Rhamnolipid biosurfactants produced by Renibacterium salmoninarum 27BN during growth on n-hexadecane

Microplastic polymer properties as deterministic factors driving terrestrial plastisphere microbiome assembly and succession in the field

Environmental microplastic (MP) is ubiquitous in aquatic and terrestrial ecosystems providing artificial habitats for microbes. Mechanisms of MP colonization, MP polymer impacts, and effects on soil microbiomes are largely unknown in terrestrial systems. Therefore, we experimentally tested the hypothesis that MP polymer type is an important deterministic factor affecting MP community assembly by incubating common MP polymer types in situ in landfill soil for 14 months. 16S rRNA gene amplicon sequencing indicated that MP polymers have specific impacts on plastisphere microbiomes, which are subsets of the soil microbiome. Chloroflexota, Gammaproteobacteria, certain Nitrososphaerota, and Nanoarchaeota explained differences among MP polymers and time points. Plastisphere microbial community composition derived from different MP diverged over time and was enriched in potential pathogens. PICRUSt predictions of pathway abundances and quantitative PCR of functional marker genes indicated that MP polymers exerted an ambivalent effect on genetic potentials of biogeochemical cycles. Overall, the data indicate that (i) polymer type as deterministic factor rather than stochastic factors drives plastisphere community assembly, (ii) MP impacts greenhouse gas metabolism, xenobiotic degradation and pathogen distribution, and (iii) MP serves as an ideal model system for studying fundamental questions in microbial ecology such as community assembly mechanisms in terrestrial environments.

Microbial surfactants: characteristics, production and broader application prospects in environment and industry

Microbial surfactants are green molecules with high surface activities having the most promising advantages over chemical surfactants including their ability to efficiently reducing surface and interfacial tension, nontoxic emulsion-based formulations, biocompatibility, biodegradability, simplicity of preparation from low cost materials such as residual by-products and renewable resources at large scales, effectiveness and stabilization under extreme conditions and broad spectrum antagonism of pathogens to be part of the biocontrol strategy. Thus, biosurfactants are universal tools of great current interest. The present work describes the major types and microbial origin of surfactants and their production optimization from agro-industrial wastes in the batch shake-flasks and bioreactor systems through solid-state and submerged fermentation industries. Various downstream strategies that had been developed to extract and purify biosurfactants are discussed. Further, the physicochemical properties and functional characteristics of biosurfactants open new future prospects for the development of efficient and eco-friendly commercially successful biotechnological product compounds with diverse potential applications in environment, industry, biomedicine, nanotechnology and energy-saving technology as well.

Supramolecular bioamphiphile facilitated bioemulsification and concomitant treatment of recalcitrant hydrocarbons in petroleum refining industry oily waste

Bioremediation of real-time petroleum refining industry oily waste (PRIOW) is a major challenge due to the poor emulsification potential and oil sludge disintegration efficiency of conventional bioamphiphile molecules. The present study was focused on the design of a covalently engineered supramolecular bioamphiphile complex (SUBC) rich in hydrophobic amino acids for proficient emulsification of hydrocarbons followed by the concomitant degradation of total petroleum hydrocarbons (TPH) in PRIOW using the hydrocarbonoclastic microbial bio-formulation system. The synthesis of SUBC was carried out by pH regulated microbial biosynthesis process and the yield was obtained to be 450.8 mg/g of petroleum oil sludge. The FT-IR and XPS analyses of SUBC revealed the anchoring of hydrophilic moieties of monomeric bioamphiphilic molecules, resulting in the formation of SUBC via covalent interaction. The SUBC was found to be lipoprotein in nature. The maximum loading capacity of SUBC onto surface modified rice hull (SMRH) was achieved to be 45.25 mg/g SMRH at the optimized conditions using RSM-CCD design. The SUBC anchored SMRH was confirmed using SEM, FT-IR, XRD and TGA analyses. The adsorption isotherm models of SUBC onto SMRH were performed. The integrated approach of SUBC-SMRH and hydrocarbonoclastic microbial bio-formulation system, emulsified oil from PRIOW by 92.86 ± 2.26% within 24 h and degraded TPH by 89.25 ± 1.75% within 4 days at the optimum dosage ratio of SUBC-SMRH (0.25 g): PRIOW (1 g): mass of microbial-assisted biocarrier material (0.05 g). The TPH degradation was confirmed by SARA fractional analysis, FT-IR, ¹H NMR and GC-MS analyses. The study suggested that the application of covalently engineered SUBC has resulted in the accelerated degradation of real-time PRIOW in a very short duration without any secondary sludge generation. Thus, the SUBC integrated approach can be considered to effectively manage the hydrocarbon contaminants from petroleum refining industries under optimal conditions.

Current biotechnologies on depolymerization of polyethylene terephthalate (PET) and repolymerization of reclaimed monomers from PET for bio-upcycling: A critical review

The production of polyethylene terephthalate (PET) has drastically increased in the past half-century, reaching 30 million tons every year. The accumulation of this recalcitrant waste now threatens diverse ecosystems. Despite efforts to recycle PET wastes, its rate of recycling remains limited, as the current PET downcycling is mostly unremunerative. To address this problem, PET bio-upcycling, which integrates microbial depolymerization of PET followed by repolymerization of PET-derived monomers into value-added products, has been suggested. This article critically reviews current understanding of microbial PET hydrolysis, the metabolic mechanisms involved in PET degradation, PET hydrolases, and their genetic improvement. Furthermore, this review includes the use of meta-omics approaches to search PET-degrading microbiomes, microbes, and putative hydrolases. The current development of biosynthetic technologies to convert PET-derived materials into value-added products is also comprehensively discussed. The integration of various depolymerization and repolymerization biotechnologies enhances the prospects of a circular economy using waste PET.

Biosequestration of lignin in municipal landfill leachate by tailored cationic lipoprotein biosurfactant through Bacillus tropicus valorized tannery solid waste

Bioremediation of municipal landfill leachate (MLL) is often intricate due to presence of refractory lignin. In the present study, it was attempted to tailor the histidine rich protein moiety of cationic lipoprotein biosurfactant (CLB) to sequester the lignin from MLL. Animal fleshing (AF), the solid waste generated in tanning industry was utilized for the production of histidine rich CLB by de novo substrate dependent synthesis pathway involving Bacillus tropicus. The optimum conditions for the maximum production of CLB were determined using response surface methodology. At the optimized conditions, the maximum yield of CLB was 217.4 mg/g AF (on dry basis). The produced histidine rich CLB was purified using Immobilized metal affinity chromatography at the optimum binding and elution conditions. The histidine residues were more pronounced in the CLB, as determined by HPLC analysis. The CLB was further characterized by SDS-PAGE, Zeta potential, XRD, FT-IR, Raman, NMR, GC-MS and TG analyses. The CLB was immobilized onto functionalized nanoporous activated bio carbon (FNABC) and the optimum immobilization capacity was found to be 211.6 mg/g FNABC. The immobilization of CLB onto FNABC was confirmed using SEM, FT-IR, XRD and TG analyses. The isotherm models, kinetic and thermodynamics studies of CLB immobilization onto FNABC were performed to evaluate its field level application. Subsequently, the CLB-FNABC was then applied for the sequestration of lignin in MLL. The maximum lignin sequestration was achieved by 92.5 mg/g CLB-FNABC at the optimized sequestration time, 180 min; pH, 5; temperature, 45 °C and mass of CLB-FNABC, 1.0 g. The sequestration of lignin by CLB- FNABC was confirmed by SEM, FT-IR and UV-Vis analyses. Further, the mechanistic study revealed the anchoring of CLB onto the surface of lignin through electrostatic interaction.

Development and Genetic Engineering of Hyper-Producing Microbial Strains for Improved Synthesis of Biosurfactants

Current research energies are fixated on the synthesis of environmentally friendly and non-hazardous products, which include finding and recognizing biosurfactants that can substitute synthetic surfactants. Microbial biosurfactants are surface-active compounds synthesized intracellularly or extracellularly. To use biosurfactants in various industries, it is essential to understand scientific engagements that demonstrate its potentials as real advancement in the 21st century. Other than applying a substantial effect on the world economic market, engineered hyper-producing microbial strains in combination with optimized cultivation parameters have made it probable for many industrial companies to receive the profits of 'green' biosurfactant innovation. There needs to be an emphasis on the worldwide state of biosurfactant synthesis, expression of biosurfactant genes in expressive host systems, the recent developments, and prospects in this line of research. Thus, molecular dynamics with respect to genetic engineering of biosynthetic genes are proposed as new biotechnological tools for development, improved synthesis, and applications of biosurfactants. For example, mutant and hyper-producing recombinants have been designed efficaciously to advance the nature, quantity, and quality of biosurfactants. The fastidious and deliberate investigation will prompt a comprehension of the molecular dynamics and phenomena in new microorganisms. Throughout the decade, valuable data on the molecular genetics of biosurfactant have been produced, and this solid foundation would encourage application-oriented yields of the biosurfactant production industry and expand its utilization in diverse fields. Therefore, the conversations among different interdisciplinary experts from various scientific interests such as microbiology, biochemistry, molecular biology, and genetics are indispensable and significant to accomplish these objectives.

Microbial biosurfactant research: time to improve the rigour in the reporting of synthesis, functional characterization and process development

The demand for microbially produced surface-active compounds for use in industrial processes and products is increasing. As such, there has been a comparable increase in the number of publications relating to the characterization of novel surface-active compounds: novel producers of already characterized surface-active compounds and production processes for the generation of these compounds. Leading researchers in the field have identified that many of these studies utilize techniques are not precise and accurate enough, so some published conclusions might not be justified. Such studies lacking robust experimental evidence generated by validated techniques and standard operating procedures are detrimental to the field of microbially produced surface-active compound research. In this publication, we have critically reviewed a wide range of techniques utilized in the characterization of surface-active compounds from microbial sources: identification of surface-active compound producing microorganisms and functional testing of resultant surface-active compounds. We have also reviewed the experimental evidence required for process development to take these compounds out of the laboratory and into industrial application. We devised this review as a guide to both researchers and the peer-reviewed process to improve the stringency of future studies and publications within this field of science.

Double bond localization in unsaturated rhamnolipid precursors 3-(3-hydroxyalkanoyloxy)alkanoic acids by liquid chromatography-mass spectrometry applying online Paternò-Büchi reaction

Lipids are biomolecules with a broad variety of chemical structures, which renders them essential not only for various biological functions but also interestingly for biotechnological applications. Rhamnolipids are microbial glycolipids with surface-active properties and are widely used biosurfactants. They are composed of one or two L-rhamnoses and up to three hydroxy fatty acids. Their biosynthetic precursors are 3-hydroxy(alkanoyloxy)alkanoic acids (HAAs). The latter are also present in cell supernatants as complex mixtures and are extensively studied for their potential to replace synthetically derived surfactants. The carbon chain lengths of HAAs determine their physical properties, such as their abilities to foam and emulsify, and their critical micelle concentration. Despite growing biotechnological interest, methods for structural elucidation are limited and often rely on hydrolysis and analysis of free hydroxy fatty acids losing the connectivity information. Therefore, a high-performance liquid chromatography-mass spectrometry method was developed for comprehensive structural characterization of intact HAAs. Information is provided on chain length and number of double bonds in each hydroxy fatty acid and their linkage by tandem mass spectrometry (MS/MS). Post-column photochemical derivatization by online Paternὸ-Büchi reaction and MS/MS fragmentation experiments generated diagnostic fragments allowing structural characterization down to the double bond position level. Furthermore, the presented experiments demonstrate a powerful approach for structure elucidation of complex lipids by tailored fragmentation.

Microbial Degradation and Valorization of Plastic Wastes

A growing accumulation of plastic wastes has become a severe environmental and social issue. It is urgent to develop innovative approaches for the disposal of plastic wastes. In recent years, reports on biodegradation of synthetic plastics by microorganisms or enzymes have sprung up, and these offer a possibility to develop biological treatment technology for plastic wastes. In this review, we have comprehensively summarized the microorganisms and enzymes that are able to degrade a variety of generally used synthetic plastics, such as polyethylene (PE), polystyrene (PS), polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PUR), and polyethylene terephthalate (PET). In addition, we have highlighted the microbial metabolic pathways for plastic depolymerization products and the current attempts toward utilization of such products as feedstocks for microbial production of chemicals with high value. Taken together, these findings will contribute to building a conception of bio-upcycling plastic wastes by connecting the biodegradation of plastic wastes to the biosynthesis of valuable chemicals in microorganisms. Last, but not least, we have discussed the challenges toward microbial degradation and valorization of plastic wastes.

Exploiting the Natural Diversity of RhlA Acyltransferases for the Synthesis of the Rhamnolipid Precursor 3-(3-Hydroxyalkanoyloxy)Alkanoic Acid

While rhamnolipids of the Pseudomonas aeruginosa type are commercially available, the natural diversity of rhamnolipids and their origin have barely been investigated. Here, we collected known and identified new rhlA genes encoding the acyltransferase responsible for the synthesis of the lipophilic rhamnolipid precursor 3-(3-hydroxyalkanoyloxy)alkanoic acid (HAA). Generally, all homologs were found in Betaproteobacteria and Gammaproteobacteria A likely horizontal gene transfer event into Actinobacteria is the only identified exception. The phylogeny of the RhlA homologs from Pseudomonas and Burkholderia species is consistent with the organism phylogeny, and genes involved in rhamnolipid synthesis are located in operons. In contrast, RhlA homologs from the Enterobacterales do not follow the organisms' phylogeny but form their own branch. Furthermore, in many Enterobacterales and Halomonas from the Oceanospirillales, an isolated rhlA homolog can be found in the genome. The RhlAs from Pseudomonas aeruginosa PA01, Pseudomonas fluorescens LMG 05825, Pantoea ananatis LMG 20103, Burkholderia plantarii PG1, Burkholderia ambifaria LMG 19182, Halomonas sp. strain R57-5, Dickeya dadantii Ech586, and Serratia plymuthica PRI-2C were expressed in Escherichia coli and tested for HAA production. Indeed, except for the Serratia RhlA, HAAs were produced with the engineered strains. A detailed analysis of the produced HAA congeners by high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) highlights the congener specificity of the RhlA proteins. The congener length varies from 4 to 18 carbon atoms, with the main congeners consisting of different combinations of saturated or monounsaturated C₁₀, C₁₂, and C₁₄ fatty acids. The results are discussed in the context of the phylogeny of this unusual enzymatic activity.IMPORTANCE The RhlA specificity explains the observed differences in 3-(3-hydroxyalkanoyloxy)alkanoic acid (HAA) congeners. Whole-cell catalysts can now be designed for the synthesis of different congener mixtures of HAAs and rhamnolipids, thereby contributing to the envisaged synthesis of designer HAAs.

Biotechnological upcycling of plastic waste and other non-conventional feedstocks in a circular economy

The envisaged circular economy requires absolute carbon efficiency and in the long run abstinence from fossil feedstocks, and integration of industrial production with end-of-life waste management. Non-conventional feedstocks arising from industrial production and societal consumption such as CO₂ and plastic waste may soon enable manufacture of multiple products from simple bulk chemicals to pharmaceuticals using biotechnology. The change to these feedstocks could be faster than expected by many, especially if the true cost, including the carbon footprint of products, is considered. The efficiency of biotechnological processes can be improved through metabolic engineering, which can help fulfill the promises of the Paris agreement.

Rhamnolipid from a Lysinibacillus sphaericus strain IITR51 and its potential application for dissolution of hydrophobic pesticides

Rhamnolipid produced from a Lysinibacillus sphaericus IITR51 was characterized and its ability for dissolution of hydrophobic pesticides were evaluated. L. sphaericus produced 1.6 g/L of an anionic biosurfactant that reduced surface tension from 72 N/m to 52 N/m with 48% emulsification index. The biosurfactant was found stable over a wide range of pH (4.0-10.0), temperature (4-100 °C), salt concentration (2-14%) and was identified as rhamnolipid. At the concentration of 90 mg/L rhamnolipid showed enhanced dissolution of α-, β-endosulfan, and γ-hexachlorocyclohexane up to 7.2, 2.9, and 1.8 folds, respectively. The bacterium utilized benzoic acid, chlorobenzene, 3- and 4-chlorobenzoic acid as sole source of carbon and was found resistant to arsenic, lead and cadmium. Furthermore, the isolated biosurfactant showed antimicrobial activities against different pathogenic bacteria. The results obtained indicate the usefulness of rhamnolipid for enhanced dissolution and thereby increasing the bioavailability.

Efficiency in hydrocarbon degradation and biosurfactant production by Joostella sp. A8 when grown in pure culture and consortia

Joostella strains are emerging candidates for biosurfactant production. Here such ability was analyzed for Joostella strain A8 in comparison with Alcanivorax strain A53 and Pseudomonas strain A6, all previously isolated from hydrocarbon enrichment cultures made of polychaete homogenates. In pure cultures Joostella sp. A8 showed the highest stable emulsion percentage (78.33%), hydrophobicity rate (62.67%), and an optimal surface tension reduction during growth in mineral medium supplemented with diesel oil (reduction of about 12mN/m), thus proving to be highly competitive with Alcanivorax and Pseudomonas strains. During growth in pure culture different level of biodegradation were detected for Alcanivorax strain A53 (52.7%), Pseudomonas strain A6 (38.2%) and Joostella strain A8 (26.8%). When growing in consortia, isolates achieved similar abundance values, with the best efficiency that was observed for the Joostella-Pseudomonas co-culture. Gas-chromatographic analysis revealed an increase in the biodegradation efficiency in co-cultures (about 90%), suggesting that the contemporary action of different bacterial species could improve the process. Results were useful to compare the efficiencies of well-known biosurfactant producers (i.e. Pseudomonas and Alcanivorax representatives) with a still unknown biosurfactant producer, i.e. Joostella, and to confirm them as optimal biosurfactant-producing candidates.

Rhamnolipids, Microbial Virulence Factors, in Alzheimer's Disease

Alzheimer's disease (AD) has been attributed to chronic bacterial infections. The recognition of human microbiota as a substantial contributor to health and disease is relatively recent and growing. During evolution, mammals live in a symbiotic state with myriads of microorganisms that survive at a diversity of tissue micro-surroundings. Microbes produce a plethora of secretory products [amyloids, lipopolysaccharides, virulence factors rhamnolipids (RLs), toxins, and a great number of neuroactive compounds]. The contribution of infectious microbial components to the pathophysiology of the human central nervous system including AD is considered potentially substantial, but the involvement of the RLs has never been reported. Here, RLs were isolated from serum and identified through various conventional methods including the colorimetric orcinol method, thin-layer chromatography, attenuated total reflection Fourier transform infrared (ATR-FTIR), and dot blot using antibodies against RLs. Dot blot demonstrated elevated RL levels in sera of AD patients compared to controls (p = 0.014). Moreover, ELISA showed similarly elevated RL levels in cerebrospinal fluid of both AD (0.188 versus 0.080) (p = 0.04) and mild cognitive impairment (0.188 versus 0.129) (p = 0.088) patients compared to healthy, and are well-correlated with the AD stages severity assessed using the Mini-Mental State Examination. These results provide conclusive evidence for the newly-reported implication of RLs in AD, adding it to the list of bacterial components, opening new avenues for AD investigation. Moreover, they strengthen and vindicate the divergence of research toward the exploration of bacterial involvement in AD generation and progression.

Adhesion of Lactobacilli and their anti-infectivity potential

The probiotic potential of lactic acid bacteria primarily point toward colonizing ability of Lactobacilli as the most important attribute for endowing all the known beneficial effects in a host. Lactobacillus species exert health-promoting function in the gastrointestinal tract through various mechanisms such as pathogen exclusion, maintenance of microbial balance, immunomodulation, and other crucial functions. It has been seen that many surface layer proteins are involved in host adhesion, and play significant role in the modification of some signaling pathways within the host cells. Interaction between different bacterial cell surface proteins and host receptor has been imperative for a better understanding of the mechanism through which Lactobacilli exert their health-promoting functions.

Microbial production of rhamnolipids: opportunities, challenges and strategies

Rhamnolipids are a class of biosurfactants which contain rhamnose as the sugar moiety linked to β-hydroxylated fatty acid chains. Rhamnolipids can be widely applied in many industries including petroleum, food, agriculture and bioremediation etc. Pseudomonas aeruginosa is still the most competent producer of rhamnolipids, but its pathogenicity may cause safety and health concerns during large-scale production and applications. Therefore, extensive studies have been carried out to explore safe and economical methods to produce rhamnolipids. Various metabolic engineering efforts have also been applied to either P. aeruginosa for improving its rhamnolipid production and diminishing its pathogenicity, or to other non-pathogenic strains by introducing the key genes for safe production of rhamnolipids. The three key enzymes for rhamnolipid biosynthesis, RhlA, RhlB and RhlC, are found almost exclusively in Pseudomonas sp. and Burkholderia sp., but have been successfully expressed in several non-pathogenic host bacteria to produce rhamnolipids in large scales. The composition of mono- and di-rhamnolipids can also be modified through altering the expression levels of RhlB and RhlC. In addition, cell-free rhamnolipid synthesis by using the key enzymes and precursors from non-pathogenic sources is thought to not only eliminate pathogenic effects and simplify the downstream purification processes, but also to circumvent the complexity of quorum sensing system that regulates rhamnolipid biosynthesis. The pathogenicity of P. aeruginosa can also be reduced or eliminated through in vivo or in vitro enzymatic degradation of the toxins such as pyocyanin during rhamnolipid production. The rhamnolipid production cost can also be significantly reduced if rhamnolipid purification step can be bypassed, such as utilizing the fermentation broth or the rhamnolipid-producing strains directly in the industrial applications of rhamnolipids.

Microbial rhamnolipid production: a critical re-evaluation of published data and suggested future publication criteria

High production cost and potential pathogenicity of Pseudomonas aeruginosa, commonly used for rhamnolipid synthesis, have led to extensive research for safer producing strains and cost-effective production methods. This has resulted in numerous research publications claiming new non-pathogenic producing strains and novel production techniques many of which are unfortunately without proper characterisation of product and/or producing strain/s. Genes responsible for rhamnolipid production have only been confirmed in P. aeruginosa, Burkholderia thailandensis and Burkholderia pseudomallei. Comparing yields in different publications is also generally unreliable especially when different methodologies were used for rhamnolipid quantification. After reviewing the literature in this area, we strongly feel that numerous research outputs have insufficient evidence to support claims of rhamnolipid-producing strains and/or yields. We therefore recommend that standards should be set for reporting new rhamnolipid-producing strains and production yields. These should include (1) molecular and bioinformatic tools to fully characterise new microbial isolates and confirm the presence of the rhamnolipid rhl genes for all bacterial strains, (2) using gravimetric methods to quantify crude yields and (3) use of a calibrated method (high-performance liquid chromatography or ultra-performance liquid chromatography) for absolute quantitative yield determination.

Production of rhamnolipids and diesel oil degradation by bacteria isolated from soil contaminated by petroleum

Biosurfactants are microbial secondary metabolites. The most studied are rhamnolipids, which decrease the surface tension and have emulsifying capacity. In this study, the production of biosurfactants, with emphasis on rhamnolipids, and diesel oil degradation by 18 strains of bacteria isolated from waste landfill soil contaminated by petroleum was analyzed. Among the studied bacteria, gram-positive endospore forming rods (39%), gram positive rods without endospores (17%), and gram-negative rods (44%) were found. The following methods were used to test for biosurfactant production: oil spreading, emulsification, and hemolytic activity. All strains showed the ability to disperse the diesel oil, while 77% and 44% of the strains showed hemolysis and emulsification of diesel oil, respectively. Rhamnolipids production was observed in four strains that were classified on the basis of the 16S rRNA sequences as Pseudomonas aeruginosa. Only those strains showed the rhlAB gene involved in rhamnolipids synthesis, and antibacterial activity against Escherichia coli, P. aeruginosa, Staphylococcus aureus, Bacillus cereus, Erwinia carotovora, and Ralstonia solanacearum. The highest production of rhamnolipids was 565.7 mg/L observed in mineral medium containing olive oil (pH 8). With regard to the capacity to degrade diesel oil, it was observed that 7 strains were positive in reduction of the dye 2,6-dichlorophenolindophenol (2,6-DCPIP) while 16 had the gene alkane mono-oxygenase (alkB), and the producers of rhamnolipids were positive in both tests. Several bacterial strains have shown high potential to be explored further for bioremediation purposes due to their simultaneous ability to emulsify, disperse, and degrade diesel oil. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 32:262-270, 2016.

Surfactants tailored by the class Actinobacteria

Globally the change towards the establishment of a bio-based economy has resulted in an increased need for bio-based applications. This, in turn, has served as a driving force for the discovery and application of novel biosurfactants. The class Actinobacteria represents a vast group of microorganisms with the ability to produce a diverse range of secondary metabolites, including surfactants. Understanding the extensive nature of the biosurfactants produced by actinobacterial strains can assist in finding novel biosurfactants with new potential applications. This review therefore presents a comprehensive overview of the knowledge available on actinobacterial surfactants, the chemical structures that have been completely or partly elucidated, as well as the identity of the biosurfactant-producing strains. Producer strains of not yet elucidated compounds are discussed, as well as the original habitats of all the producer strains, which seems to indicate that biosurfactant production is environmentally driven. Methodology applied in the isolation, purification and structural elucidation of the different types of surface active compounds, as well as surfactant activity tests, are also discussed. Overall, actinobacterial surfactants can be summarized to include the dominantly occurring trehalose-comprising surfactants, other non-trehalose containing glycolipids, lipopeptides and the more rare actinobacterial surfactants. The lack of structural information on a large proportion of actinobacterial surfactants should be considered as a driving force to further explore the abundance and diversity of these compounds. This would allow for a better understanding of actinobacterial surface active compounds and their potential for biotechnological application.

Rhamnolipid biosurfactants: evolutionary implications, applications and future prospects from untapped marine resource

Rhamnolipid-biosurfactants are known to be produced by the genus Pseudomonas, however recent literature reported that rhamnolipids (RLs) are distributed among diverse microbial genera. To integrate the evolutionary implications of rhamnosyl transferase among various groups of microorganisms, a comprehensive comparative motif analysis was performed amongst bacterial producers. Findings on new RL-producing microorganism is helpful from a biotechnological perspective and to replace infective P. aeruginosa strains which ultimately ensure industrially safe production of RLs. Halotolerant biosurfactants are required for efficient bioremediation of marine oil spills. An insight on the exploitation of marine microbes as the potential source of RL biosurfactants is highlighted in the present review. An economic production process, solid-state fermentation using agro-industrial and industrial waste would increase the scope of biosurfactants commercialization. Potential and prospective applications of RL-biosurfactants including hydrocarbon bioremediation, heavy metal removal, antibiofilm activity/biofilm disruption and greener synthesis of nanoparticles are highlighted in this review.

Oil degradation and biosurfactant production by the deep sea bacterium Dietzia maris As-13-3

Recent investigations of extreme environments have revealed numerous bioactive natural products. However, biosurfactant-producing strains from deep sea extreme environment are largely unknown. Here, we show that Dietzia maris As-13-3 isolated from deep sea hydrothermal field could produce di-rhamnolipid as biosurfactant. The critical micelle concentration (CMC) of the purified di-rhamnolipid was determined to be 120 mgL⁻¹, and it lowered the surface tension of water from 74 ± 0.2 to 38 ± 0.2 mN m⁻¹. Further, the alkane metabolic pathway-related genes and di-rhamnolipid biosynthesis-related genes were also analyzed by the sequencing genome of D. maris As-13-3 and quantitative real-time PCR (Q-PCR), respectively. Q-PCR analysis showed that all these genes were induced by n-Tetradecane, n-Hexadecane, and pristane. To the best of our knowledge, this is first report about the complete pathway of the di-rhamnolipid synthesis process in the genus Dietzia. Thus, our study provided the insights into Dietzia in respects of oil degradation and biosurfactant production, and will help to evaluate the potential of Dietzia in marine oil removal.

Influence of salinity and temperature on the activity of biosurfactants by polychaete-associated isolates

Influence of different parameters on biosurfactant (BS) activity was carried out on strains that were isolated from the polychaetes Megalomma claparedei, Sabella spallanzanii and Branchiomma luctuosum and additional 30 strains that were previously identified as potential BS producers from crude oil enrichments of the same polychaete specimens. The selection of BS-producing strains from polychaete natural samples was carried out by using standard screening tests. The BS activity by each isolate was evaluated for the effect of salinity and temperature on emulsion production and surface tension reduction, during incubation in mineral medium supplemented with tetradecane or diesel oil. All isolates showed a similar time course of BS activity, and the latter was more influenced by salinity rather than temperature. Some of the BS producers belonged to genera that have not (i.e. Citricoccus, Cellulophaga, Tenacibaculum and Maribacter) or have poorly been (Psychrobacter, Vibrio, and Pseudoalteromonas) reported as able to produce BSs. This is remarkable as some of them have previously been detected in hydrocarbon-enriched samples. Results confirm that filter-feeding polychaetes are an efficient source for the isolation of BS producers.

Lipoprotein biosurfactant production from an extreme acidophile using fish oil and its immobilization in nanoporous activated carbon for the removal of Ca2+and Cr3+in aqueous solution

Lipoprotein biosurfactant from extreme acidophile using fish oil and its immobilization in nanoporous activated carbon for removal of metal ions.

Inhibition of pathogenic bacterial biofilm by biosurfactant produced by Lysinibacillus fusiformis S9

A biosurfactant producing microbe isolated from a river bank was identified as Lysinibacillus fusiformis S9. It was identified with help of biochemical tests and 16S rRNA gene phylogenetic analysis. The biosurfactant S9BS produced was purified and characterized as glycolipid. The biosurfactant showed remarkable inhibition of biofilm formation by pathogenic bacteria like Escherichia coli and Streptococcus mutans. It was interesting to note that at concentration of 40 μg ml(-1) the biosurfactant did not show any bactericidal activity but restricted the biofilm formation completely. L. fusiformis is reported for the first time to produce a glycolipid type of biosurfactant capable of inhibiting biofilm formation by pathogenic bacteria. The biosurfactant inhibited bacterial attachment and biofilm formation equally well on hydrophilic as well as hydrophobic surfaces like glass and catheter tubing. This property is significant in many biomedical applications where the molecule should help in preventing biofouling of surfaces without being toxic to biotic system.

Potential benefit of surfactants in a hydrocarbon contaminated soil washing process: fluorescence spectroscopy based assessment

Soil washing is an ex situ soil remediation treatment process. The purpose of soil washing is to clean the major gravel and sand fractions, concentrating contamination into the fine silt and clay fractions. The addition of surfactants can improve the efficiency of this method. Here we report the use of UV fluorescence spectroscopy to assess the hydrocarbon cleaning process as a rapid and cost effective alternative to gas chromatography. Three wash solutions were tested on a total petroleum hydrocarbon contaminated soil: water, Sea Power 101 (SP101) at 1% (v/v) and Tween80 at 0.5% (w/v). The most effective to wash the gravel and sand was SP101 (54 and 65% improvement over the water only wash, respectively) which moved contamination to the silt fraction (94% of contaminants). Tween80 appeared not to enhance TPH removal efficiency from the gravel and sand fractions but did concentrate TPH in the effluent (95% more than water wash). In addition to TPH removal from gravel and sand, SP101 also showed potential benefit in the soil washing sedimentation process, enhancing sludge/water volume separation by 10% over the water only wash. In summary, fluorescence spectroscopy proved an effective technique to compare TPH removal efficiencies as part of soil washing laboratory based treatability testing.

Microbial communities to mitigate contamination of PAHs in soil--possibilities and challenges: a review

BACKGROUND, AIM, AND SCOPE

Although highly diverse and specialized prokaryotic and eukaryotic microbial communities in soil degrade polycyclic aromatic hydrocarbons (PAHs), most of these are removed slowly. This review will discuss the biotechnological possibilities to increase the microbial dissipation of PAHs from soil as well as the main biological and biotechnological challenges.

DISCUSSION AND CONCLUSIONS

Microorganism provides effective and economically feasible solutions for soil cleanup and restoration. However, when the PAHs contamination is greater than the microbial ability to dissipate them, then applying genetically modified microorganisms might help to remove the contaminant. Nevertheless, it is necessary to have a more holistic review of the different individual reactions that are simultaneously taking place in a microbial cell and of the interactions microorganism-microorganism, microorganism-plant, microorganism-soil, and microorganisms-PAHs.

PERSPECTIVES

Elucidating the function of genes from the PAHs-polluted soil and the study in pure cultures of isolated PAHs-degrading organisms as well as the generation of microorganisms in the laboratory that will accelerate the dissipation of PAHs and their safe application in situ have not been studied extensively. There is a latent environmental risk when genetically engineered microorganisms are used to remedy PAHs-contaminated soil.

Rhamnolipids: diversity of structures, microbial origins and roles

Rhamnolipids are glycolipidic biosurfactants produced by various bacterial species. They were initially found as exoproducts of the opportunistic pathogen Pseudomonas aeruginosa and described as a mixture of four congeners: alpha-L-rhamnopyranosyl-alpha-L-rhamnopyranosyl-beta-hydroxydecanoyl-beta-hydroxydecanoate (Rha-Rha-C(10)-C(10)), alpha-L-rhamnopyranosyl-alpha-L-rhamnopyranosyl-beta-hydroxydecanoate (Rha-Rha-C(10)), as well as their mono-rhamnolipid congeners Rha-C(10)-C(10) and Rha-C(10). The development of more sensitive analytical techniques has lead to the further discovery of a wide diversity of rhamnolipid congeners and homologues (about 60) that are produced at different concentrations by various Pseudomonas species and by bacteria belonging to other families, classes, or even phyla. For example, various Burkholderia species have been shown to produce rhamnolipids that have longer alkyl chains than those produced by P. aeruginosa. In P. aeruginosa, three genes, carried on two distinct operons, code for the enzymes responsible for the final steps of rhamnolipid synthesis: one operon carries the rhlAB genes and the other rhlC. Genes highly similar to rhlA, rhlB, and rhlC have also been found in various Burkholderia species but grouped within one putative operon, and they have been shown to be required for rhamnolipid production as well. The exact physiological function of these secondary metabolites is still unclear. Most identified activities are derived from the surface activity, wetting ability, detergency, and other amphipathic-related properties of these molecules. Indeed, rhamnolipids promote the uptake and biodegradation of poorly soluble substrates, act as immune modulators and virulence factors, have antimicrobial activities, and are involved in surface motility and in bacterial biofilm development.

Screening concepts for the isolation of biosurfactant producing microorganisms

This chapter gives an overview of current methods for the isolation of biosurfactant producing microbes. The common screening methods for biosurfactants are presented. Sampling and isolation of bacteria are the basis for screening of biosurfactant producing microbes. Hydrocarbon-contaminated sites are the most promising for the isolation of biosurfactant producing microbes, but many strains have also been isolated from undisturbed sites. In subsequent steps the isolates have to be characterized in order to identify the strains which are interesting for a further investigation. Several techniques have been developed for identifying biosurfactant producing strains. Most of them are directly based on the surface or interfacial activity of the culture supernatant. Apart from that, some screening methods explore the hydrophobicity of the cell surface. This trait also gives an indication on biosurfactant production. In recent years automation and miniaturization have led to the development of high throughput methods for screening. High throughput screening (HTS) for analyzing large amounts of potential candidates or whole culture collections is reflected in the end. However, no new principals have been introduced by HTS methods.

Effect of biosurfactants on the aqueous solubility of PCE and TCE

The effect of biosurfactants on the solubility of tetrachloroethylene (PCE) and trichloroethylene (TCE) was studied in batch experiments pertaining to their use for solubilization and mobilization of such contaminants in surfactant enhanced aquifer remediation. Biosurfactants, rhamnolipid and surfactin used in solubility studies were synthesized in our laboratory by Pseudomonas aeruginosa (MTCC 2297) and Bacillus subtilis (MTCC 2423), respectively. The efficiency of the biosurfactants in solubilizing the chlorinated solvents was compared to that of synthetic surfactants. The Weight Solubilization Ratio (WSR) values for solubilization of PCE and TCE by biosurfactants were very high compared to the values obtained for synthetic surfactants. Surfactin proved to be a better surfactant over rhamnolipid. The WSR of surfactin on solubilization of PCE and TCE were 3.83 and 12.5, respectively, whereas the values obtained for rhamnolipid were 2.06 and 8.36. The solubility of the chlorinated solvents by biosurfactants was considerably affected by the changes in pH. The aqueous solubility of PCE and TCE increased tremendously with decrease in pH. The solubility of biosurfactants was observed to decrease with the pH, favoring partitioning of surfactants into the chlorinated solvents in significant amounts at lower pH. The excessive accumulation of biosurfactants at the interface facilitated interfacial tension reductions resulting in higher solubility of the chlorinated solvents at pH less than 7.