Structure-Activity Relationship Assessment of Sophorolipid Ester Derivatives against Model Bacteria Strains

Exploring Biosurfactants as Antimicrobial Approaches

Antibacterial resistance is one of the most important global threats to human health. Several studies have been performed to overcome this problem and infection-preventive approaches appear as promising solutions. Novel antimicrobial preventive molecules are needed and microbial biosurfactants have been explored in that scope. Considering their structure, these biomolecules can be divided into different classes, glycolipids and lipopeptides being the most studied. Besides their antimicrobial activity, biosurfactants have the advantage of being biocompatible, biodegradable, and non-toxic, which favor their application in several areas, including the health sector. Often, the most difficult infections to fight are associated with biofilm formation, particularly in medical devices. Strategies to overcome micro-organism attachment are thus emergent, and it is possible to take advantage of the antimicrobial/antibiofilm properties of biosurfactants to produce surfaces that are more resistant to the deposition/attachment of bacteria. Approaches such as the covalent bond of biosurfactants to the medical device surface leading to repulsive physical-chemical interactions or contact killing can be selected. Simpler strategies such as the absorption of biosurfactants on surfaces are also possible, eliminating micro-organisms in the vicinity. This review will focus on the physical and chemical characteristics of biosurfactants, their antimicrobial activity, antimicrobial/antibiofilm approaches, and finally on their structure-activity relationship.

ASLdC3: A Derivative of Acidic Sophorolipid Disrupts Mitochondrial Function, Induces ROS Generation, and Inhibits Biofilm Formation in Candida albicans

Fungal infections account for more than 140 million cases of severe and life-threatening conditions each year, causing approximately 1.7 million deaths annually. Candida albicans and related species are the most common human fungal pathogens, causing both superficial (mucosal and cutaneous) and life-threatening invasive infections (candidemia) with a 40-75% mortality rate. Among many virulence factors of Candida albicans, morphological transition from yeast to hyphae, secretion of hydrolytic enzymes, and formation of biofilms are considered to be crucial for pathogenicity. However, the arsenals for the treatment against these pathogens are restricted to only a few classes of approved drugs, the efficacy of which is being compromised by host toxicity, fungistatic activity, and the emergence of drug resistance. In this study, we have described the development of a molecule, exhibiting excellent antifungal activity (MIC 8 μg/mL), by tailoring acidic sophorolipids with aryl alcohols via enzyme catalysis. This novel derivative, ASLdC3, is a surface-active compound that lowers the surface tension of the air-water interface up to 2-fold before reaching the critical micelle concentration of 25 μg/mL. ASLdC3 exhibits excellent antibiofilm properties against Candida albicans and other nonalbicans Candida species. The molecule primarily exhibits its antifungal activity by perturbing mitochondrial function through the alteration of the mitochondrial membrane potential (MMP) and generation of reactive oxygen species (ROS). The ROS damages fungal cell membrane function and cell wall integrity, eventually leading to cell death. ASLdC3 was found to be nontoxic in in vitro assay and nonhemolytic. Besides, it does not cause toxicity in the C. elegans model. Our study provides a valuable foundation for the potential of acidic sophorolipid as a nontoxic, biodegradable precursor for the design and synthesis of novel molecules for use as antimicrobial drugs as well as for other clinical applications.

Synergistic Treatment of Breast Cancer by Combining the Antimicrobial Peptide Piscidin with a Modified Glycolipid

Piscidin 3 (P3), a peptide produced by fish, and a hexyl ester-modified sophorolipid (SL-HE), have individually shown promise as antimicrobial and anticancer drugs. A recent report by our team revealed that combining P3 with SL-HE in a 1:8 molar ratio resulted in an 8-fold enhancement in peptide activity, while SL-HE improved by 25-fold its antimicrobial activity against the Gram-positive microorganism Bacillus cereus. Extending these findings, the same P3/SL-HE combination was assessed on two breast cancer cell lines: BT-474, a hormonally positive cell line, and MDA-MB-231, an aggressive triple-negative cell line. The results demonstrated that the 1:8 molar ratio of P3/SL-HE synergistically enhances the anticancer effects against both tumorigenic breast cell lines. Mechanistic studies indicate the activation of an intrinsic apoptotic cell death mechanism through an increase in reactive oxygen species and mitochondrial dysfunction and a secondary programmed necrotic pathway that involves pore formation in the plasma membrane. When a fibroblast cell line, CCD1065SK HDF, was utilized to determine selectivity, the synergistic SL-HE/P3 combination exhibited a protective property compared to the use of SL-HE alone and therefore afforded vastly improved selectivity indices. Given the promising results reported herein, the synergistic combination of P3/SL-HE constitutes a novel strategy that merits further study for the treatment of breast cancer.

Amide Modification of Glycolipid Biosurfactants as Promising Biocompatible Antibacterial Agents

Discovering new antibacterial agents is crucial to addressing the increasing risk of bacterial infections induced by antimicrobial resistance in food and agricultural industries. Here, biocompatible acidic-type sophorolipids (ASLs) and glucolipids (GLs) prepared via chemical modification of natural sophorolipids from fermentation were functionalized via amide modification for use as potential antibacterial agents. It was found that the arginine methyl ester derivative of GLs (GLs-d-Arg-OMe) showed excellent antibacterial activity, killing more than 99.99% of Escherichia coli at 200 mg/L. The sterilization dosage of the GLs against Bacillus subtilis, Bacillus cereus, and Staphylococcus aureus was 16-64 mg/L, in contrast to 32-64 mg/L for the fungus Candida albicans. In particular, GLs-d-Arg-OMe showed the best biocompatibility with a therapeutic index of up to 18. It was shown that amide modification of glycolipids can effectively improve antibacterial activity while maintaining biocompatibility, which can be exploited for the development of novel antibiotics in food and agricultural fields.

Tuning the antimicrobial activity of microbial glycolipid biosurfactants through chemical modification

Sophorolipids, glycolipid biosurfactants derived from microorganisms such as Starmerella bombicola, possess distinctive surface-active and bioactive properties, holding potential applications in cosmetics, pharmaceuticals and bioremediation. However, the limited structural variability in wild-type sophorolipids restricts their properties and applications. To address this, metabolic engineering efforts have allowed to create a portfolio of molecules. In this study, we went one step further by chemically modifying microbially produced sophorosides, produced by an engineered S. bombicola. Twenty-four new sophoroside derivatives were synthesized, including sophoroside amines with varying alkyl chain lengths (ethyl to octadecyl) on the nitrogen atom and their corresponding quaternary ammonium salts. Additionally, six different microbially produced glycolipid biosurfactants were hydrogenated to achieve fully saturated lipid tails. These derivatives, along with microbially produced glycolipids and three benchmark biosurfactants (di-rhamnolipids, alkyl polyglucosides, cocamidopropyl betaine), were assessed for antimicrobial activity against bacteria (Bacillus subtilis, Staphylococcus aureus, Listeria monocytogenes, Escherichia coli, Pseudomonas aeruginosa) and yeast (Candida albicans). Results indicated that microbially produced glycolipids, such as bola sophorosides, acidic sophorolipids and acidic glucolipids exhibit selective antimicrobial activity against the test organisms. Conversely, lactonic sophorolipids, sophoroside amines and quaternary ammonium salts display a broad antimicrobial activity. N-octyl, N-dodecyl and N-octadecyl derivatives exhibit the lowest minimal inhibitory concentrations, ranging from 0.014 to 20.0 mg mL-1. This study demonstrates the potential synergy of thoughtful biotechnology and targeted chemistry to precisely tailor glycolipid biosurfactants to meet specific requirements across applications.

Microbial Biosurfactants: Antimicrobial Activity and Potential Biomedical and Therapeutic Exploits

The rapid emergence of multidrug-resistant pathogens worldwide has raised concerns regarding the effectiveness of conventional antibiotics. This can be observed in ESKAPE pathogens, among others, whose multiple resistance mechanisms have led to a reduction in effective treatment options. Innovative strategies aimed at mitigating the incidence of antibiotic-resistant pathogens encompass the potential use of biosurfactants. These surface-active agents comprise a group of unique amphiphilic molecules of microbial origin that are capable of interacting with the lipidic components of microorganisms. Biosurfactant interactions with different surfaces can affect their hydrophobic properties and as a result, their ability to alter microorganisms' adhesion abilities and consequent biofilm formation. Unlike synthetic surfactants, biosurfactants present low toxicity and high biodegradability and remain stable under temperature and pH extremes, making them potentially suitable for targeted use in medical and pharmaceutical applications. This review discusses the development of biosurfactants in biomedical and therapeutic uses as antimicrobial and antibiofilm agents, in addition to considering the potential synergistic effect of biosurfactants in combination with antibiotics. Furthermore, the anti-cancer and anti-viral potential of biosurfactants in relation to COVID-19 is also discussed.

Host Defense Peptide Piscidin and Yeast-Derived Glycolipid Exhibit Synergistic Antimicrobial Action through Concerted Interactions with Membranes

Developing new antimicrobials as alternatives to conventional antibiotics has become an urgent race to eradicate drug-resistant bacteria and to save human lives. Conventionally, antimicrobial molecules are studied independently even though they can be cosecreted in vivo. In this research, we investigate two classes of naturally derived antimicrobials: sophorolipid (SL) esters as modified yeast-derived glycolipid biosurfactants that feature high biocompatibility and low production cost; piscidins, which are host defense peptides (HDPs) from fish. While HDPs such as piscidins target the membrane of pathogens, and thus result in low incidence of resistance, SLs are not well understood on a mechanistic level. Here, we demonstrate that combining SL-hexyl ester (SL-HE) with subinhibitory concentration of piscidins 1 (P1) and 3 (P3) stimulates strong antimicrobial synergy, potentiating a promising therapeutic window. Permeabilization assays and biophysical studies employing circular dichroism, NMR, mass spectrometry, and X-ray diffraction are performed to investigate the mechanism underlying this powerful synergy. We reveal four key mechanistic features underlying the synergistic action: (1) P1/3 binds to SL-HE aggregates, becoming α-helical; (2) piscidin-glycolipid assemblies synergistically accumulate on membranes; (3) SL-HE used alone or bound to P1/3 associates with phospholipid bilayers where it induces defects; (4) piscidin-glycolipid complexes disrupt the bilayer structure more dramatically and differently than either compound alone, with phase separation occurring when both agents are present. Overall, dramatic enhancement in antimicrobial activity is associated with the use of two membrane-active agents, with the glycolipid playing the roles of prefolding the peptide, coordinating the delivery of both agents to bacterial surfaces, recruiting the peptide to the pathogenic membranes, and supporting membrane disruption by the peptide. Given that SLs are ubiquitously and safely used in consumer products, the SL/peptide formulation engineered and mechanistically characterized in this study could represent fertile ground to develop novel synergistic agents against drug-resistant bacteria.

Sophorolipid Candidates Demonstrate Cytotoxic Efficacy against 2D and 3D Breast Cancer Models

Sophorolipids are biosurfactants derived from the nonpathogenic yeasts such as Starmerella bombicola with potential efficacy in anticancer applications. Simple and cost-effective synthesis of these drugs makes them a promising alternative to traditional chemotherapeutics, pending their success in preliminary drug-screening. Drug-screening typically utilizes 2D cell monolayers due to their simplicity and ease of high-throughput assessment. However, 2D assays fail to capture the complexity and 3D context of the tumor microenvironment and have consequently been implicated in the high percentage of drugs investigated in vitro that later fail in clinical trials. Herein, we screened two sophorolipid candidates and a clinically-used chemotherapeutic, doxorubicin, on in vitro breast cancer models ranging from 2D monolayers to 3D spheroids, employing optical coherence tomography to confirm these morphologies. We calculated corresponding IC50 values for these drugs and found one of the sophorolipids to have comparable toxicities to the chemotherapeutic control. Our findings show increased drug resistance associated with model dimensionality, such that all drugs tested showed that 3D spheroids exhibited higher IC50 values than their 2D counterparts. These findings demonstrate promising preliminary data to support the use of sophorolipids as a more affordable alternative to traditional clinical interventions and demonstrate the importance of 3D tumor models in assessing drug response.

Preparing Biosurfactant Glucolipids from Crude Sophorolipids via Chemical Modifications and Their Potential Application in the Food Industry

This investigation developed a novel strategy for efficiently preparing glucolipids (GLs) by chemically modifying crude sophorolipids. Running this strategy, crude sophorolipids were effectively transformed into GLs through deglycosylation and de-esterification, with a yield of 54.1%. The acquired GLs were then purified via stepwise extractions, and 66.2% of GLs with 95% purity was recovered. GLs are more hydrophobic and present a stronger surface activity than acidic sophorolipids (ASLs). More importantly, these GLs displayed a superior antimicrobial activity to that of ASLs against the tested Gram-positive food pathogens, with a minimum inhibitory concentration of 32-64 mg/L, except against E. coli . This activity of GLs is pH-dependent and especially more powerful under acidic conditions. The mechanism involved is possibly associated with the more efficient adsorption of GLs, as demonstrated by the hydrophobicity of the cell membrane. These GLs could be used as antimicrobial agents for food preservation and health in the food industry.

Sustainable bio-based antimicrobials derived from fatty acids: Synthesis, safety, and efficacy

Some conventional sanitizers and antibiotics used in food industry may be of concerns due to generation of toxic byproducts, impact on the environment, and the emergence of antibiotic resistance bacteria. Bio-based antimicrobials can be an alternative to conventional sanitizers since they are produced from renewable resources, and the bacterial resistance to these compounds is of less concern than those of currently used antibiotics. Among the bio-based antimicrobial compounds, those produced via either fermentation or chemical synthesis by covalently or electrovalently attaching specific moieties to the fatty acid have drawn attention in recent years. Disaccharide, arginine, vitamin B1, and phenolics are linked to fatty acids resulting in the production of sophorolipid, lauric arginate ethyl ester, thiamin dilauryl sulfate, and phenolic branched-chain fatty acid, respectively, all of which are reported to exhibit antimicrobial activity by targeting the cell membrane of the bacteria. Also, studies that applied these compounds as food preservatives by combining them with other compounds or treatments have been reviewed regarding extending the shelf life and inactivating foodborne pathogens of foods and food products. In addition, the phenolic branched-chain fatty acids, which are relatively new compounds compared to the others, are highlighted in this review.

Formulation of a glycolipid:lipopeptide mixture as biosurfactant-based dispersant and development of a low-cost glycolipid production process

Biosurfactant-based dispersants were formulated by mixing glycolipids from Weissella cibaria PN3 and lipopeptides from Bacillus subtilis GY19 to enhance the synergistic effect and thereby achieve hydrophilic-lipophilic balance. The proportions of each biosurfactant and dispersant-to-oil ratios (DORs) were varied to obtain the appropriated formulations. The most efficient glycolipid:lipopeptide mixtures (F1 and F2) had oil displacement activities of 81–88% for fuel and crude oils. The baffled flask test of these formulations showed 77–79% dispersion effectiveness at a DOR of 1:25. To reduce the cost of the dispersant, this study optimized the glycolipid production process by using immobilized cells in a stirred tank fermenter. Semicontinuous glycolipid production was carried out conveniently for 3 cycles. Moreover, food wastes, including waste coconut water and waste frying oil, were found to promote glycolipid production. Glycolipids from the optimized process and substrates had similar characteristics but 20–50% lower cost than those produced from basal medium with soybean oil in shaking flasks. The lowest cost dispersant formulation (F2*) contained 10 g/L waste-derived cell-bound glycolipid and 10 g/L lipopeptide and showed high dispersion efficiency with various oils. Therefore, this biosurfactant-based dispersant could be produced on a larger scale for further application.