Mechanism-specific and whole-organism ecotoxicity of mono-rhamnolipids

DoE-based medium optimization for improved biosurfactant production with Aureobasidium pullulans

Polyol lipids (a.k.a. liamocins) produced by the polyextremotolerant, yeast-like fungus Aureobasidium pullulans are amphiphilic molecules with high potential to serve as biosurfactants. So far, cultivations of A. pullulans have been performed in media with complex components, which complicates further process optimization due to their undefined composition. In this study, we developed and optimized a minimal medium, focusing on biosurfactant production. Firstly, we replaced yeast extract and peptone in the best-performing polyol lipid production medium to date with a vitamin solution, a trace-element solution, and a nitrogen source. We employed a design of experiments approach with a factor screening using a two-level-factorial design, followed by a central composite design. The polyol lipid titer was increased by 56% to 48 g L-1, and the space-time yield from 0.13 to 0.20 g L-1 h-1 in microtiter plate cultivations. This was followed by a successful transfer to a 1 L bioreactor, reaching a polyol lipid concentration of 41 g L-1. The final minimal medium allows the investigation of alternative carbon sources and the metabolic pathways involved, to pinpoint targets for genetic modifications. The results are discussed in the context of the industrial applicability of this robust and versatile fungus.

Sustainable rhamnolipids production in the next decade - Advancing with Burkholderia thailandensis as a potent biocatalytic strain

Rhamnolipids are one of the most promising eco-friendly green glycolipids for bio-replacements of commercially available fossil fuel-based surfactants. However, the current industrial biotechnology practices cannot meet the required standards due to the low production yields, expensive biomass feedstocks, complicated processing, and opportunistic pathogenic nature of the conventional rhamnolipid producer strains. To overcome these problems, it has become important to realize non-pathogenic producer substitutes and high-yielding strategies supporting biomass-based production. We hereby review the inherent characteristics of Burkholderia thailandensis E264 which favor its competence towards such sustainable rhamnolipid biosynthesis. The underlying biosynthetic networks of this species have unveiled unique substrate specificity, carbon flux control and rhamnolipid congener profile. Acknowledging such desirable traits, the present review provides critical insights towards metabolism, regulation, upscaling, and applications of B. thailandensis rhamnolipids. Identification of their unique and naturally inducible physiology has proved to be beneficial for achieving previously unmet redox balance and metabolic flux requirements in rhamnolipids production. These developments in part are targeted by the strategic optimization of B. thailandensis valorizing low-cost substrates ranging from agro-industrial byproducts to next generation (waste) fractions. Accordingly, safer bioconversions can propel the industrial rhamnolipids in advanced biorefinery domains to promote circular economy, reduce carbon footprint and increased applicability as both social and environment friendly bioproducts.

Rhamnolipids and fengycins, very promising amphiphilic antifungal compounds from bacteria secretomes, act on Sclerotiniaceae fungi through different mechanisms

Rhamnolipids (RLs) and fengycins (FGs) are amphiphilic lipid compounds from bacteria secretomes proposed to replace synthetic pesticides for crop protection. They both display plant defense triggering properties and direct antimicrobial activities. In particular, they have well reported antifungal effects against phytopathogenic fungi. RLs and FGs are considered to act through a direct interaction with membrane lipids and a destabilization of microorganism plasma membrane, thereby limiting the risk of resistance emergence. The main objective of this work was to gain insights in the antimycelial mode of action of these metabolites to promote them as environment and human health friendly biocontrol solutions. Their biocidal effects were studied on two Sclerotiniaceae fungi responsible for diseases in numerous plant species worldwide. We show here that different strains of Botrytis cinerea and Sclerotinia sclerotiorum have opposite sensitivities to RLs and FGs on plate experiments. Overall, B. cinerea is more sensitive to FGs while S. sclerotiorum is more sensitive to RLs. Electron microscopy observations demonstrated that RLs induce mycelial destructuring by asperities emergence and hyphal fusions whereas FGs promote swelling and formation of vesicle-like structures due to vacuole fusions and autophagy. Permeability studies, phosphatidylserine externalization and reactive oxygen species production assessments showed a programmed cell death triggering by RLs at medium concentrations (until 50 μg mL⁻¹) and necrosis characteristics at higher concentration. Programmed cell death was always observed on hyphae treated with FGs. Quantifications of mycelial ergosterol content indicated that a higher ergosterol rate in S. sclerotiorum correlates with increasing sensitivity to RLs. Oppositely, a lower ergosterol rate in B. cinerea correlates with increasing sensitivity to FGs, which was confirmed by ergosterol biosynthesis inhibition with tebuconazole. This gain of knowledge will help to better understand the mode of action of RLs and FGs to fight specific plant fungal diseases.

Automated Search For The Low-lying Energy Isomers of Rhamnolipids and Related Organometallic Complexes

Rhamnolipids (RMLs) are a widely studied biosurfactant due to their high biodegradability and environmentally friendly pro duction. However, the knowledge of the structure-property relationship of RMLs is imperative for the design of highly efficient applications. Aiming to a better understanding of it at a molecular level, we performed an automated search for low energy structures of the most abundant RMLs, namely, Rha-C 10 , Rha-C 10 -C 10 , Rha-Rha-C 10 and Rha-Rha-C 10 -C 10 and their respective C 2 -congeners. Besides that, selected neutral metal complexes were also considered. We found that several low-energy congeners have internal hydrogen bonds. Moreover, geometries in "closed" conformation were always more stable than "open" ones. Fi nally, the energy differences between open and closed conformations of K + , Ni 2 + , Cu 2 + and Zn 2 + complexes were found to be 23.5 kcal mol -1 , 62.8 kcal mol -1 , 24.3 kcal mol -1 and 41.6 kcal mol -1 , respectively, indicating a huge structural reorganization after the complex formation.

Continuous Flow Glycolipid Synthesis Using a Packed Bed Reactor

Glycolipids are a class of biodegradable biosurfactants that are non-toxic and based on renewables, making them a sustainable alternative to petrochemical surfactants. Enzymatic synthesis allows a tailor-made production of these versatile compounds using sugar and fatty acid building blocks with rationalized structures for targeted applications. Therefore, glycolipids can be comprehensively designed to outcompete conventional surfactants regarding their physicochemical properties. However, enzymatic glycolipid processes are struggling with both sugars and fatty acid solubilities in reaction media. Thus, continuous flow processes represent a powerful tool in designing efficient syntheses of sugar esters. In this study, a continuous enzymatic glycolipid production catalyzed by Novozyme 435® is presented as an unprecedented concept. A biphasic aqueous–organic system was investigated, allowing for the simultaneous solubilization of sugars and fatty acids. Owing to phase separation, the remaining non-acylated glucose was easily separated from the product stream and was refed to the reactor forming a closed-loop system. Productivity in the continuous process was higher compared to a batch one, with space–time yields of up to 1228 ± 65 µmol/L/h. A temperature of 70 °C resulted in the highest glucose-6-O-decanoate concentration in the Packed Bed Reactor (PBR). Consequently, the design of a continuous biocatalytic production is a step towards a more competitive glycolipid synthesis in the aim for industrialization.

Rhamnolipid Nano-Micelles versus Alcohol-Based Hand Sanitizer: A Comparative Study for Antibacterial Activity against Hospital-Acquired Infections and Toxicity Concerns

Hospital-acquired infections (HAIs) are considered to be a major global healthcare challenge, in large part because of the development of microbial resistance to currently approved antimicrobial drugs. HAIs are frequently preventable through infection prevention and control measures, with hand hygiene as a key activity. Improving hand hygiene was reported to reduce the transmission of healthcare-associated pathogens and HAIs. Alcohol-based hand sanitizers are commonly used due to their rapid action and broad spectrum of microbicidal activity, offering protection against bacteria and viruses. However, their frequent administration has been reported to be associated with many side effects, such as skin sensitivity, skin drying, and cracks, which promote further skin infections. Thus, there is an essential need to find alternative approaches to hand sanitation. Rhamnolipids are glycolipids produced by Pseudomonas aeruginosa, and were shown to have broad antimicrobial activity as biosurfactants. We have previously demonstrated the antimicrobial activity of rhamnolipid nano-micelles against selected drug-resistant Gram-negative (Salmonella Montevideo and Salmonella Typhimurium) and Gram-positive bacteria (Staphylococcus aureus, Streptococcus pneumoniae). To the best of our knowledge, the antimicrobial activity of rhamnolipid nano-micelles in comparison to alcohol-based hand sanitizers against microorganisms commonly causing HAIs in Egypt—such as Acinetobacter baumannii and Staphylococcus aureus—has not yet been studied. In the present work, a comparative study of the antibacterial activity of rhamnolipid nano-micelles versus alcohol-based hand sanitizers was performed, and their safety profiles were also assessed. It was demonstrated that rhamnolipid nano-micelles had a comparable antibacterial activity to alcohol-based hand sanitizer, with a better safety profile, i.e., rhamnolipid nano-micelles are unlikely to cause any harmful effects on the skin. Thus, rhamnolipid nano-micelles could be recommended to replace alcohol-based hand sanitizers; however, they must still be tested by healthcare workers in healthcare settings to ascertain their antimicrobial activity and safety.

A plea for the integration of Green Toxicology in sustainable bioeconomy strategies - Biosurfactants and microgel-based pesticide release systems as examples

A key aspect of the transformation of the economic sector towards a sustainable bioeconomy is the development of environmentally friendly alternatives for hitherto used chemicals, which have negative impacts on environmental health. However, the implementation of an ecotoxicological hazard assessment at early steps of product development to elaborate the most promising candidates of lowest harm is scarce in industry practice. The present article introduces the interdisciplinary proof-of-concept project GreenToxiConomy, which shows the successful application of a Green Toxicology strategy for biosurfactants and a novel microgel-based pesticide release system. Both groups are promising candidates for industrial and agricultural applications and the ecotoxicological characterization is yet missing important information. An iterative substance- and application-oriented bioassay battery for acute and mechanism-specific toxicity within aquatic and terrestrial model species is introduced for both potentially hazardous materials getting into contact with humans and ending up in the environment. By applying in silico QSAR-based models on genotoxicity, endocrine disruption, skin sensitization and acute toxicity to algae, daphnids and fish, individual biosurfactants resulted in deviating toxicity, suggesting a pre-ranking of the compounds. Experimental toxicity assessment will further complement the predicted toxicity to elaborate the most promising candidates in an efficient pre-screening of new substances.

Comparative study on interfacial and foaming properties of glycolipids in relation to the gas applied for foam generation

Glycolipids are biosurfactants with a wide range of structural diversity. They are biodegradable, based on renewables, ecocompatible and exhibit high surface activity. Still, studies comparing glycolipids and conventional surfactants in terms of interfacial properties and foaming performance are lacking. Here, we compared interfacial and foaming properties of microbial and enzymatically synthesized glycolipids to those of the widely-used, conventional surfactant sodium dodecyl sulfate (SDS). The enzymatically produced sorbose monodecanoate, as well as microbially produced di-rhamno-di-lipids exhibited high foam stabilizing properties, similar to those of SDS. However, sophorolipid and mono-rhamno-di-lipids did not produce metastable foams. An appropriate selection of head and tail groups depending on the application of interest is therefore necessary. Then, glycolipids can serve as an ecofriendly and efficient alternative to petroleum-based surfactants, even at substantially lower concentrations than e.g. SDS. Moreover, the influence of three foaming gases on the foaming properties of the glycolipids was evaluated. Slightly higher foam stability and lower coarsening rates were determined for sorbose monodecanoate when using nitrogen as the foaming gas instead of air. Foams generated with carbon dioxide were not metastable, no matter which surfactant was used.

Parameters Influencing Lipase-Catalyzed Glycolipid Synthesis by (Trans-)Esterification Reaction

Glycolipids are biodegradable, non-toxic surfactants with a wide range of applications. Enzymatic esterification or transesterification facilitated in reaction media of low water activity is a reaction strategy for the production of tailor-made glycolipids as a high structural diversity can be achieved. Organic solvents, ionic liquids, and deep eutectic solvents have been applied as reaction media. However, several challenges need to be addressed for efficient (trans-)esterification reactions, especially for the lipophilization of polar substrates. Therefore, crucial parameters in (trans-)esterification reactions in conventional and non-conventional media are discussed and compared in this review with a special focus on glycolipid synthesis.

Rhamnolipids Nano-Micelles as a Potential Hand Sanitizer

COVID-19 is a pandemic disease caused by the SARS-CoV-2, which continues to cause global health and economic problems since emerging in China in late 2019. Until now, there are no standard antiviral treatments. Thus, several strategies were adopted to minimize virus transmission, such as social distancing, face covering protection and hand hygiene. Rhamnolipids are glycolipids produced formally by Pseudomonas aeruginosa and as biosurfactants, they were shown to have broad antimicrobial activity. In this study, we investigated the antimicrobial activity of rhamnolipids against selected multidrug resistant bacteria and SARS-CoV-2. Rhamnolipids were produced by growing Pseudomonas aeruginosa strain LeS3 in a new medium formulated from chicken carcass soup. The isolated rhamnolipids were characterized for their molecular composition, formulated into nano-micelles, and the antibacterial activity of the nano-micelles was demonstrated in vitro against both Gram-negative and Gram-positive drug resistant bacteria. In silico studies docking rhamnolipids to structural and non-structural proteins of SARS-CoV-2 was also performed. We demonstrated the efficient and specific interaction of rhamnolipids with the active sites of these proteins. Additionally, the computational studies suggested that rhamnolipids have membrane permeability activity. Thus, the obtained results indicate that SARS-CoV-2 could be another target of rhamnolipids and could find utility in the fight against COVID-19, a future perspective to be considered.

Valorization of biodiesel side stream waste glycerol for rhamnolipids production by Pseudomonas aeruginosa RS6

Biodiesel side stream waste glycerol was identified as a cheap carbon source for rhamnolipids (RLs) production which at the same time could improve the management of waste. The present study aimed to produce RLs by using Pseudomonas aeruginosa RS6 utilizing waste glycerol as a substrate and to evaluate their physico-chemicals properties. Fermentation conditions such as temperature, initial medium pH, waste glycerol concentration, nitrogen sources and concentrations resulted in different compositions of the mono- and di-RLs produced. The maximum RLs production of 2.73 g/L was obtained when P. aeruginosa RS6 was grown in a basal salt medium supplemented with 1% waste glycerol and 0.2 M sodium nitrate at 35 °C and pH 6.5. At optimal fermentation conditions, the emulsification index (E₂₄) values of cooking oil, diesel oil, benzene, olive oil, petroleum, and kerosene were all above E₂₄₌50%. The surface tension reduction obtained from 72.13 mN/m to 29.4-30.4 mN/m was better than the surface activity of some chemical-based surfactants. The RLs produced possessed antimicrobial activities against Gram-negative and Gram-positive bacteria with values ranging from 37% to 77% of growth inhibition when 1 mg/mL of RLs was used. Concentrations of RLs below 1500 μg/mL did not induce phytotoxicity effects on the tested seeds (Vigna radiata) compared to the chemical-based- surfactant, SDS. Furthermore, RLs tested on zebrafish (Danio rerio) embryos only exhibited low acute toxicity with an LC₅₀ value of 72.97 μg/mL at 48 h of exposure suggesting a green and eco-biochemical worthy of future applications to replace chemical-based surfactants.

Impact of the number of rhamnose moieties of rhamnolipids on the structure, lateral organization and morphology of model biomembranes

Various studies have described remarkable biological activities and surface-active properties of rhamnolipids, leading to their proposed use in a wide range of industrial applications. Here, we report on a study of the effects of monorhamnolipid RhaC₁₀C₁₀ and dirhamnolipid RhaRhaC₁₀C₁₀ incorporation into model membranes of varying complexity, including bacterial and heterogeneous model biomembranes. For comparison, we studied the effect of HAA (C₁₀C₁₀, lacking a sugar headgroup) partitioning into these membrane systems. AFM, confocal fluorescence microscopy, DSC, and Laurdan fluorescence spectroscopy were employed to yield insights into the rhamnolipid-induced morphological changes of lipid vesicles as well as modifications of the lipid order and lateral membrane organization of the model biomembranes upon partitioning of the different rhamnolipids. The partitioning of the three rhamnolipids into phospholipid bilayers changes the phase behavior, fluidity, lateral lipid organization and morphology of the phospholipid membranes dramatically, to what extent, depends on the headgroup structure of the rhamnolipid, which affects its packing and hydrogen bonding capacity. The incorporation into giant unilamellar vesicles (GUVs) of a heterogeneous anionic raft membrane system revealed budding of domains and fission of daughter vesicles and small aggregates for all three rhamnolipids, with major destabilization of the lipid vesicles upon insertion of RhaC₁₀C₁₀, and also formation of huge GUVs upon the incorporation of RhaRhaC₁₀C₁₀. Finally, we discuss the results with regard to the role these biosurfactants play in biology and their possible impact on applications, ranging from agricultural to pharmaceutical industries.

Toxicity Profiling of Biosurfactants Produced by Novel Marine Bacterial Strains

Surface active agents (SAAs), currently used in modern industry, are synthetic chemicals produced from non-renewable sources, with potential toxic impacts on humans and the environment. Thus, there is an increased interest for the identification and utilization of natural derived SAAs. As such, the marine environment is considered a promising source of biosurfactants with low toxicity, environmental compatibility, and biodegradation compared to their synthetic counterparts. MARISURF is a Horizon 2020 EU-funded project aiming to identify and functionally characterize SAAs, derived from a unique marine bacterial collection, towards commercial exploitation. Specifically, rhamnolipids produced by Marinobacter MCTG107b and Pseudomonas MCTG214(3b1) strains were previously identified and characterized while currently their toxicity profile was assessed by utilizing well-established methodologies. Our results showed a lack of cytotoxicity in in vitro models of human skin and liver as indicated by alamar blue and propidium iodide assays. Additionally, the use of the single gel electrophoresis assay, under oxidative stress conditions, revealed absence of any significant mutagenic/anti-mutagenic potential. Finally, both 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonicacid) (ABTS) and 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) cell-free assays, revealed no significant anti-oxidant capacity for neither of the tested compounds. Consequently, the absence of significant cytotoxicity and/or mutagenicity justifies their commercial exploitation and potential development into industrial end-user applications as natural and environmentally friendly biosurfactants.

Rhamnolipids produced by Pseudomonas: from molecular genetics to the market

Rhamnolipids are biosurfactants with a wide range of industrial applications that entered into the market a decade ago. They are naturally produced by Pseudomonas aeruginosa and some Burkholderia species. Occasionally, some strains of different bacterial species, like Pseudomonas chlororaphis NRRL B-30761, which have acquired RL-producing ability by horizontal gene transfer, have been described. P. aeruginosa, the ubiquitous opportunistic pathogenic bacterium, is the best rhamnolipids producer, but Pseudomonas putida has been used as heterologous host for the production of this biosurfactant with relatively good yields. The molecular genetics of rhamnolipids production by P. aeruginosa has been widely studied not only due to the interest in developing overproducing strains, but because it is coordinately regulated with the expression of different virulence-related traits by the quorum-sensing response. Here, we highlight how the research of the molecular mechanisms involved in rhamnolipid production have impacted the development of strains that are suitable for industrial production of this biosurfactant, as well as some perspectives to improve these industrial useful strains.

Importance of the C12 Carbon Chain in the Biological Activity of Rhamnolipids Conferring Protection in Wheat against Zymoseptoria tritici

The hemibiotrophic fungus Zymoseptoria tritici, responsible for Septoria tritici blotch, is currently the most devastating foliar disease on wheat crops worldwide. Here, we explored, for the first time, the ability of rhamnolipids (RLs) to control this pathogen, using a total of 19 RLs, including a natural RL mixture produced by Pseudomonas aeruginosa and 18 bioinspired RLs synthesized using green chemistry, as well as two related compounds (lauric acid and dodecanol). These compounds were assessed for in vitro antifungal effect, in planta defence elicitation (peroxidase and catalase enzyme activities), and protection efficacy on the wheat-Z. tritici pathosystem. Interestingly, a structure-activity relationship analysis revealed that synthetic RLs with a 12 carbon fatty acid tail were the most effective for all examined biological activities. This highlights the importance of the C12 chain in the bioactivity of RLs, likely by acting on the plasma membranes of both wheat and Z. tritici cells. The efficacy of the most active compound Rh-Est-C12 was 20-fold lower in planta than in vitro; an optimization of the formulation is thus required to increase its effectiveness. No Z. tritici strain-dependent activity was scored for Rh-Est-C12 that exhibited similar antifungal activity levels towards strains differing in their resistance patterns to demethylation inhibitor fungicides, including multi-drug resistance strains. This study reports new insights into the use of bio-inspired RLs to control Z. tritici.

Heterologous Rhamnolipid Biosynthesis: Advantages, Challenges, and the Opportunity to Produce Tailor-Made Rhamnolipids

The first heterologous expression of genes responsible for the production of rhamnolipids was already implemented in the mid-1990s during the functional identification of the rhlAB operon. This was the starting shot for multiple approaches to establish the rhamnolipid biosynthesis in different host organisms. Since most of the native rhamnolipid producing organisms are human or plant pathogens, the intention for these ventures was the establishment of non-pathogenic organisms as heterologous host for the production of rhamnolipids. The pathogenicity of producing organisms is one of the bottlenecks for applications of rhamnolipids in many industrial products especially foods and cosmetics. The further advantage of heterologous rhamnolipid production is the circumvention of the complex regulatory network, which regulates the rhamnolipid biosynthesis in wild type production strains. Furthermore, a suitable host with an optimal genetic background to provide sufficient amounts of educts allows the production of tailor-made rhamnolipids each with its specific physico-chemical properties depending on the contained numbers of rhamnose sugar residues and the numbers, chain length and saturation degree of 3-hydroxyfatty acids. The heterologous expression of rhl genes can also enable the utilization of unusual carbon sources for the production of rhamnolipids depending on the host organism.

Interfacial and Foaming Properties of Tailor-Made Glycolipids-Influence of the Hydrophilic Head Group and Functional Groups in the Hydrophobic Tail

Glycolipids are a class of biodegradable surfactants less harmful to the environment than petrochemically derived surfactants. Here we discuss interfacial properties, foam stability, characterized in terms of transient foam height, gas volume fraction and bubble diameter as well as texture of seven enzymatically synthesized surfactants for the first time. Glycolipids consisting of different head groups, namely glucose, sorbitol, glucuronic acid and sorbose, combined with different C10 acyl chains, namely decanoate, dec-9-enoate and 4-methyl-nonanoate are compared. Equilibrium interfacial tension values vary between 24.3 and 29.6 mN/m, critical micelle concentration varies between 0.7 and 3.0 mM. In both cases highest values were found for the surfactants with unsaturated or branched tail groups. Interfacial elasticity and viscosity, however, were significantly reduced in these cases. Head and tail group both affect foam stability. Foams from glycolipids with sorbose and glucuronic acid derived head groups showed higher stability than those from surfactants with glucose head group, sorbitol provided lowest foam stability. We attribute this to different head group hydration also showing up in the time to reach equilibrium interfacial adsorption. Unsaturated tail groups reduced whereas branching enhanced foam stability compared to the systems with linear, saturated tail. Moreover, the tail group strongly influences foam texture. Glycolipids with unsaturated tail groups produced foams quickly collapsing even at smallest shear loads, whereas the branched tail group yielded a higher modulus than the linear tails. Normalized shear moduli for the systems with different head groups varied in a narrow range, with the highest value found for decylglucuronate.

Killing Two Birds With One Stone - Strain Engineering Facilitates the Development of a Unique Rhamnolipid Production Process

High-titer biosurfactant production in aerated fermenters using hydrophilic substrates is often hampered by excessive foaming. Ethanol has been shown to efficiently destabilize foam of rhamnolipids, a popular group of biosurfactants. To exploit this feature, we used ethanol as carbon source and defoamer, without introducing novel challenges for rhamnolipid purification. In detail, we engineered the non-pathogenic Pseudomonas putida KT2440 for heterologous rhamnolipid production from ethanol. To obtain a strain with high growth rate on ethanol as sole carbon source at elevated ethanol concentrations, adaptive laboratory evolution (ALE) was performed. Genome re-sequencing allowed to allocate the phenotypic changes to emerged mutations. Several genes were affected and differentially expressed including alcohol and aldehyde dehydrogenases, potentially contributing to the increased growth rate on ethanol of 0.51 h^-1 after ALE. Further, mutations in genes were found, which possibly led to increased ethanol tolerance. The engineered rhamnolipid producer was used in a fed-batch fermentation with automated ethanol addition over 23 h, which resulted in a 3-(3-hydroxyalkanoyloxy)alkanoates and mono-rhamnolipids concentration of about 5 g L^-1. The ethanol concomitantly served as carbon source and defoamer with the advantage of increased rhamnolipid and biomass production. In summary, we present a unique combination of strain and process engineering that facilitated the development of a stable fed-batch fermentation for rhamnolipid production, circumventing mechanical or chemical foam disruption.

Environmentally friendly rhamnolipid production for petroleum remediation

Biosurfactants have potential applications in the remediation of petroleum-contaminated sites. Several strategies can be used to reduce the production costs of these surfactants and make the process more environmentally friendly. In this study, we combined some of these strategies to produce the rhamnolipid-type biosurfactant, including the use of the genetically modified strain Pseudomonas aeruginosa-estA, an industrial coproduct as a carbon source, a simple and low-cost medium, and a simple downstream process. The process resulted in a high yield (17.6 g L^-1), even using crude glycerin as the carbon source, with substrate in product conversion factor (Y_RML/s) of 0.444. The cell-free supernatant (CFS) was not toxic to Artemia salina and selected mammalian cell lineages, suggesting that it can be used directly in the environment without further purification steps. Qualitative analysis showed that CFS has excellent dispersion in the oil-displacement test, emulsifying (IE₂₄ = 65.5%), and tensoactive properties. When salinity, temperature and pressure were set to seawater conditions, the values for interfacial tension between crude oil and water were below 1.0 mN m^-1. Taken together, these results demonstrate that it is possible to obtain a nontoxic crude rhamnolipid product, with high productivity, to replace petroleum-based surfactants in oil spill cleanups and other environmental applications.

Synthetic Mono-Rhamnolipids Display Direct Antifungal Effects and Trigger an Innate Immune Response in Tomato against Botrytis Cinerea

Natural rhamnolipids are potential biocontrol agents for plant protection against bacterial and fungal diseases. In this work, we synthetized new synthetic mono-rhamnolipids (smRLs) consisting in a rhamnose connected to a simple acyl chain and differing by the nature of the link and the length of the lipid tail. We then investigated the effects of these ether, ester, carbamate or succinate smRL derivatives on Botrytis cinerea development, symptoms spreading on tomato leaves and immune responses in tomato plants. Our results demonstrate that synthetic smRLs are able to trigger early and late immunity-related plant defense responses in tomato and increase plant resistance against B. cinerea in controlled conditions. Structure-function analysis showed that chain length of the lipidic part and type of acyl chain were critical to smRLs immune activity and to the extent of symptoms caused by the fungus on tomato leaves.

Biological properties and structural characterization of a novel rhamnolipid like-biosurfactants produced by Lactobacillus casei subsp. casei TM1B

Biosurfactants are microbial surface-active compounds with antimicrobial and antioxidant activities that display a range of physiological functions. In this study, a strain isolated from a Cameroonian fermented milk "pendidam" and identified as Lactobacillus casei subsp. casei TM1B was used for biosurfactants production. The biosurfactants produced by L. casei TM1B with molasses as the substrate had a good surface (40.77 mN/m) and emulsifying (84.50%) activities. The scavenging of the ABTS^+• radical (IC₅₀ value of 0.60 ± 0.03 mg/mL) by the biosurfactants was found to be higher than that of DPPH^• radical (IC₅₀ value of 0.97 ± 0.13 mg/mL). The maximum chelating activity of biosurfactants (82.29%) was observed at 3.5 mg/mL. The biologically active compound of the biosurfactants produced by L. casei TM1B was identified as 2,5-O-methylrhamnofuranosyl-palmitate, a novel rhamnolipid-like biosurfactant by using chemical, Fourier transform infrared spectroscopy, gas chromatography-mass spectrometry, and NMR analysis. The biosurfactants were bactericidal against several Gram-negative and Gram-positive pathogens (minimum inhibitory concentration values ranged from 3.22 to 12.83 mg/mL), and scanning electron microscope analysis revealed bacterial cell walls and membranes as main targets.

Optimization of Glycolipid Synthesis in Hydrophilic Deep Eutectic Solvents

Glycolipids are considered an alternative to petrochemically based surfactants because they are non-toxic, biodegradable, and less harmful to the environment while having comparable surface-active properties. They can be produced chemically or enzymatically in organic solvents or in deep eutectic solvents (DES) from renewable resources. DES are non-flammable, non-volatile, biodegradable, and almost non-toxic. Unlike organic solvents, sugars are easily soluble in hydrophilic DES. However, DES are highly viscous systems and restricted mass transfer is likely to be a major limiting factor for their application. Limiting factors for glycolipid synthesis in DES are not generally well understood. Therefore, the influence of external mass transfer, fatty acid concentration, and distribution on initial reaction velocity in two hydrophilic DES (choline:urea and choline:glucose) was investigated. At agitation speeds of and higher than 60 rpm, the viscosity of both DES did not limit external mass transfer. Fatty acid concentration of 0.5 M resulted in highest initial reaction velocity while higher concentrations had negative effects. Fatty acid accessibility was identified as a limiting factor for glycolipid synthesis in hydrophilic DES. Mean droplet sizes of fatty acid-DES emulsions can be significantly decreased by ultrasonic pretreatment resulting in significantly increased initial reaction velocity and yield (from 0.15 ± 0.03 μmol glucose monodecanoate/g DES to 0.57 ± 0.03 μmol/g) in the choline: urea DES. The study clearly indicates that fatty acid accessibility is a limiting factor in enzymatic glycolipid synthesis in DES. Furthermore, it was shown that physical pretreatment of fatty acid-DES emulsions is mandatory to improve the availability of fatty acids.

Semipurified Rhamnolipid Mixes Protect Brassica napus Against Leptosphaeria maculans Early Infections

The rapeseed crop (Brassica napus) has to cope with fungal diseases that significantly impacts yields. In particular, the fungal pathogen Leptosphaeria maculans, the causal agent of blackleg disease (also named Phoma stem canker), is a worldwide issue to this crop. Considering environmental concerns, it is essential to propose alternative natural compounds for rapeseed crop protection to reduce chemical fungicide use. Here we report data showing the efficacy of semipurified rhamnolipid (RL) mixes from bacterial origin to protect rapeseed against L. maculans at early stages of infection in controlled conditions. In addition, we show that RL solutions have excellent adhesion properties when sprayed onto rapeseed leaves, without adding any adjuvant. We demonstrate that RL mixes display direct antimycelial properties against the pathogen and stimulate plant defense responses in rapeseed. Our results validate, a preventive action of low RL concentrations to protect rapeseed against L. maculans and a curative effect in specific conditions when applied after the inoculation of the pathogen spores. Semipurified RL mixes therefore appear to be real cost-effective compounds that could be used in fields as biocontrol products to fight L. maculans early infections of rapeseed.

Effects of Burkholderia thailandensis rhamnolipids on the unicellular algae Dunaliella tertiolecta

The effects of rhamnolipids (RLs) produced and further purified from Burkholderia thailandensis, on the unicellular microalgae Dunaliella tertiolecta were investigated, in terms of RLs ability to affect algal growth, photosynthetic apparatus structure and energy flux, round and through photosystems II and I. Specifically, 24-48 h RLs-treated algae (RLs at concentrations ranged from 5 to 50 mg L^-1) showed significantly decreased levels of growth rate, while increased levels of Chl a and b were obtained only in 72-96 h RLs-treated algae. Similarly, although no changes were obtained in the Chl a/b ratio and almost all chlorophyll fluorescence parameters over time, yields of electron transport (ϕR₀, ϕE₀) and respective performance index (PI_total) were negatively affected at 72 and 96 h. Based on those findings, it seems that the inhibitory effect of RLs on the algae growth rate after 24 and 48 h and the gradual attenuation of the phenomenon (after 72 h of exposure), may indicate the initial response of the organism, as well as algae ability to overcome, since RLs showed no effects on algae photosynthetic ability. Those findings reveal for the first time that RLs from Burkholderia thailandensis are not harmful for Dunaliella tertiolecta. However, further studies with the use of more aquatic species could be essential for assessing the RLs-mediated effects on aquatic biota.

Marine Biosurfactants: Biosynthesis, Structural Diversity and Biotechnological Applications

Biosurfactants are amphiphilic secondary metabolites produced by microorganisms. Marine bacteria have recently emerged as a rich source for these natural products which exhibit surface-active properties, making them useful for diverse applications such as detergents, wetting and foaming agents, solubilisers, emulsifiers and dispersants. Although precise structural data are often lacking, the already available information deduced from biochemical analyses and genome sequences of marine microbes indicates a high structural diversity including a broad spectrum of fatty acid derivatives, lipoamino acids, lipopeptides and glycolipids. This review aims to summarise biosyntheses and structures with an emphasis on low molecular weight biosurfactants produced by marine microorganisms and describes various biotechnological applications with special emphasis on their role in the bioremediation of oil-contaminated environments. Furthermore, novel exploitation strategies are suggested in an attempt to extend the existing biosurfactant portfolio.

Toxicological and ecotoxicological evaluation of the water quality in a large and eutrophic freshwater lake of China

Effect-based methods (EBMs) are recommended as holistic approach for diagnosis and monitoring of water quality; however, the application of EBMs is still scare in China. In the present study, water quality of the freshwater lake Taihu (China) was investigated by EBMs. Different types of water samples were collected from three bays of the lake during 2015, 2016 and 2017. A battery of seven effect-based bioassays, including both specific and non-specific toxicity assays, was used. The bioassay battery was recently suggested based on joint activities of the EU project SOLUTIONS and the NORMAN network on emerging pollutants and is also under discussion for being implemented into monitoring activities in the context of the European Water Framework Directive (WFD). Adverse effects were observed towards the primary producer, primary consumer and fish, indicating the potential ecotoxicity of water in Taihu Lake. Mutagenic and estrogenic effects were found in the Ames fluctuation assay and ERα CALUX (Chemically Activated Luciferase Gene-eXpression) assay, respectively, highlighting the potential risks on human health. Algal growth inhibition and mutagenic effects can be observed during each of the three years. Acute toxicity towards Daphnia magna and estrogen receptor agonistic effects were found in at least one of the samples collected in 2016 and 2017, but not in 2015. The endpoints for fish toxicity in the Danio rerio fish embryo test included both lethal and additionally several sublethal effects (only for samples from 2017) and were not compared between years. Algal growth inhibition, fish embryo toxicity, mutagenic effect and estrogenicity were observed in each of the three bays, while Daphnia acute toxicity was only found in Zhushan Bay. Taking together, this study provides a big picture on the water quality of Taihu Lake. The battery of effect-based tools is promising to be a routine for water quality monitoring in China.

Ecotoxicity of Nitrogen, Sulfur, or Oxygen Heterocycles and Short-Chained Alkyl Phenols Commonly Detected in Contaminated Groundwater

Nitrogen, sulfur, or oxygen heterocyclic aromatic hydrocarbons (NSO-HETs) and short-chained alkyl phenols (SCAPs) are commonly detected in groundwater at contaminated sites and in the surrounding environment. It is now scientific consensus that these chemicals pose a risk to human and ecosystem health. However, toxicity data are comparably fragmentary, and only few studies have addressed the ecotoxicity of NSO-HETs and SCAPs in a systematic and comparative fashion. To overcome this shortcoming, we tested 18 SCAPs, 16 NSO-HETs, as well as the homocyclic hydrocarbons indane and indene in the Microtox® assay with Aliivibrio fischeri, the growth inhibition test with Desmodesmus subspicatus, the acute immobilization assay with Daphnia magna, as well as the fish embryo toxicity test with embryos of the zebrafish (Danio rerio). Because of the physicochemical properties of the tested chemicals (limited water solubility, volatility, and sorption to test vessels), actual exposure concentrations in test media and their dissipation over time were analytically quantified by means of gas chromatography with mass spectrometry. Analytically corrected effect levels (median effect and lethal concentrations) ranged from 0.017 to 180 mg L^-1 , underlining the environmental relevance of some NSO-HETs and SCAPs. Para-substituted phenols showed the overall greatest toxicities in all 4 toxicity tests. We provide, for the first time, a complete high-quality data set in support of better environmental risk assessments of these chemicals. Environ Toxicol Chem 2019;38:1343-1355. © 2019 SETAC.

Biodegradability and toxicity of monorhamnolipid biosurfactant diastereomers

Synthetic monorhamnolipids differ from biologically produced material because they are produced as single congeners, depending on the β-hydroxyalkanoic acid used during synthesis. Each congener is produced as one of four possible diastereomers resulting from two chiral centers at the carbinols of the lipid tails [(R,R), (R,S), (S,R) and (S,S)]. We compare the biodegradability (CO2 respirometry), acute toxicity (Microtox assay), embryo toxicity (Zebrafish assay), and cytotoxicity (xCELLigence and MTS assays) of synthetic rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10) monorhamnolipids against biosynthesized monorhamnolipid mixtures (bio-mRL). All Rha-C10-C10 diastereomers and bio-mRL were inherently biodegradable ranging from 34 to 92% mineralized. The Microtox assay showed all Rha-C10-C10 diastereomers and bio-mRL are slightly toxic according to the US EPA ecotoxicity categories with 5 min EC50 values ranging from 39.6 to 87.5 μM. The zebrafish assay showed that of 22 developmental endpoints tested, only mortality was observed at 120 h post fertilization; all Rha-C10-C10 diastereomers and bio-mRL caused significant mortality at 640 μM, except the Rha-C10-C10 (R,R) which showed no developmental effects. xCELLigence and MTS showed IC50 values ranging from 103.4 to 191.1 μM for human lung cell line H1299 after 72 h exposure. These data provide key information regarding Rha-C10-C10 diastereomers that is pertinent when considering potential applications.

Integrating bioassays, chemical analysis and in silico techniques to identify genotoxicants in surface water

Identification of hazardous compounds, as the first step of water protection and regulation, is still challenged by the difficulty to establish a linkage between toxic effects and suspected contaminants. Genotoxic compounds are one type of highly relevant toxicants in surface water, which may attack the DNA and lead to cancer in individual organism, or even damaged germ cells to be passed on to future generations. Thus, the establishment of a linkage between genotoxic effects and genotoxicant is important for environmental toxicologists and chemists. For this purpose, in the present study in silico methods were integrated with bioassays, chemical analysis and literature information to identify genotoxicants in surface water. Large volume water samples from 22 sampling sites of the Danube were collected and subjected to biological and chemical analysis. Samples from the most toxic sites (JDS32, JDS44 and JDS63) induced significant genotoxic effects in the micronucleus assay, and two of them caused mutagenicity in the Ames fluctuation assay. Chemical analysis showed that 68 chemicals were detected in these most toxic samples. Literature findings and in silico techniques using the OECD QSAR Toolbox and the ChemProp software package revealed genotoxic potentials for 29 compounds out of 68 targeted chemicals. To confirm the integrative technical data, the micronucleus assay and the Ames fluctuation assay were applied with artificial mixtures of those compounds and the raw water sample extracts. The results showed that 18 chemicals explained 48.5% of the genotoxicity in the micronucleus assay. This study highlights the capability of in silico techniques in linking adverse biological effect to suspicious hazardous compounds for the identification of toxicity drivers, and demonstrates the genotoxic potential of pollutants in the Danube.

Mechanisms for rhamnolipids-mediated biodegradation of hydrophobic organic compounds

The widespread existence of hydrophobic organic compounds (HOCs) in soil and water poses a potential health hazard to human, such as skin diseases, heart diseases, carcinogenesis, etc. Surfactant-enhanced bioremediation has been regarded as one of the most viable technologies to treat HOCs contaminated soil and groundwater. As a biosurfactant that has been intensively studied, rhamnolipids have shown to enhance biodegradation of HOCs in the environment, however, the underlying mechanisms are not fully disclosed. In this paper, properties and production of rhamnolipids are summarized. Then effects of rhamnolipids on the biodegradation of HOCs, including solubilization, altering cell affinity to HOCs, and facilitating microbial uptake are reviewed in detail. Special attention is paid to how rhamnolipids change the bioavailability of HOCs, which are crucial for understanding the mechanism of rhamnolipids-mediated biodegradation. The biodegradation and toxicity of rhamnolipids are also discussed. Finally, perspectives and future research directions are proposed. This review adds insight to rhamnolipids-enhanced biodegradation process, and helps in application of rhamnolipids in bioremediation.

Rhamnolipids From Pseudomonas aeruginosa Are Elicitors Triggering Brassica napus Protection Against Botrytis cinerea Without Physiological Disorders

Rhamnolipids (RLs) are amphiphilic molecules naturally produced by some bacteria with a large range of biological activities. Although some studies report their potential interest in plant protection, evaluation of their effects and efficiency on annual crops of worldwide agronomic interest is lacking. The main objective of this work was to investigate their elicitor and protective activities on rapeseed crop species while evaluating their physiological effects. Here we report that RLs from Pseudomonas aeruginosa secretome trigger an effective protection of Brassicanapus foliar tissues toward the fungus Botrytis cinerea involving the combination of plant defense activation and direct antimicrobial properties. We demonstrated their ability to activate canonical B.napus defense responses including reactive oxygen species production, expression of defense genes, along with callose deposits and stomatal closure as efficient physical protections. In addition, microscopic cell death observations and electrolyte leakage measurements indicated that RLs trigger a hypersensitive response-like defense in this plant. We also showed that foliar spray applications of RLs do not induce deleterious physiological consequences on plant growth or chlorophyll content and that RL protective properties are efficient on several grown cultivars of rapeseed. To our knowledge, this is the first report of RLs as an elicitor that suppresses fungal disease on tissues of an annual crop species under greenhouse conditions. Our results highlight the dual mode of action of these molecules exhibiting plant protection activation and antifungal activities and demonstrate their potential for crop cultures as environmental-friendly biocontrol solution.

Semi-correlations combined with the index of ideality of correlation: a tool to build up model of mutagenic potential

Mutagenicity is the ability of a substance to induce mutations. This hazardous ability of a substance is decisive from point of view of ecotoxicology. The number of substances, which are used for practical needs, grows every year. Consequently, methods for at least preliminary estimation of mutagenic potential of new substances are necessary. Semi-correlations are a special case of traditional correlations. These correlations can be named as "correlations along two parallel lines." This kind of correlation has been tested as a tool to predict selected endpoints, which are represented by only two values: "inactive/active" (0/1). Here this approach is used to build up predictive models for mutagenicity of large dataset (n = 3979). The so-called index of ideality of correlation (IIC) has been tested as a statistical criterion to estimate the semi-correlation. Three random splits of experimental data into the training, invisible-training, calibration, and validation sets were analyzed. Two models were built up for each split: the first model based on optimization without the IIC and the second model based on optimization where IIC is involved in the Monte Carlo optimization. The statistical characteristics of the best model (calculated with taking into account the IIC) n = 969; sensitivity = 0.8050; specificity = 0.9069; accuracy = 0.8648; Matthews's correlation coefficient = 0.7196 (using IIC). Thus, the use of IIC improves the statistical quality of the binary classification models of mutagenic potentials (Ames test) of organic compounds.

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.

Sodium chloride effect on the aggregation behaviour of rhamnolipids and their antifungal activity

In this work, the antifungal activity of rhamnolipids produced by Pseudomonas aeruginosa #112 was evaluated against Aspergillus niger MUM 92.13 and Aspergillus carbonarius MUM 05.18. It was demonstrated that the di-rhamnolipid congeners were responsible for the antifungal activity exhibited by the crude rhamnolipid mixture, whereas mono-rhamnolipids showed a weak inhibitory activity. Furthermore, in the presence of NaCl (from 375 mM to 875 mM), the antifungal activity of the crude rhamnolipid mixture and the purified di-rhamnolipids was considerably increased. Dynamic Light Scattering studies showed that the size of the structures formed by the rhamnolipids increased as the NaCl concentration increased, being this effect more pronounced in the case of di-rhamnolipids. These results were confirmed by Confocal Scanning Laser Microscopy, which revealed the formation of giant vesicle-like structures (in the µm range) by self-assembling of the crude rhamnolipid mixture in the presence of 875 mM NaCl. In the case of the purified mono- and di-rhamnolipids, spherical structures (also in the µm range) were observed at the same conditions. The results herein obtained demonstrated a direct relationship between the rhamnolipids antifungal activity and their aggregation behaviour, opening the possibility to improve their biological activities for application in different fields.

Improving the reliability of aquatic toxicity testing of hydrophobic chemicals via equilibrium passive dosing - A multiple trophic level case study on bromochlorophene

The main objective of the present study was to improve the reliability and practicability of aquatic toxicity testing of hydrophobic chemicals based upon the model substance bromochlorophene (BCP). Therefore, we adapted a passive dosing format to test the toxicity of BCP at different concentrations and in multiple test systems with aquatic organisms of various trophic levels. At the same time, the method allowed for the accurate determination of exposure concentrations (i.e., in the presence of exposed organisms; C_test) and freely dissolved concentrations (i.e., without organisms present; C_free) of BCP in all tested media. We report on the joint adaptation of three ecotoxicity tests - algal growth inhibition, Daphnia magna immobilization, and fish-embryo toxicity - to a silicone O-ring based equilibrium passive dosing format. Effect concentrations derived by passive dosing methods were compared with corresponding effect concentrations derived by standard co-solvent setups. The passive dosing format led to EC₅₀-values in the lower μgL^-1 range for algae, daphnids, and fish embryos, whereas increased effect concentrations were measured in the co-solvent setups for algae and daphnids. This effect once more shows that passive dosing might offer advantages over standard methods like co-solvent setups when it comes to a reliable risk assessment of hydrophobic substances. The presented passive dosing setup offers a facilitated, practical, and repeatable way to test hydrophobic chemicals on their toxicity to aquatic organisms, and is an ideal basis for the detailed investigation of this important group of chemicals.

Novel insights into biosynthesis and uptake of rhamnolipids and their precursors

The human pathogenic bacterium Pseudomonas aeruginosa produces rhamnolipids, glycolipids with functions for bacterial motility, biofilm formation, and uptake of hydrophobic substrates. Rhamnolipids represent a chemically heterogeneous group of secondary metabolites composed of one or two rhamnose molecules linked to one or mostly two 3-hydroxyfatty acids of various chain lengths. The biosynthetic pathway involves rhamnosyltransferase I encoded by the rhlAB operon, which synthesizes 3-(3-hydroxyalkanoyloxy)alkanoic acids (HAAs) followed by their coupling to one rhamnose moiety. The resulting mono-rhamnolipids are converted to di-rhamnolipids in a third reaction catalyzed by the rhamnosyltransferase II RhlC. However, the mechanism behind the biosynthesis of rhamnolipids containing only a single fatty acid is still unknown. To understand the role of proteins involved in rhamnolipid biosynthesis the heterologous expression of rhl-genes in non-pathogenic Pseudomonas putida KT2440 strains was used in this study to circumvent the complex quorum sensing regulation in P. aeruginosa. Our results reveal that RhlA and RhlB are independently involved in rhamnolipid biosynthesis and not in the form of a RhlAB heterodimer complex as it has been previously postulated. Furthermore, we demonstrate that mono-rhamnolipids provided extracellularly as well as HAAs as their precursors are generally taken up into the cell and are subsequently converted to di-rhamnolipids by P. putida and the native host P. aeruginosa. Finally, our results throw light on the biosynthesis of rhamnolipids containing one fatty acid, which occurs by hydrolyzation of typical rhamnolipids containing two fatty acids, valuable for the production of designer rhamnolipids with desired physicochemical properties.

Rhamnolipid Biosurfactant against Fusarium verticillioides to Control Stalk and Ear Rot Disease of Maize

Antifungal activity of rhamnolipids (RLs) has been widely studied against many plant pathogenic fungi, but not against Fusarium verticillioides, a major pathogen of maize (Zea mays L.). F. verticillioides causes stalk and ear rot of maize or asymptomatically colonizes the plant and ears resulting in moderate to heavy crop loss throughout the world. F. verticillioides produces fumonisin mycotoxins, reported carcinogens, which makes the contaminated ears unsuitable for consumption. In this study, the RL produced using glucose as sole carbon source was characterized by FTIR and LCMS analyses and its antifungal activity against F. verticillioides was evaluated in vitro on maize stalks and seeds. Further, the effect of RL on the mycelia of F. verticillioides was investigated by scanning electron microscopy which revealed visible damage to the mycelial structure as compared to control samples. In planta, treatment of maize seeds with a RL concentration of 50 mg l-1 resulted in improved biomass and fruiting compared to those of healthy control plants and complete suppression of characteristic disease symptoms and colonization of maize by F. verticillioides. The study highlights the potential of RLs to be used for an effective biocontrol strategy against colonization of maize plant by F. verticillioides.