RL characteristics and solubilization efficiency, represented as logPRL = f(carbon and nitrogen source, parameters of biosynthesis) and logMSR = f(solubilizate, rhamnolipid (e.g. logPRL), parameters of solubilization), respectively. The models, characterized by robust R2 values of respectively 0.581-0.997 and 0.804, enabled the ranking of descriptors based on their significance and impact-positive or negative-on the predicted values. These models have been translated into ready-to-use calculators, tools designed to streamline the selection process for identifying a biosurfactant optimally suited for intended applications.

High mono-rhamnolipids production by a novel isolate Pseudomonas aeruginosa LP20 from oily sludge: characterization, optimization, and potential application

This study aims to isolate microbial strains for producing mono-rhamnolipids with high proportion. Oily sludge is rich in petroleum and contains diverse biosurfactant-producing strains. A biosurfactant-producing strain LP20 was isolated from oily sludge, identified as Pseudomonas aeruginosa based on phylogenetic analysis of 16S rRNA. High-performance liquid chromatography-mass spectrometry results indicated that biosurfactants produced from LP20 were rhamnolipids, mainly containing Rha-C8-C10, Rha-C10-C10, Rha-Rha-C8-C10, Rha-Rha-C10-C10, Rha-C10-C12:1, and Rha-C10-C12. Interestingly, more mono-rhamnolipids were produced by strain LP20 with a relative abundance of 64.5%. Pseudomonas aeruginosa LP20 optimally produced rhamnolipids at a pH of 7.0 and a salinity of 0.1% using glycerol and nitrate. The culture medium for rhamnolipids by strain LP20 was optimized by response surface methodology. LP20 produced rhamnolipids up to 6.9 g L-1, increased by 116%. Rhamnolipids produced from LP20 decreased the water surface tension to 28.1 mN m-1 with a critical micelle concentration of 60 mg L-1. The produced rhamnolipids emulsified many hydrocarbons with EI24 values higher than 56% and showed antimicrobial activity against Staphylococcus aureus and Cladosporium sp. with inhibition rates 48.5% and 17.9%, respectively. Pseudomonas aeruginosa LP20 produced more proportion of mono-rhamnolipids, and the LP20 rhamnolipids exhibited favorable activities and promising potential in microbial-enhanced oil recovery, bioremediation, and agricultural biocontrol.

Electrodialysis treatment of rhamnolipids hydrolysate and its waste water for use as water-soluble fertilizer

Rhamnolipids can serve as a precursor for rhamnose production, but using ion exchange resin in purifying rhamnolipids hydrolysate results in excessive high-salinity wastewater, making the process environmentally and economically unfeasible. This study introduced electrodialysis technology as an alternative for purifying rhamnolipids hydrolysate, significantly reducing wastewater to less than 5 % compared to the resin method. To achieve zero wastewater discharge, the electrodialysis-treated wastewater was repurposed into a water-soluble fertilizer containing 7.1 g/L of rhamnolipids, 11.4 g/L of fatty acid, 2.4 g/L of amino acid, and 8.2 g/L of potassium. Unlike traditional fertilizers, the nutritional components with rhamnolipids showed remarkable potential in enhancing tomato plant growth, flowering, and fruit quality. Taken together, the electrodialysis treatment of rhamnolipids hydrolysate largely reduced the water volume, the economic cost, and took a full use of the final wastewater as efficient water-soluble fertilizers, making it applicable for large-scale rhamnose production.

Chemobiocatalytic transfromation of biomass into furfurylamine with mixed amine donor in an eco-friendly medium

Biobased furfurylamine (FAM) is a versatile platform molecule for producing additives, pharmaceuticals, and pesticides. Recombinant E. coli HNND-AlaDH was created by co-expressing L-alanine dehydrogenase (AlaDH) and mutated Aspergillus terreus ω-transaminase (HNND), aiming to convert furfural (FUR) into FAM using inexpensive L-alanine and isopropylamine as mixed amine donors. In ChCl:FA:OA (10 wt%), pineapple peel, bagasse, barley shell, peanut shell, and corn stalk could be efficiently transformed into FUR under 170 °C for 10 min. Pineapple peel produced a high titer of FUR (183.3 mM). Additionally, the viscosity, surface tension and polarity of ChCl:FA:OA were explored. The biomass-derived FUR was fully transformed to FAM by HNND-AlaDH with amine donor (1:1:1 of L-Ala/isopropylamine/FUR mol/mol/mol) within 300 min. Accordingly, the FAM productivity was 0.58 g/(g xylan in pineapple peel). This chemobiocatalytic strategy established through the combination of chemocatalysis and biocatalysis could be applied to convert renewable biomass into valuable organic amines.

Effects of nutrient injection on the Xinjiang oil field microbial community studied in a long core flooding simulation device

Microbial Enhanced Oil Recovery (MEOR) is an option for recovering oil from depleted reservoirs. Numerous field trials of MEOR have confirmed distinct microbial community structure in diverse production wells within the same block. The variance in the reservoir microbial communities, however, remains ambiguously documented. In this study, an 8 m long core microbial flooding simulation device was built on a laboratory scale to study the dynamic changes of the indigenous microbial community structure in the Qizhong Block, Xinjiang oil field. During the MEOR, there was an approximate 34% upswing in oil extraction. Based on the 16S rRNA gene high-throughput sequencing, our results indicated that nutrition was one of the factors affecting the microbial communities in oil reservoirs. After the introduction of nutrients, hydrocarbon oxidizing bacteria became active, followed by the sequential activation of facultative anaerobes and anaerobic fermenting bacteria. This was consistent with the hypothesized succession of a microbial ecological "food chain" in the reservoir, which preliminarily supported the two-step activation theory for reservoir microbes transitioning from aerobic to anaerobic states. Furthermore, metagenomic results indicated that reservoir microorganisms had potential functions of hydrocarbon degradation, gas production and surfactant production. Understanding reservoir microbial communities and improving oil recovery are both aided by this work.

Genome mining of Pseudomonas spp. hints towards the production of under-pitched secondary metabolites

The recent advances in omics and computational analysis have enabled the capacity to identify the exclusive strain-specific metabolites and novel biosynthetic gene clusters. This study analyzed eight strains of P. aurantiaca including GS1, GS3, GS4, GS6, GS7, FS2, ARS38, PBSt2, one strain of P. chlororaphis RP4, one strain of P. aeruginosa (At1RP4), and one strain of P. fluorescens (RS1) for the production of rhamnolipids, quorum-sensing signals, and osmolytes. Seven rhamnolipid derivatives were variably detected in fluorescent pseudomonads. These rhamnolipids included Rha-C10-C8, Rha-Rha-C10-C10, Rha-C10-C12db, Rha-C10-C10, Rha-Rha-C10-C12, Rha-C10-C12, and Rha-Rha-C10-C12db. Pseudomonas spp. also showed the variable production of osmoprotectants including N-acetyl glutaminyl glutamine amide (NAGGN), betaine, ectoine, and trehalose. Betaine and ectoine were produced by all pseudomonads, however, NAGGN and trehalose were observed by five and three strains, respectively. Four strains including P. chlororaphis (RP4), P. aeruginosa (At1RP4), P. fluorescens (RS1), and P. aurantiaca (PBSt2) were exposed to 1- 4% NaCl concentrations and evaluated for the changes in phenazine production profile which were negligible. AntiSMASH 5.0 platform showed 50 biosynthetic gene clusters in PB-St2, of which 23 (45%) were classified as putative gene clusters with ClusterFinder algorithm, five (10%) were classified as non-ribosomal peptides synthetases (NRPS), five (10%) as saccharides, and four (8%) were classified as putative fatty acids. The genomic attributes and comprehensive insights into the metabolomic profile of these Pseudomonas spp. strains showcase their phytostimulatory, phyto-protective, and osmoprotective effects of diverse crops grown in normal and saline soils.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03607-x.

Rhamnolipid Self-Aggregation in Aqueous Media: A Long Journey toward the Definition of Structure–Property Relationships

The need to protect human and environmental health and avoid the widespread use of substances obtained from nonrenewable sources is steering research toward the discovery and development of new molecules characterized by high biocompatibility and biodegradability. Due to their very widespread use, a class of substances for which this need is particularly urgent is that of surfactants. In this respect, an attractive and promising alternative to commonly used synthetic surfactants is represented by so-called biosurfactants, amphiphiles naturally derived from microorganisms. One of the best-known families of biosurfactants is that of rhamnolipids, which are glycolipids with a headgroup formed by one or two rhamnose units. Great scientific and technological effort has been devoted to optimization of their production processes, as well as their physicochemical characterization. However, a conclusive structure–function relationship is far from being defined. In this review, we aim to move a step forward in this direction, by presenting a comprehensive and unified discussion of physicochemical properties of rhamnolipids as a function of solution conditions and rhamnolipid structure. We also discuss still unresolved issues that deserve further investigation in the future, to allow the replacement of conventional surfactants with rhamnolipids.

Microbial conversion of agro-processing waste (peanut meal) to rhamnolipid by Pseudomonas aeruginosa: solid-state fermentation, water extraction, medium optimization and potential applications

The high cost and severe foam in rhamnolipid fermentation are still bottlenecks for its industrial production and application. Non-foaming production of rhamnolipid by Pseudomonas aeruginosa FA1 was explored in solid-state fermentation using the agro-processing waste (peanut meal) as low-cost substrate. An environmental-friendly extraction method was developed to harvest rhamnolipid from solid-state culture. Strain FA1 produced 265.4 ± 8.2 mg rhamnolipid using 10 g peanut meal. HPLC-MS results revealed that 7 rhamnolipid homologues were produced, mainly including Rha-C₈-C₁₀ and Rha-Rha-C₁₀-C₁₀. Nitrate was the optimal nitrogen source. Peanut meal, MgSO₄ and CaCl₂ were significant factors for rhamnolipid production in solid-state fermentation. Rhamnolipid production was enhanced 31 % using the solid-state medium optimized by response surface method. The produced rhamnolipid reduced water surface tension to 28.1 ± 0.2 mN/m with a critical micelle concentration of 70 mg/L. The crude oil was emulsified with an emulsification index of 75.56 ± 1.29 %. The growth of tested bacteria and fungi was inhibited.

Enhanced production of surfactin using cassava wastewater and hydrophobic inducers: a prospection on new homologues

Biosurfactants are amphipathic molecules that can be applied in a wide range of areas. The cost of production limits the industrial application of biosurfactants. Nevertheless, the biosurfactant productivity can be easily enhanced by inducers. This work aimed to investigate the effect of hydrophobic inducers on surfactin production by B. subtilis ATCC 6633 using cassava wastewater as low-cost culture medium. The submerged cultivation was carried out at 30 °C, 150 rpm for 72 h. The fermentation parameters used were bacterial growth, consumption of sugars, and surfactin production, including surfactin homologues. The surface tension decreased by 40% after 12 h, when compared to control. Depletion of sugars was observed in all experiments. Palmitic acid led to the highest yield in terms of surfactin production (≈ 1.3 g·L^- 1 of pure surfactin). The inducers triggered the production of new surfactin homologues, that represent, potentially, new biological activities.

Cassava wastewater valorization for the production of biosurfactants: surfactin, rhamnolipids, and mannosileritritol lipids

The global production of cassava was estimated at ca. 303 million tons. Due to this high production, the cassava processing industry (cassava flour and starch) generates approximately ca. 0.65 kg of solid residue and ca. 25.3 l of wastewater per kg of fresh processed cassava root. The composition of the liquid effluent varies according to its origin; for example, the effluent from cassava flour production, when compared to the wastewater from the starch processing, presents a higher organic load (ca. 12 times) and total cyanide (ca. 29 times). It is worthy to highlight the toxicity of cassava residues regarding cyanide presence, which could generate disorders with acute or chronic symptoms in humans and animals. In this sense, the development of simple and low-cost eco-friendly methods for the proper treatment or reuse of cassava wastewater is a challenging, but promising path. Cassava wastewater is rich in macro-nutrients (proteins, starch, sugars) and micro-nutrients (iron, magnesium), enabling its use as a low-cost culture medium for biotechnological processes, such as the production of biosurfactants. These compounds are amphipathic molecules synthesized by living cells and can be widely used in industries as pharmaceutical agents, for microbial-enhanced oil recovery, among others. Amongst these biosurfactants, surfactin, rhamnolipids, and mannosileritritol lipids show remarkable properties such as antimicrobial, biodegradability, demulsifying and emulsifying capacity. However, the high production cost restricts the massive biosurfactant applications. Therefore, this study aims to present the state of the art and challenges in the production of biosurfactants using cassava wastewater as an alternative culture medium.

Optimization and characterization of rhamnolipids produced by Pseudomonas aeruginosa ATCC 9027 using molasses as a substrate

The present study aims to evaluate the growth potential of the P. aeruginosa ATCC9027 strain with molasses as the sole carbon source to produce rhamnolipids. The influence of the cultivation time and substrate concentration on biosurfactant production was investigated by using a complete 3-level factorial design, with the rhamnolipid concentration as the variable response. The strain was able to produce the biosurfactant in all design conditions tested, producing 758.04 mg/L rhamnolipids with 7% v/v substrate concentration in a cultivation time of 120 h. The substrate concentration used in the cultivation step directly influenced the biosurfactant production, and, even with the decrease in biomass growth, the biosurfactant production continued to increase. High Performance Liquid Chromatography (HPLC) revealed the presence of 62.3% mono- (RL1) and 37.6% di-rhamnolipids (RL3). The stability tests showed that the biosurfactant has good performance in extreme conditions of temperature, pH and saline concentration. The emulsification index was also evaluated for several oils and hydrocarbons, obtaining emulsification rates of up to 84.9% for the burnt motor oil. In addition, rhamnolipid showed a good ability to remove spilled oil from the sand, removing 58.51% of burnt motor oil and 70.09% of post-frying soybean oil. The results indicate that molasses, an agro-industrial residue abundant in Brazil, can be used as the only carbon source for quality rhamnolipid production when under optimized conditions, therefore presenting itself as a management option for this residue and, at the same time, providing the production product with high added value.

Comparative study on antimicrobial activity of mono-rhamnolipid and di-rhamnolipid and exploration of cost-effective antimicrobial agents for agricultural applications

Background

Chemical pesticides have defects in crop diseases control, such as narrow antimicrobial spectrum, chemicals residue risk and harm to farmland ecosystem. Antimicrobial agents from microbial sources are highly interested in agriculture. Studies showed that rhamnolipid biosurfactants possessed certain antimicrobial activity. The structural differences in rhamnolipid inevitably affect their activities. But the antimicrobial effect of mono-rhamnolipid and di-rhamnolipid is unknown. Rhamnolipid with unique structure can be produced using specific microbial cell factory.

Results

Different types of rhamnolipid were produced from different Pseudomonas aeruginosa strains. Rha-C₁₀-C₁₀ and Rha-Rha-C₁₀-C₁₀ were the main homologues in the separated mono-rhamnolipid and di-rhamnolipid, respectively. Both mono-rhamnolipid and di-rhamnolipid exhibited certain antimicrobial activity against the tested microbial strains, especially the fungi and Gram-positive bacteria. But mono-rhamnolipid was superior to di-rhamnolipid, with inhibition zone diameters larger than 25 mm and inhibition rate higher than 90%. The IC50 values of mono-rhamnolipid were lower than 5 mg/L against the tested bacterium and fungus, whereas the IC50 values of di-rhamnolipid were ranged from 10 mg/L to 20 mg/L. Mono-rhamnolipid stimulated the tested strains to generate higher level of intracellular ROS. Mono-rhamnolipid exhibited better antimicrobial activity to the potential agricultural pathogens, such as Alternaria alternata, Pantoea agglomerans and Cladosporium sp. The mono-rhamnolipid crude extract of strain P. aeruginosa SGΔrhlC can replace the separated mono-rhamnolipid. After 50 times dilution, the fermentation broth of the mono-rhamnolipid producing strain SGΔrhlC exhibited equal antimicrobial effect to mono-rhamnolipid (200 mg/L). Prospects of mono-rhamnolipid were also discussed for antimicrobial applications in agriculture.

Conclusions

This work discovered that mono-rhamnolipid was superior to di-rhamnolipid on antimicrobial activity for agricultural applications. Mono-rhamnolipid is an excellent candidate for agricultural biocontrol. The knockout strain P. aeruginosa SGΔrhlC is an excellent microbial cell factory for high producing mono-rhamnolipid. Its mono-rhamnolipid crude extract and its diluted fermentation broth are cost-effective antimicrobial agents. This work provided new insights to develop green and efficient antimicrobial agents for agricultural applications.

Limited Role of Rhamnolipids on Cadmium Resistance for an Endogenous-Secretion Bacterium

Rhamnolipids, a type of biosurfactant, represent a potential strategy for both enhancing organismic resistance and in situ remediation of heavy metals contaminations. In-depth study of the mechanism of rhamnolipids synthesis in response to heavy metals stress, is indispensable for a wide use of biosurfactant-secreting microbes in bioremediation. In this study, we employed the wild-type and the rhlAB deficient strain (ΔrhlAB) of Pseudomonas aeruginosa, a prototypal rhamnolipids-producing soil microorganism, to investigate its responses to cadmium resistance based on its physicochemical, and physiological properties. Compared with the wild-type strain, the ΔrhlAB were more sensitive to Cd-stress at low Cd concentration (<50 mg/L), whereas there was little difference in sensitivity at higher Cd concentrations, as shown by spot titers and cell viability assays. Secreted rhamnolipids reduced intracellular Cd²⁺ accumulation to alleviate Cd²⁺ stress, whereas endogenous rhamnolipids played a limited role in alleviating Cd²⁺ stress. Synthesized rhamnolipids exhibited a higher critical micelle concentration (CMC) (674.1 mg/L) and lower emulsification index (4.7%) under high Cd-stress, while these parameters showed no obvious changes. High Cd-stress resulted in high hydrophilic wild-type bacterial surface and lower bioremediation ability. This study could advance a deeper understanding of the mechanism of cadmium resistance and provide a theoretical foundation for the application of biosurfactant and biosurfactant-secreted bacterium in contaminant bioremediation.

Surface-Active Compounds Produced by Microorganisms: Promising Molecules for the Development of Antimicrobial, Anti-Inflammatory, and Healing Agents

Surface-active compounds (SACs), biomolecules produced by bacteria, yeasts, and filamentous fungi, have interesting properties, such as the ability to interact with surfaces as well as hydrophobic or hydrophilic interfaces. Because of their advantages over other compounds, such as biodegradability, low toxicity, antimicrobial, and healing properties, SACs are attractive targets for research in various applications in medicine. As a result, a growing number of properties related to SAC production have been the subject of scientific research during the past decade, searching for potential future applications in biomedical, pharmaceutical, and therapeutic fields. This review aims to provide a comprehensive understanding of the potential of biosurfactants and emulsifiers as antimicrobials, modulators of virulence factors, anticancer agents, and wound healing agents in the field of biotechnology and biomedicine, to meet the increasing demand for safer medical and pharmacological therapies.

Revealing the mechanisms of rhamnolipid enhanced hydrogen production from dark fermentation of waste activated sludge

Rhamnolipid (RL), as an environmentally compatible biosurfactant, has been used to enhance waste activated sludge (WAS) fermentation. However, the effect of RL on hydrogen accumulation in anaerobic fermentation remains unclear. Therefore, this work targets to investigate the mechanism of RL-based dark fermentation system on hydrogen production of WAS. It was found that the maximum yield of hydrogen increased from 1.76 ± 0.26 to 11.01 ± 0.30 mL/g VSS (volatile suspended solids), when RL concentration increased from 0 to 0.10 g/g TSS (total suspended solids). Further enhancement of RL level to 0.12 g/g TSS slightly reduced the production to 10.80 ± 0.28 mL/g VSS. Experimental findings revealed that although RL could be degraded to generate hydrogen, it did not play a major role in enhancing hydrogen accumulation. Mechanism analysis suggested that RL decreased the surface tension between sludge liquid and hydrophobic compounds, thus accelerating the solubilization of WAS, improving the proportion of biodegradable substances which could be used for subsequent hydrogen production. Regardless of the fact that adding RL suppressed all the fermentation processes, the inhibition effect of processes associated with hydrogen consumption was much severer than that of hydrogen production. Further investigations of microbial community revealed that RL enriched the relative abundance of hydrogen producers e.g., Romboutsia but reduced that of hydrogen consumers like Desulfobulbus and Caldisericum.

Biodegradation of waste cooking oil and simultaneous production of rhamnolipid biosurfactant by Pseudomonas aeruginosa P7815 in batch and fed-batch bioreactor

Biosurfactants are non-toxic, surface-active biomolecules capable of reducing surface tension (ST) and emulsifying interface at a comparably lower concentration than commercial surfactants. Yet, poor yield, costlier substrates, and complex cultivation processes limit their commercial applications. This study focuses on producing biosurfactants by Pseudomonas aeruginosa P7815 in batch and fed-batch bioreactor systems using waste cooking oil (WCO) as the sole carbon source. The batch study showed a 92% of WCO biodegradation ability of P. aeruginosa producing 11 g L^-1 of biosurfactant. To enhance this biosurfactant production, a fed-batch oil feeding strategy was opted to extend the stationary phase of the bacterium and minimize the effects of substrate deprivation. An enhanced biosurfactant production of 16 g L^-1 (i.e. 1.5 times of batch study) was achieved at a feed rate of 5.7 g L^-1d^-1 with almost 94% of WCO biodegradation activity. The biosurfactant was characterized as rhamnolipid using Fourier transform infrared spectroscopy (FTIR), and its interfacial characterization showed ST reduction to 29 ± 1 mN m^-1 and effective emulsification stability at pH value of 4, temperature up to 40 °C and salinity up to 40 g L^-1. The biosurfactant exhibited antibacterial activity with minimum inhibitory concentration (MIC) values of 100 µg mL^-1 and 150 µg mL^-1 for pathogenic E. hirae and E. coli, respectively. These findings suggest that biodegradation of WCO by P. aeruginosa in a fed-batch cultivation strategy is a potential alternative for the economical production of biosurfactants, which can be further explored for biomedical, cosmetics, and oil washing/recovery applications.

Application of surfactants in papermaking industry and future development trend of green surfactants

In this work, the application of chemical surfactants, including cooking aids, detergents, surface sizing agents, and deinking agents as core components, is introduced in the wet end of pulping and papermaking. This method for the combined application of enzymes and surfactants has expanded, promoting technological updates and improving the effect of surfactants in practical applications. Finally, the potential substitution of green surfactants for chemical surfactants is discussed. The source, classification, and natural functions of green surfactants are introduced, including plant extracts, biobased surfactants, fermentation products, and woody biomass. These green surfactants have advantages over their chemically synthesized counterparts, such as their low toxicity and biodegradability. This article reviews the latest developments in the application of surfactants in different paper industry processes and extends the methods of use. Additionally, the application potential of green surfactants in the field of papermaking is discussed. KEY POINTS: • Surfactants as important chemical additives in papermaking process are reviewed. • Deinking technologies by combined of surfactants and enzymes are reviewed. • Applications of green surfactant in papermaking industry are prospected.

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.

Exploiting Microbes in the Petroleum Field: Analyzing the Credibility of Microbial Enhanced Oil Recovery (MEOR)

Crude oil is a major energy source that is exploited globally to achieve economic growth. To meet the growing demands for oil, in an environment of stringent environmental regulations and economic and technical pressure, industries have been required to develop novel oil salvaging techniques. The remaining ~70% of the world’s conventional oil (one-third of the available total petroleum) is trapped in depleted and marginal reservoirs, and could thus be potentially recovered and used. The only means of extracting this oil is via microbial enhanced oil recovery (MEOR). This tertiary oil recovery method employs indigenous microorganisms and their metabolic products to enhance oil mobilization. Although a significant amount of research has been undertaken on MEOR, the absence of convincing evidence has contributed to the petroleum industry’s low interest, as evidenced by the issuance of 400+ patents on MEOR that have not been accepted by this sector. The majority of the world’s MEOR field trials are briefly described in this review. However, the presented research fails to provide valid verification that the microbial system has the potential to address the identified constraints. Rather than promising certainty, MEOR will persist as an unverified concept unless further research and investigations are carried out.

Production and analysis of capsules containing microorganisms consortiated for future application in petroleum bioremediation

Oil spills cause severe environmental and economic impacts, so the use of bioremediation techniques has been widely studied to solve this problem. Due to its complex composition, the oil prevents the full action of microorganisms, and in this way, the microbial consortium encapsulation technique is an innovation in the use of bacteria and biomass in the face of possible oil degradation, with the possibility of overcoming techniques such as bio-enhancement and biostimulation in the face of factors such as nutrient availability, oxygenation and temperature. Therefore, this work aims to produce capsules containing microbiological consortium and analyze its characteristics using the techniques TGA, DSC, FESEM, viable cell count, emulsification index and surface tension, in order to propose the best conditions to be applied. TGA and DSC results showed that the capsules have thermal stability in the range of 25-40 °C. Viable cell counts were more effective in capsules containing 1% (w/v) sodium alginate, and the emulsification index showed a large increase (80%) from day 5, as well as surface tension had a large drop (48%) in the same period. The increase in the emulsification index is caused by the increase in the production of biosurfactants (amphipathic molecules) by the bacteria consortium and this offers a greater contact between the microorganisms and the oil, providing best conditions for the degradation of oil. Therefore, all analyzes showed excellent results for future application in oil spills.

Advanced Simulation of Removing Chromium from a Synthetic Wastewater by Rhamnolipidic Bioflotation Using Hybrid Neural Networks with Metaheuristic Algorithms

This work aims at presenting an advanced simulation approach for a novel rhamnolipidic-based bioflotation process to remove chromium from wastewater. For this purpose, the significance of key influential operating variables including initial solution pH (2, 4, 6, 8, 10 and 12), rhamnolipid to chromium ratio (RL:Cr = 0.010, 0.025, 0.050, 0.075 and 0.100), reductant (Fe) to chromium ratio (Fe:Cr of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0), and air flowrate (50, 100, 150, 200 and 250 mL/min) were investigated and evaluated using Analysis of Variance (ANOVA) method. The RL as both collector and frother was produced using a pure strain of Pseudomonas aeruginosa MA01 under specific conditions. The bioflotation tests were carried out within a bubbly regimed column cell with the dimensions of 60 × 5.70 × 0.1 cm. Four optimization techniques based on Artificial Neural Network (ANN) including Cuckoo, genetic, firefly and biogeography-based optimization algorithms were applied to 113 experiments to identify the optimum values of studied factors. The ANOVA results revealed that all four variables influence the bioflotation performance through a non-linear trend. Their influences, except for aeration rate, were found statistically significant (p-value < 0.05), and all parameters followed the normal distribution according to Anderson-Darlin (AD) criterion. Maximum chromium removal of about 98% was achieved at pH of 6, rhamnolipid to chromium ratio of 0.05, air flowrate of 150 mL/min, and Fe to Cr ratio of 1.0. Flotation kinetics study indicated that chromium bioflotation follows the first-order kinetic model with a rate of 0.023 sec-1. According to the statistical assessment of the model accuracy, the firefly algorithm (FFA) with a structure of 4-9-1 yielded the highest level of reliability with the mean squared, root mean squared, percentage errors and correlation coefficient values of test-data of 0.0038, 0.0617, 3.08% and 96.92%, respectively. These values were evidences of the consistency of the well-structured ANN method to simulate the process.

Bio-cleaning Efficiency of Rhamnolipids Produced from Native Pseudomonas aeruginosa Grown on Agro-industrial By-products for Liquid Detergent Formulation

The cleaning activity of surface-active agents such as rhamnolipids (RLs) requires utmost effectiveness and is employed abundantly in various industries, particularly laundry cleaning, detergents, and cosmetics. In the current study, RLs were produced from Pseudomonas aeruginosa isolated from oil-contaminated soil using a minimal medium amended with agro-industrial by-products of refinery vegetable oil wastes (comprising of unsaturated types of fatty acids as determined by GC analysis) and dairy whey. The results showed that an amount of 5.72 g/L of RLs were obtained, while lower concentrations were obtained using chemically defined carbon sources. Ten congeners of mono- and di-RLs were detected by LC-MS, and they reduced the surface tension of water to 26 mN/m with a critical micelle concentration of 33 mg/L. Furthermore, the produced RLs showed promising cleaning and detergency properties in the removal of different stains on tested fabrics with a Stain Removal Index (SRI) of 17.45%. Moreover, an efficient cleaning was obtained when RLs were applied to a liquid detergent formulation model, and a cleaning power (∆E) of 245.95 and SRI of 36.28% were achieved. The present work showed that the produced RLs could be exploited as a powerful and alternative eco-friendly cleaning agent in many industries.

Foaming of rhamnolipids fermentation: impact factors and fermentation strategies

Rhamnolipids have recently attracted considerable attentions because of their excellent biosurfactant performance and potential applications in agriculture, environment, biomedicine, etc., but severe foaming causes the high cost of production, restraining their commercial production and applications. To reduce or eliminate the foaming, numerous explorations have been focused on foaming factors and fermentation strategies, but a systematic summary and discussion are still lacking. Additionally, although these studies have not broken through the bottleneck of foaming, they are conducive to understanding the foaming mechanism and developing more effective rhamnolipids production strategies. Therefore, this review focuses on the effects of fermentation components and control conditions on foaming behavior and fermentation strategies responded to the severe foaming in rhamnolipids fermentation and systematically summarizes 6 impact factors and 9 fermentation strategies. Furthermore, the potentialities of 9 fermentation strategies for large-scale production are discussed and some further strategies are suggested. We hope this review can further facilitate the understanding of foaming factors and fermentation strategies as well as conducive to developing the more effective large-scale production strategies to accelerate the commercial production process of rhamnolipids.

Enhanced production of mono-rhamnolipid in Pseudomonas aeruginosa and application potential in agriculture and petroleum industry

Differences in the rhamnolipid structures must result in its different activities, thus affecting its application effect. The rhlC gene in Pseudomonas aeruginosa SG was knocked out to construct strain P. aeruginosa SGΔrhlC. Rhamnolipid production was enhanced by 23.3% through knocking out rhlC gene. P. aeruginosa SGΔrhlC produced 14.22 g/L of rhamnolipid using glycerol and nitrate. Five kinds of mono-rhamnolipid but no di-rhamnolipid were produced by strain SGΔrhlC. The main rhamnolipid homologues were Rha-C₁₀-C₁₀, Rha-C₁₀-C_12:1 and Rha-C₁₀-C₁₂. Mono-rhamnolipid exhibited better antimicrobial activity to Escherichia coli, Staphylococcus aureus, Aspergillus niger and Penicillium chrysogenum. Rhamnolipid produced from strain SGΔrhlC showed greater emulsifying activity to crude oil with EI₂₄ of 84.73%. Rhamnolipid produced from strain SGΔrhlC efficiently washed oily sludge at 35 °C. High-producing strain P. aeruginosa SGΔrhlC and its produced mono-rhamnolipid are more promising in agriculture and petroleum industry. This study is a step forward to the tailor-made biosynthesis and application of rhamnolipid.

Biosurfactant inducers for enhanced production of surfactin and rhamnolipids: an overview

Biosurfactants can be widely used in industries as pharmaceutical agents, for microbial enhanced oil recovery, crop biostimulation, among others. Surfactin and rhamnolipids are well-known biosurfactants. These compounds have several advantages over chemical surfactants, however they are not economically competitive, since their production cost is up to 12 times higher than chemical surfactants. In this sense, an interesting approach is to replace synthetic culture medium, which represents ≈ 30% of the production cost by agro-industrial wastes. In addition, biosurfactant productivity can be easily enhanced by inductor supplementation into culture medium that triggers biosurfactant metabolism. Biosurfactant inducers are mainly a pool of hydrophobic molecules (e.g. olive oil-saturated and unsaturated fatty acids, proteins and vitamins). Nevertheless, there is little information on inducer effects of specific molecules (e.g. oleic acid). In general, hydrophobic inducers lead to higher fatty acid chain lengths (biosurfactant chemical structure). Therefore, the aim of this review was to critically discuss the current state of the art and future trends on biosurfactant production, in particular biosurfactant inducers. Taking into account the last 10 years, there is a clear lack of information on correlation between "inducers" or "hydrophobic inducers" AND "biosurfactants", since only 13 documents were found (Scopus database). Thus, it is essential to deeply investigate all inducer effects on biosurfactant production, mainly yield and chemical structure.

Rhamnolipids from Planococcus spp. and their mechanism of action against pathogenic bacteria

Two bacterial species with the ability to produce biosurfactants were isolated from a pesticide contaminated soil and identified as Planococcus rifietoensis IITR53 and Planococcus halotolerans IITR55. Formation of froth indicating the surfactant production was observed when grown in basal salt medium containing 2% glucose. The culture supernatant after 72 h showed reduction in surface tension from 72 N/m to 46 and 42 N/m for strain IITR53 and IITR55 with emulsification index of 51 and 54% respectively. The biosurfactant identified as rhamnolipid based on liquid chromatography-mass spectrometry analysis, was found to inhibit the growth of both gram- positive and negative pathogenic bacteria. Both the rhamnolipids at 40 mg/mL exhibited the release of extracellular DNA and protein content. Also at one third of the MIC, a significant generation of reactive oxygen species was recorded. These rhamnolipids effectively emulsified different vegetable oils suggesting their possible utilization as antimicrobial agent.