A pH shift induces high-titer liamocin production in Aureobasidium pullulans

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.

Liamocins overproduction via the two-pH stage fermentation and anti-Aspergillus flavus activity of Massoia lactone

Aureobasidium melanogenum was found to be grown the best at the constant pH 7.0 and to produce the highest amount of liamocins at the constant pH 3.0. Therefore, the wild type strain A. melanogenum 9-1 and the engineered strain V33 constructed in the laboratory were grown at the constant pH 7.0 for 48 h, then, they were continued to be cultivated at the constant pH 3.0. Under such conditions, A. melanogenum 9-1 produced 36.51 ± 0.55 g L-1 of liamocin and its cell mass was 27.43 ± 0.63 and 6.00 ± 0.11 g L-1 of glucose was left in the finished medium within 168 h while the engineered strain V33 secreted 70.86 ± 2.04 g L-1 of liamocin, its cell mass was 31.63 ± 0.74 g L-1 , 0.16 ± 0.01 g L-1 of glucose was maintained in the finished medium. Then, Massoia lactone was released from the produced liamocins. The released Massoia lactone loaded in the nanoemulsions could be used to actively damage cell wall and cell membrane of both spores and mycelia of Aspergillus flavus, leading to its cell necrosis. Massoia lactone loaded in the nanoemulsions also actively inhibited cell growth of A. flavus, its conidia production and aflatoxin biosynthesis on peanuts, indicating that Massoia lactone loaded in the nanoemulsions had highly potential application in controlling cell growth of A. flavus and aflatoxin biosynthesis in foods and feedstuffs.

Liamocin biosynthesis is induced by an autogenous host acid activation in Aureobasidium melanogenum

It has been known that maximal liamocin production must be carried out at low environmental pH (around 3.0). In this study, it was found that the low pH was mainly caused by the secreted citric acid which is one precursor of acetyl-CoA for liamocin biosynthesis. Determination of citric acid in the culture, deletion, complementation and overexpression of the CEXA gene encoding specific citrate exporter demonstrated that the low pH was indeed caused by the secreted citric acid. Deletion, complementation and overexpression of the ACL gene encoding ATP-citric acid lyase and effects of different initial pHs and added citric acid showed that the low pH in the presence of citric acid was suitable for lysis of intracellular citric acid, liamocin production and expression of the PACC gene encoding the pH signaling transcription factor PacC. This meant that the PACC gene was an acid-expression gene. Deletion, complementation and overexpression of the PACC gene indicated that expression of the key gene cluster GAL1-EST1-PKS1 for liamocin biosynthesis was driven by the pH signaling transcription factor PacC and there was weak nitrogen catabolite repression on liamocin biosynthesis at the low pH. That was why liamocin biosynthesis was induced at a low pH in the presence of citric acid. The mechanisms of the enhanced liamocin biosynthesis by the autogenous host acid activation, together with the pH signaling pathway, were proposed.

Transcriptome Analysis Reveals the Regulation of Aureobasidium pullulans under Different pH Stress

Aureobasidium pullulans (A. pullulans), a commonly found yeast-like fungus, exhibits adaptability to a wide range of pH environments. However, the specific mechanisms and regulatory pathways through which A. pullulans respond to external pH remain to be fully understood. In this study, we first sequenced the whole genome of A. pullulans using Nanopore technology and generated a circle map. Subsequently, we explored the biomass, pullulan production, melanin production, and polymalic acid production of A. pullulans when cultivated at different pH levels. We selected pH 4.0, pH 7.0, and pH 10.0 to represent acidic, neutral, and alkaline environments, respectively, and examined the morphological characteristics of A. pullulans using SEM and TEM. Our observations revealed that A. pullulans predominantly exhibited hyphal growth with thicker cell walls under acidic conditions. In neutral environments, it primarily displayed thick-walled spores and yeast-like cells, while in alkaline conditions, it mainly assumed an elongated yeast-like cell morphology. Additionally, transcriptome analysis unveiled that A. pullulans orchestrates its response to shifts in environmental pH by modulating its cellular morphology and the expression of genes involved in pullulan, melanin, and polymalic acid synthesis. This research enhances the understanding of how A. pullulans regulates itself in diverse pH settings and offers valuable guidance for developing and applying engineered strains.

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

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

Liamocin overproduction by the mutants of Aureobasidium melanogenum 9-1 for effectively killing spores of the pathogenic fungi from diseased human skin by Massoia lactone

Liamocins and Massoia lactone have many applications. In this study, the glucose-derepressed mutant Δcrea5 in which the CREA gene was removed could produce 36.5 g/L of liamocins. Furthermore, overexpression of the MSN2 gene in the mutant Δcrea5 made the transformant M60 produce 41.4 g/L of liamocins and further overexpression of the GAL1 gene in the transformant M60 rendered the transformant G40 to produce 49.5 ± 0.4 g/L of liamocins during the 10-L fermentation while their wild type strain 9-1 made only 26.3 g/L of liamocins. The expressed transcription activators Msn2 and Gal1 were localized in the nuclei, promoting expression of the genes responsible for liamocins biosynthesis and sugar transport. Massoia lactone prepared from the produced liamocins could actively kill the spores of the pathogenic fungi from the diseased human skin by inhibiting spore germination and causing cellular necrosis of the fungal spores.

Simultaneous quantitative assays of 15 ginsenosides from 119 batches of ginseng samples representing 12 traditional Chinese medicines by ultra-high performance liquid chromatography coupled with charged aerosol detector

Despite the extensive consumption of ginseng, precise quality control of different ginseng products is highly challenging due to the containing of ginsenosides in common for different Panax species or different parts (e.g. root, leaf, and flower) of a same species. Herein we performed a comparative investigation of diverse ginseng products by simultaneously assaying 15 saponins (notoginsenoside R1, ginsenosides Rg1, -Re, -Rf, -Ra2, -Rb1, -Rc, -Ro, -Rb2, -Rb3, -Rd, 20(R)-ginsenoside Rg3, 24(R)-pseudoginsenoside F11, chikusetsusaponins IV, and -IVa) using an ultra-high-performance liquid chromatography/charged aerosol detector (UHPLC-CAD) approach. Twelve Panax-derived ginseng products (involving P. ginseng root, P. quinquefolius root, P. notoginseng root, Red ginseng, P. ginseng leaf, P. quinquefolius leaf, P. notoginseng leaf, P. ginseng flower, P. quinquefolius flower, P. notoginseng flower, P. japonicus root, and P. japonicus var. major root) were considered. Benefiting from the condition optimization, the baseline resolution of 15 ginsenosides was achieved on a CORTECS UPLC Shield RP18 column. This method was validated as specific, precise (0.81-1.94% intra-day variation; 0.86-2.35% inter-day variation), and accurate (recovery: 90.73-107.5%), with good linearity (R² > 0.999), high sensitivity (limit of detection: 0.02-0.21 μg; limit of quantitation: 0.04-0.42 μg) and sample stability (1.49-4.74%). Its application to 119 batches of ginseng samples unveiled vital information enabling the authentication of these different ginseng products. Detection of ginsenosides by CAD exhibited superiority over UV in sensitivity and the ability to monitor chromophore-free structures. Large-scale comparative studies by quantifying multiple markers provide methodological reference to the precise quality control of herbal medicine.

Liamocins biosynthesis, its regulation in Aureobasidium spp., and their bioactivities

Liamocins synthesized by Aureobasidium spp. are glycolipids composed of a single mannitol or arabitol headgroup linked to either three, four or even six 3,5-dihydroxydecanoic ester tail-groups. The highest titer of liamocin achieved was over 40.0 g/L. The substrates for liamocins synthesis include glucose, sucrose, xylose, mannitol, and others. The Pks1 is responsible for the biosynthesis of the tail-group 3,5-dihydroxydecanoic acid, both mannitol dehydrogenase (MDH) and mannitol 1-phosphate 5-dehydrogenase (MPDH) catalyze the mannitol biosynthesis and the arabitol biosynthesis is controlled by arabitol dehydrogenase (ArDH). The ester bond formation between 3,5-dihydroxydecanoic acid and mannitol or arabitol is catalyzed by the esterase (Est1). Liamocin biosynthesis is regulated by the specific transcriptional activator (Gal1), global transcriptional activator (Msn2), various signaling pathways, acetyl-CoA flux while Pks1 activity is controlled by PPTase activity. The synthesized liamocins have high bioactivity against the pathogenic bacteria Streptococcus spp. and some kinds of cancer cells while Massoia lactone released liamocins which exhibited obvious antifungal and anticancer activities. Therefore, liamocins and Massoia lactone have many applications in various sectors of biotechnology.

Fungal biosurfactants, from nature to biotechnological product: bioprospection, production and potential applications

Biosurfactants are in demand by the global market as natural commodities that can be added to commercial products or use in environmental applications. These biomolecules reduce the surface/interfacial tension between fluid phases and exhibit superior stability to chemical surfactants under different physico-chemical conditions. Biotechnological production of biosurfactants is still emerging. Fungi are promising producers of these molecules with unique chemical structures, such as sophorolipids, mannosylerythritol lipids, cellobiose lipids, xylolipids, polyol lipids and hydrophobins. In this review, we aimed to contextualize concepts related to fungal biosurfactant production and its application in industry and the environment. Concepts related to the thermodynamic and physico-chemical properties of biosurfactants are presented, which allows detailed analysis of their structural and application. Promising niches for isolating biosurfactant-producing fungi are presented, as well as screening methodologies are discussed. Finally, strategies related to process parameters and variables, simultaneous production, process optimization through statistical and genetic tools, downstream processing and some aspects of commercial products formulations are presented.

Coproduction of polymalic acid and liamocins from two waste by-products from the xylitol and gluconate industries by Aureobasidium pullulans

The coproduction of polymalic acid (PMA) and liamocins, two important metabolites secreted by Aureobasidium pullulans, from two waste by-products from the xylitol and gluconate industries was investigated in shake flasks and fermentors, confirming that waste xylose mother liquor (WXML) could be utilized as an economical feedstock without any pretreatment. Gluconate could strengthen carbon flux and NADPH supply for the synergetic biosynthesis of PMA and liamocins. High PMA and liamocin titers of 82.9 ± 2.1 and 28.3 ± 2.7 g/L, respectively, were obtained from the coupled WXML and waste gluconate mother liquor (WGML) in batch fermentation, with yields of 0.84 and 0.25 g/g, respectively. These results are comparable to those obtained from renewable feedstocks. Economic assessment of the process revealed that PMA and liamocins could be coproduced from two by-products at costs of $1.48/kg or $0.67/kg (with liamocins credit), offering an economic and sustainable process for the application of waste by-products.

Genetic evidences for the core biosynthesis pathway, regulation, transport and secretion of liamocins in yeast-like fungal cells

So far, it has been still unknown how liamocins are biosynthesized, regulated, transported and secreted. In this study, a highly reducing polyketide synthase (HR-PKS), a mannitol-1-phosphate dehydrogenase (MPDH), a mannitol dehydrogenase (MtDH), an arabitol dehydrogenase (ArDH) and an esterase (Est1) were found to be closely related to core biosynthesis of extracellular liamocins in Aureobasidium melanogenum 6-1-2. The HR-PKS was responsible for biosynthesis of 3,5-dihydroxydecanoic acid. The MPDH and MtDH were implicated in mannitol biosynthesis and the ArDH was involved in arabitol biosynthesis. The Est1 catalyzed ester bond formation of them. A phosphopantetheine transferase (PPTase) activated the HR-PKS and a transcriptional activator Ga11 activated expression of the PKS1 gene. Therefore, deletion of the PKS1 gene, all the three genes encoding MPDH, MtDH and ArDH, the EST1, the gene responsible for PPTase and the gene for Ga11 made all the disruptants (Δpks13, Δpta13, Δest1, Δp12 and Δg11) totally lose the ability to produce any liamocins. A GLTP gene encoding a glycolipid transporter and a MDR1 gene encoding an ABC transporter took part in transport and secretion of the produced liamocins into medium. Removal of the GLTP gene and the MDR1 gene resulted in a Δgltp1 mutant and a Δmdr16 mutant, respectively, that lost the partial ability to secrete liamocins, but which cells were swollen and intracellular lipid accumulation was greatly enhanced. Hydrolysis of liamocins released 3,5-dihydroxydecanoic acid, mannitol, arabitol and acetic acid. We proposed a core biosynthesis pathway, regulation, transport and secretion of liamocins in A. melanogenum.

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.

Screening and isolation of the liamocin-producing yeast Aureobasidium melanogenum using xylose as the sole carbon source

Xylose, the main component of xylan, is the second most abundant sugar in nature after glucose. Consequently, xylose represents an attractive feedstock for the production of value-added compounds such as biosurfactants (BSs), which are produced by various bacteria and yeasts. In this study, we screened and isolated yeast strains that synthesize BSs using xylose as the sole carbon source. We applied matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) to screen for BS-producing yeasts and isolated eight strains as the liamocin producers. Two of the eight strains, AS37 and SK25, were identified as Aureobasidium melanogenum, which is known as black yeasts, by based on 26S ribosomal RNA gene sequences. Both strains produced a wide variety of liamocin structures from not only xylose but also glucose and sucrose. According to the MALDI-TOF MS analysis, signals corresponding to sodium ion adducts of di-, tri-, tetra-, penta- and hexa-acylated C6-liamocins and di-, tri- and tetra-acylated C5-liamocins were detected. In addition, their mono-acetylated form was also detected. The dominant sugar component of liamocins produced by strains AS37 and SK25 is mannitol as estimated by HPLC analysis. This is the first report to describe the screening of liamocins-producing yeasts using xylose as the sole carbon source.