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BACKGROUND: The yeast Starmerella bombicola is renowned for its highly efficient sophorolipid production, reaching titers and productivities of (over) 200 g/L and 2 g/(L h), respectively. This inherent efficiency has led to the commercialization of sophorolipids. While the sophorolipid biosynthetic pathway has been elucidated a few years ago, in this study, it is revisited and true key intermediates are revealed. RESULTS: Recently, Starmerella bombicola strains developed and evaluated in the past were reevaluated unveiling unexpected findings. The AT enzyme encoded in the sophorolipid biosynthetic gene cluster is the only described enzyme known to acetylate sophorolipids, while the SBLE enzyme encoded by the SBLE gene is described to catalyze the conversion of (acetylated) acidic sophorolipids into lactonic sophorolipids. A double knockout of both genes was described to result in the generation of bolaform sophorolipids. However, new experiments performed with respective S. bombicola strains Δsble, Δat Δsble, and ∆at revealed inconsistencies with the current understanding of the SL pathway. It was observed that the ∆sble strain produces mainly bolaform sophorolipids with higher acetylation degrees instead of acidic sophorolipids. Furthermore, the ∆at strain produces predominantly bolaform sophorolipids and lactonic sophorolipids with lower acetylation degrees, while the ∆at ∆sble strain predominantly produces bolaform sophorolipids with lower acetylation degrees. These results indicate that the AT enzyme is not the only enzyme responsible for acetylation of sophorolipids, while the SBLE enzyme performs an intramolecular transesterification reaction on bolaform glycolipids instead of an esterification reaction on acidic sophorolipids. These findings, together with recent in vitro data, led us to revise the sophorolipid biosynthetic pathway. CONCLUSIONS: Bolaform sophorolipids instead of acidic sophorolipids are the key intermediates in the biosynthetic pathway towards lactonic sophorolipids. Bolaform sophorolipids are found in very small amounts in extracellular S. bombicola wild type broths as they are very efficiently converted into lactonic sophorolipids, while acidic sophorolipids build up as they cannot be converted. Furthermore, acetylation of sophorolipids is not exclusively performed by the AT enzyme encoded in the sophorolipid biosynthetic gene cluster and acetylation of bolaform sophorolipids promotes their transesterification. These findings led to the revision of the industrially relevant sophorolipid biosynthetic pathway.

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Surfactants are amphiphilic substances that induce surface tension reduction, washing, and emulsification and are used for various purposes. Recently, biosurfactants manufactured from renewable resources and with high biocompatibility have gained increasing attention. Sophorolipid (SL), a type of biosurfactant derived from Starmerella bombicola, possesses detergency and emulsification properties, making it suitable for household and personal care applications. However, there are limited toxicological data on SLs. In this study, we conducted cytotoxicity and skin-irritation tests using SLs, revealing that cytotoxicity and skin irritation induced by SLs were extremely low (logIC 50 = 4.76 mg/L) and equivalent to those of solvents and oils used in personal care products.

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BACKGROUND: Sophorolipids (SLs) are a class of natural, biodegradable surfactants that found their way as ingredients for environment friendly cleaning products, cosmetics and nanotechnological applications. Large-scale production relies on fermentations using the yeast Starmerella bombicola that naturally produces high titers of SLs from renewable resources. The resulting product is typically an extracellular mixture of acidic and lactonic congeners. Previously, we identified an esterase, termed Starmerella bombicola lactone esterase (SBLE), believed to act as an extracellular reverse lactonase to directly use acidic SLs as substrate. RESULTS: We here show based on newly available pure substrates, HPLC and mass spectrometric analysis, that the actual substrates of SBLE are in fact bola SLs, revealing that SBLE actually catalyzes an intramolecular transesterification reaction. Bola SLs contain a second sophorose attached to the fatty acyl group that acts as a leaving group during lactonization. CONCLUSIONS: The biosynthetic function by which the Starmerella bombicola 'lactone esterase' converts acidic SLs into lactonic SLs should be revised to a 'transesterase' where bola SL are the true intermediate. This insights paves the way for alternative engineering strategies to develop designer surfactants.

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Sophorolipids (SLs) are promising glycolipid biosurfactants as they are easily produced and functional. SLs from microorganisms are comprised of mixtures of multiple derivatives that have different structures and properties, including well-known acidic and lactonic SL (ASLs and LSLs, respectively). In this study, we established a method for analyzing all SL derivatives in the products of Starmerella bombicola, a typical SL-producing yeast. Detailed component analyses of S. bombicola products were carried out using reversed-phase high-performance liquid chromatography and mass spectrometry. Methanol was used as the eluent as it is a good solvent for all SL derivatives. With this approach, it was possible to not only quantify the ratio of the main components of ASL, LSL, and SL glycerides but also confirm trace components such as SL mono-glyceride and bola-form SL (sophorose at both ends); notably, this is the first time these components have been isolated and identified successfully in naturally occurring SLs. In addition, our results revealed a novel SL derivative in which a fatty acid is bonded in series to the ASL, which had not been reported previously. Using the present analysis method, it was possible to easily track compositional changes in the SL components during culture. Our results showed that LSL and ASL are produced initially and that SL glycerides accumulate from the middle stage during the fermentation process. KEY POINTS: • An easy and detailed component analysis method for sophorolipids (SLs) is introduced. • Multiple SL derivatives were identified different from known SLs. • A novel hydrophobic acidic SL was isolated and characterized.

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Sophorolipids (SLs) are surface active compounds that have excellent surface-lowering properties. SLs were produced by Starmerella bombicola (CGMCC1576) yeast with sunflower seed oil, fried waste oil, cooked tung oil and raw tung oil used as hydrophobic carbon sources. The results showed that the strain could use sunflower seed oil and fried waste oil as hydrophobic carbon sources to produce SLs, and the yields were 44.52 and 39.09 gl-1. It could not be used as cooked tung oil and raw tung oil. The analysis by high-performance liquid chromatography/high resolution mass spectrometry (HPLC-MS/MS) showed that the main composition and structure of SLs produced by fermentation using fried waste oil were similar to that of sunflower seed oil as hydrophobic carbon source. The yield of SLs was the highest when the fried waste oil was used as hydrophobic carbon source, glucose (8%), waste oil (6%) and yeast (0.3%). When fried waste oil was used as a hydrophobic carbon source in a parallel 4-strand fermentation tank (FT), the combination with the largest yield and the most cost saving was that 3% of fried waste oil was added into the initial medium, and another 3% was again added after 72 h of fermentation. The total yield of SLs was 121.28 gl-1, and the yield of lactone SLs was 48.07 gl-1.

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Cyanobacterial harmful algal blooms (CyanoHABs) pose significant threats to human health and natural ecosystems worldwide, primarily caused by water eutrophication, increased surface water temperature, and co-occurring microorganisms. Urgent action is needed to develop an eco-friendly solution to effectively curb the proliferation of CyanoHABs. Sophorolipids (SLs) are fully biodegradable biosurfactants synthesized by Starmerella bombicola. They can be classified into lactone and acid types. The lactone type displays strong antimicrobial activity, while the acid type exhibits good solubility, which make them ideal agents for mitigating CyanoHABs. Nevertheless, the broad utilization of SLs are hindered by their expensive production costs and the absence of effective genetic editing tools in the native host. In this study, we constructed recombinant strains capable of producing either acidic or lactonic SLs using the CRISPR-Cas9 gene editing system. The yields of acidic and lactonic SLs reached 53.64 g/L and 45.32 g/L in a shaking flask, respectively. In a 5 L fermenter, acidic SLs reached 129.7 g/L using low-cost glucose and rapeseed oil as substrates. The addition of 5 mg/L lactonic SLs effectively degraded cyanobacteria within 30 min, and a ratio of 8.25:1.75 of lactonic to acidic SLs showed the highest degradation efficiency. This study offers a safe and promising solution for CyanoHABs treatment.

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A large-scale application of sophorolipids (SLs) was blocked by their high production cost. One feasible way to reduce the cost of SL production is to develop cheap feedstocks as the substrates for SL fermentation. In the present work, cottonseed molasses (CM), a waste from raffinose production, was used as the hydrophilic substrate;, and cottonseed oil (CO) was used as a hydrophobic substrate for SL production by Starmerella bombicola CGMCC 1576. The primary optimization of carbon sources, nitrogen source and inorganic salts, produced 57.6 ± 2.3 g/L of total SLs and 24.0 ± 1.2 g/L of lactonic SLs on CM and CO, almost equal to the titer of SLs produced from glucose and oleic. A response surface method was applied to optimize the fermentation medium for growth and SL production of S. bombicola. The production of total SLs reached 58.4 ± 3.4 g/L, and lactonic SLs were elevated to more than 25.0 ± 1.9 g/L. HPLC-MS analysis showed that the compositions of SLs produced by S. bombicola on CM and CO were very similar to those on glucose and oleic acid. These results suggested that cottonseed molasses and cottonseed oil can be used as renewable cheap substrates for the reduced-cost production of SLs.

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Sophorolipids are biobased and biodegradable glycolipid surface-active agents contributing to the shift from petroleum to biobased surfactants, associated with clear environmental benefits. However, their production cost is currently too high to allow commercialisation. Therefore, a continuous sophorolipid production process was evaluated, i.e., a retentostat with an external filtration unit. Despite an initial increase in volumetric productivity, productivity eventually declined to almost 0 g L-1 h-1. Following comprehensive metabolomics on supernatant obtained from a standardised retentostat, we hypothesised exhaustion of the N-starvation-induced autophagy as the main mechanism responsible for the decline in bolaform sophorolipid productivity. Thirty-six metabolites that correlate with RNA/protein autophagy and high sophorolipid productivity were putatively identified. In conclusion, our results unveil a plausible cause of this bola sophorolipid productivity decline in an industrially relevant bioreactor set-up, which may thus impact majorly on future yeast biosurfactant regulation studies and the finetuning of bola sophorolipid production processes.

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Saccharomyces cerevisiae is the model organism to most yeast researchers, and information obtained from its physiology is generally extrapolated to other yeasts. Studies on fatty acid transport in S. cerevisiae are based on the expression of both native fatty acid export genes as well as heterologous proteins. Starmerella bombicola, on the other hand, is an oleaginous yeast of industrial relevance but its fatty acid transport mechanisms are unknown. In this study, we attempt to use existing knowledge from S. cerevisiae to study fatty acid transport in S. bombicola, but the obtained results differ from those observed in S. cerevisiae. First, we observed that deletion of SbPRY1 in S. bombicola leads to higher fatty acid export, the opposite effect to the one previously observed for the Pry homologues in S. cerevisiae. Second, following reports that human FATP1 could export fatty acids and alcohols in S. cerevisiae, we expressed FATP1 in a fatty acid-accumulating S. bombicola strain. However, FATP1 reduced fatty acid export in S. bombicola, most likely due to its acyl-CoA synthetase activity. These results not only advance knowledge on fatty acid physiology of S. bombicola, but also improve our understanding of S. cerevisiae and its limitations as a model organism.

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BACKGROUND: Microbial derived-surfactants display low eco-toxicity, diverse functionality, high biodegradability, high specificity, and stability under extreme conditions. Sophorolipids are emerging as key biosurfactants of yeast origins, used in various industrial sectors to lower surface tension. Recently, sophorolipid complexes have been applied in biomedicals and agriculture to eradicate infectious problems related to human and plant fungal pathogens. This study aimed to characterize the functional properties and antifungal activities of sophorolipids produced by a newly characterized Starmerella riodocensis GT-SL1R sp. nov. strain. RESULTS: Starmerella riodocensis GT-SL1R sp. nov. strain was belonged to Starmerella clade with 93.12% sequence similarity using the ITS technique for strain identification. Sophorolipids production was examined, using co-carbon substrates glucose and palm oil, with a yield on the substrate between 30 and 46%. Using shake-flasks, the S. riodocensis GT-SL1R strain produced biosurfactants with an emulsification activity of 54.59% against kerosene compared to the S. bombicola BCC5426 strain with an activity of 60.22%. Maximum productivities of GT-SL1R and the major sophorolipid-producer S. bombicola were similar at 0.8 gl-1 h-1. S. riodocensis GT-SL1R produced mixed forms of lactonic and acidic sophorolipids, shown by TCL, FTIR, and HPLC. Importantly, the complex sophorolipid mixture displayed antifungal activity against an opportunistic yeast pathogen Candida albicans by effectively reducing hyphal and biofilm formation. CONCLUSIONS: Sophorolipids derived from S. riodocensis demonstrate potential industrial and biomedical applications as green surfactant and antifungal agent. Since numerous renewable bioresources and industrial wastes could be used by microbial cell factories in the biosynthesis of biosurfactants to reduce the production cost, sophorolipids hold a promising alternative to current antimicrobials in treatments against infectious diseases in humans, animals, and plants.

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AIMS: The objective of this study was to evaluate the antibacterial effect of sophorolipids in combination with palmarosa essential oil and to develop a cosmetic formulation against acne-causing bacteria. METHODS AND RESULTS: The antibacterial activity of sophorolipids, palmarosa oil and their combined effect was evaluated by broth microdilution and checkerboard methods. Antioxidant activity was determined by the DPPH method. The results showed that the compounds presented antibacterial activity against Staphylococcus aureus and Staphylococcus epidermidis. The combination of sophorolipid and palmarosa oil resulted in synergistic and additive interaction reducing the concentration needed for the effectiveness against S. aureus and S. epidermidis, to 98.4% and 50%, respectively. The compounds interaction showed an additive effect for antioxidant activity. The cosmetic formulation without any chemical preservative presents antibacterial activity against S. aureus, S. epidermidis and Cutibacterium acnes. The pH values and organoleptic characteristics of formulations remained stable under all conditions tested. CONCLUSIONS: The association of sophorolipids and palmarosa oil resulted in a self-preserving cosmetic formulation with great stability, and effective antioxidant and antibacterial activities against acne-causing micro-organisms. SIGNIFICANCE AND IMPACT OF THE STUDY: This study showed the development of an effective multifunctional cosmetic formulation with natural preservatives to treat acne vulgaris and other skin infections.

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Wide ranges of microorganisms produce glycosylated lipids (GL), which are characterized by their tensio-active properties. Therefore, they can be used in different industrial applications as biosurfactants, such as food, agriculture, cosmetics, and health products among others. Two GL biosurfactants, rhamnolipids (RL) and sophorolipids (SL), are now commercially available and share a significant part of the biosurfactant market that in 2017 represented about 2.5% of the total surfactants market, estimated at 15 million tons globally.In this chapter, we present a general overview of GL biosurfactants in terms of their diversity and the microorganisms that produce them. Additionally, we focus on the more detailed description of RL, SL, mannosylerythritol lipids (MEL), and cellobiose lipids (CL).Pseudomonas aeruginosa, the ubiquitous opportunistic pathogenic bacterium, is the best RL producer, but other non-pathogenic bacteria like Burkholderia thailandensis and Pseudomonas chlororaphis NRRL B-30761 are also capable of producing them naturally. In addition, Pseudomonas putida has been used as heterologous host to produce RL with good yields. Here we describe the biosynthetic pathway for RL production, the genes involved in its synthesis, and some of the challenges for producing a homogeneous RL product in high quantities that is suitable for specific applications.SL, MEL, and CL are some of the GL biosurfactants produced in high quantities by fungi, like Starmerella bombicola, Moesziomyces aphidis, or Ustilago maydis. We provide an overview of some of their characteristics, insights on the metabolic pathways involved in their synthesis and genetic modifications performed to increase their production, as well as fermentation and purification strategies and some of their applications.

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Gene editing technology plays an extremely significant role in synthetic biology and metabolic engineering. Traditional genetic manipulation methods, such as homologous recombination, however, are inefficient, time-consuming, and barely feasible when disrupting multiple genes simultaneously. Starmerella bombicola, a nonconventional yeast that overproduces sophorolipids, lacks convenient genetic tools for engineering strains. Here, we developed an efficient CRISPR-Cas9 genome editing technology by combining molecular element mining and expression system optimization for S. bombicola. This CRISPR-Cas9 system improved the efficiency of gene-integration/target gene-introducing disruption by homology-directed repair and realized the multi-gene simultaneous disruptions. Based on this CRISPR-Cas9 system, we also further constructed an engineered strain via the in vivo assembly of multiple DNA fragments (10 kb) that can produce acid-type sophorolipids. These results showed that the CRISPR-Cas9 system may be an efficient and convenient strategy to perform genetic manipulation in S. bombicola.

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Sophorolipids (SLs), an important biosurfactant produced by S. bombicola, were one of the most potential substitutes for chemical surfactants. Few reports on the transcriptional regulation of SLs synthesis and the engineered strains with high-yield SLs were available. In this study, a Rim9-like protein (Rlp) and three transcription factors (ztf1, leu3, gcl) were mined and analyzed, and a progressive enhancement of SLs production was achieved through cumulative knockouts of three genes. The sophorolipid production of ΔrlpΔleu3Δztf1 reached 97.44 g/L, increased by 50.51% than that of the wild-type strain. Compared with the wild-type strain, the flow of glucose to SLs synthesis pathways was increased, and the synthesis of branched-chain amino acids was reduced in ΔrlpΔleu3Δztf1. The amount of UDP-glucose, the substrate for two glycosyltransferases, also increased, and the expression level of the key genes sble and UGPase for SLs synthesis increased by 2.2 times, respectively. The multiple-gene knockout strategy was proved to be highly effective to construct the engineered strain with high-yield SLs production, and this strain was a superior strain for industrial fermentation of SLs and reduced SLs production costs.

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Ricinoleic acid (RA) is a long-chain hydroxy fatty acid produced from castor bean that is used in the manufacturing of a variety of industrial products. The demand for RA keeps increasing due to its broad applications. However, due to the presence of a potent toxin ricin, the native oilseed plant is not an ideal source for hydroxy fatty acid production. Although there have been significant efforts on engineering different microorganisms for heterologous production of RA, all had very limited success. The main reason for this is the exhibited toxicity of the intracellularly accumulated RA. To avoid this issue, we genetically modified a Starmerella bombicola strain by engineering its native sophorolipid production pathway to direct the synthesized RA bound with sophorolipid to be secreted out of the cell, followed by acid hydrolysis to recover RA. The engineered S. bombicola strain expressing the heterologous codon-optimized oleate hydroxylase-encoding gene from ergot fungus Claviceps purpurea resulted in a record production titer of RA at about 2.96 g/L. Thus, this work highlights a new strategy to produce a high level of hydroxy fatty acids in engineered yeast through a sophorolipid intermediate, enabling a new biocatalysis platform for the future.

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Starmerella bombicola is a non-conventional yeast mainly known for its capacity to produce high amounts of the glycolipids 'sophorolipids'. Although its product has been used as biological detergent for a couple of decades, the genetics of S. bombicola are still largely unknown. Computational analysis of the yeast's genome enabled us to identify 254 putative transporter genes that make up the entire transportome. For each of them, a potential substrate was predicted using homology analysis, subcellular localization prediction and RNA sequencing in different stages of growth. One transporter family is of exceptional importance to this yeast: the ATP Binding Cassette (ABC) transporter Superfamily, because it harbors the main driver behind the highly efficient sophorolipid export. Furthermore, members of this superfamily translocate a variety of compounds ranging from antibiotics to hydrophobic molecules. We conducted an analysis of this family by creating deletion mutants to understand their role in the export of hydrophobic compounds, antibiotics and sophorolipids. Doing this, we could experimentally confirm the transporters participating in the efflux of medium chain fatty alcohols, particularly decanol and undecanol, and identify a second sophorolipid transporter that is located outside the sophorolipid biosynthetic gene cluster.

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Sophorolipids are highly active green surfactants (glycolipid biosurfactants) getting tremendous appreciation worldwide due to their low toxicity, biodegradability, broad spectrum of applications, and significant biotechnological potential. Sophorolipids are mainly produced by an oleaginous budding yeast Starmerella bombicola using low-cost substrates. Therefore, the recent state-of-art literature information about S. bombicola yeast is hereby provided, especially the underlying production pathways, biosynthetic gene cluster, and regulatory enzymes. Moreover, the S. bombicola offers flexibility for regulating the structural diversity of sophorolipids, either genetically or by varying fermentative conditions. The emergence of advanced technologies like 'Omics and CRISPR/Cas have certainly boosted rational engineering research for designing high-performing platform strains. Therefore, currently available genetic engineering tools in S. bombicola were reviewed, thereby opening up exciting new possibilities for improving the overall bioproduction titers, structural variability, and stability of sophorolipids. Finally, some technical perspectives to address the current challenges were discussed.

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Biodegradable and biobased surface active agents are renewable and environmentally friendly alternatives to petroleum derived or oleochemical surfactants. However, they are accompanied by relatively high production costs. In this study, the aim was to reduce the production costs for an innovative type of microbial biosurfactant: bolaform sophorolipids, produced by the yeast Starmerella bombicola ΔsbleΔat. A novel continuous retentostat set-up was performed whereby continuous broth microfiltration retained the biomass in the bioreactor while performing an in situ product separation of bolaform sophorolipids. Although a mean volumetric productivity of 0.56 g L-1 h-1 was achieved, it was not possible to maintain this productivity, which collapsed to almost 0 g L-1 h-1. Therefore, two process adaptations were evaluated, a sequential batch strategy and a phosphate limitation alleviation strategy. The sequential batch set-up restored the mean volumetric productivity to 0.66 g L-1 h-1 for an additional 132 h but was again followed by a productivity decline. A similar result was obtained with the phosphate limitation alleviation strategy where a mean volumetric productivity of 0.54 g L-1 h-1 was reached, but a productivity decline was also observed. Whole genome variant analysis uncovered no evidence for genomic variations for up to 1306 h of retentostat cultivation. Untargeted metabolomics analysis identified 8-hydroxyguanosine, a biomarker for oxidative RNA damage, as a key metabolite correlating with high bolaform sophorolipid productivity. This study showcases the application of a retentostat to increase bolaform sophorolipid productivity and lays the basis of a multi-omics platform for in depth investigation of microbial biosurfactant production with S. bombicola.

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Sophorolipids (SLs) are homologous microbial secondary metabolites produced by Starmerella bombicola and have been widely applied in many industrial fields. The biosynthesis of SLs is a highly aerobic process and is often limited by low dissolved oxygen (DO) levels. In this study, the Vitreoscilla hemoglobin (VHb) gene was transformed into S. bombicola O-13-1 by homologous recombination to alleviate oxygen limitation. VHb expression improved the intracellular oxygen utilization efficiency under either oxygen-rich or oxygen-limited conditions. In shake flask culture, the production of SLs was higher in the recombinant (VHb+) strain than in the wild-type (VHb-) strain, while the oxygen uptake rate of the recombinant (VHb+) strain was significantly lower than that of the wild-type (VHb-) strain. In a 5 L bioreactor, the production of SLs did not increase significantly, but the DO level in the fermentation broth of the VHb+ strain was 21.8% higher than that of VHb- strain under oxygen-rich conditions. Compared to wide-type strains (VHb-), VHb expression enhanced SLs production by 25.1% in the recombinants (VHb+) under oxygen-limited conditions. In addition, VHb expression raised the transcription levels of key genes involved in the electron transfer chain (NDH, SDH, COX), TCA cycle (CS, ICD, KDG1) and SL synthesis (CYP52M1 and UGTA1) in the recombinant (VHb+) strains. VHb expression in S. bombicola could enhance SLs biosynthesis and intracellular oxygen utilization efficiency by increasing ATP production and cellular respiration. Our findings highlight the potential use of VHb to improve the oxygen utilization efficiency of S. bombicola in the industrial-scale production of SLs using industrial and agricultural by-products like molasses and waste oil as fermentation feedstock.

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The yeast Starmerella bombicola distinguishes itself from other yeasts by its potential of producing copious amounts of the secondary metabolites sophorolipids (SLs): these are glycolipid biosurfactants composed out of a(n) (acetylated) sophorose moiety and a lipid tail. Although SLs are the subject of numerous research papers and have been commercialized, e.g., in eco-friendly cleaning solutions, the natural function of SLs still remains elusive. This research article investigates several hypotheses for why S. bombicola invests that much energy in the production of SLs, and we conclude that the main natural function of SLs in S. bombicola is niche protection: (1) the extracellular storage of an energy-rich, yet metabolically less accessible carbon source that can be utilized by S. bombicola upon conditions of starvation with (2) antimicrobial properties. In this way, S. bombicola creates a dual advantage in competition with other microorganisms. Additionally, SLs can expedite growth on rapeseed oil, composed of triacylglycerols which are hydrophobic substrates present in the yeasts' environment, for a non-SL producing strain (Δcyp52M1). It was also found that-at least under lab conditions-SLs do not provide protection against high osmotic pressure prevalent in sugar-rich environments such as honey or nectar present in the natural habitat of S. bombicola.