The Physicochemical and Functional Properties of Biosurfactants: A Review

Surfactants, also known as surface-active agents, have emerged as an important class of compounds with a wide range of applications. However, the use of chemical-derived surfactants must be restricted due to their potential adverse impact on the ecosystem and the health of human and other living organisms. In the past few years, there has been a growing inclination towards natural-derived alternatives, particularly microbial surfactants, as substitutes for synthetic or chemical-based counterparts. Microbial biosurfactants are abundantly found in bacterial species, predominantly Bacillus spp. and Pseudomonas spp. The chemical structures of biosurfactants involve the complexation of lipids with carbohydrates (glycolipoproteins and glycolipids), peptides (lipopeptides), and phosphates (phospholipids). Lipopeptides, in particular, have been the subject of extensive research due to their versatile properties, including emulsifying, antimicrobial, anticancer, and anti-inflammatory properties. This review provides an update on research progress in the classification of surfactants. Furthermore, it explores various bacterial biosurfactants and their functionalities, along with their advantages over synthetic surfactants. Finally, the potential applications of these biosurfactants in many industries and insights into future research directions are discussed.

Quantitative determination of rhamnolipid using HPLC-UV through carboxyl labeling

Rhamnolipid, as a low-toxic, biodegradable and environmentally friendly biosurfactant, has broad application prospects in many industries. However, the quantitative determination of rhamnolipid is still a challenging task. Here, a new sensitive method for the quantitative analysis of rhamnolipid based on a simple derivatization reaction was developed. In this study, 3-[3'-(l-rhamnopyranosyloxy) decanoyloxy] decanoic acid (Rha-C10-C10) and 3-[3'-(2'-O-α-l-rhamnopyranosyloxy) decanoyloxy] decanoic acid (Rha-Rha-C10-C10) were utilized as the representative rhamnolipids. Liquid chromatography-mass spectrometry and high-performance liquid chromatography-ultra violet results showed that these two compounds were successfully labeled with 1 N1-(4-nitrophenyl)-1,2-ethylenediamine. There was an excellent linear relationship between rhamnolipid concentration and peak area of labeled rhamnolipid. The detection limits of the Rha-C10-C10 and Rha-Rha-C10-C10 were 0.018 mg/L (36 nmol/L) and 0.014 mg/L (22 nmol/L), respectively. The established amidation method was suitable for the accurate analysis of rhamnolipids in the biotechnological process. The method had good reproducibility with the relative standard deviation of 0.96% and 0.79%, respectively, and sufficient accuracy with a recovery of 96%-100%. This method was applied to quantitative analysis of 10 rhamnolipid homologs metabolized by Pseudomonas aeruginosa LJ-8. The single labeling method was used for the quantitative analysis of multiple components, which provided an effective method for the quality evaluation of other glycolipids with carboxyl groups.

Isolation, identification, and screening of biosurfactant-producing and hydrocarbon-degrading bacteria from oil and gas industrial waste

Qatar is one of the biggest oil and gas producers in the world, coupled with it is challenging environmental conditions (high average temperature: >40 °C, low annual rainfall: 46.71 mm, and high annual evaporation rate: 2200 mm) harbors diverse microbial communities that are novel and robust, with the potential to biodegrade hydrocarbons. In this study, we collected hydrocarbon contaminated sludge, wastewater and soil samples from oil and gas industries in Qatar. Twenty-six bacterial strains were isolated in the laboratory from these samples using high saline conditions and crude oil as the sole carbon source. A total of 15 different bacterial genera were identified in our study that have not been widely reported in the literature or studied for their usage in the biodegradation of hydrocarbons. Interestingly, some of the bacteria that were identified belonged to the same genus however, demonstrated variable growth rates and biosurfactant production. This indicates the possibility of niche specialization and specific evolution to acquire competitive traits for better survival. The most potent strain EXS14, identified as Marinobacter sp., showed the highest growth rate in the oil-containing medium as well as the highest biosurfactant production. When this strain was further tested for biodegradation of hydrocarbons, the results showed that it was able to degrade 90 to 100% of low and medium molecular weight hydrocarbons and 60 to 80% of high molecular weight (C35 to C50) hydrocarbons. This study offers many promising leads for future studies of microbial species and their application for the treatment of hydrocarbon contaminated wastewater and soil in the region and in other areas with similar environmental conditions.

Antimicrobial activity of dietary supplements based on bacterial lysate of Lactobacillus rhamnosus DV

Introduction

According to WHO, antibiotic resistance is increasing to hazardous levels worldwide. Candidiasis often occurs after taking antibiotics. Therefore, antibiotic resistance is a global problem and searching for antibacterial agents is necessary.

Aim

To determine the antimicrobial activity of bacterial lysate of Lactobacillus (L.) rhamnosus DV separately and with plant extracts against bacterial and yeast test cultures.

Material and methods

Antimicrobial activity of Del-Immune V® (cell wall and DNA fragments from a L. rhamnosus DV) separately and with cinnamon, beetroot, and blackcurrant extracts was determined by the minimum inhibitory concentration (MIC). Twofold serial dilutions determined the MIC in previously prepared meat-peptone broth (MPB) for bacteria and liquid wort for yeast. In the study, gram-negative (Escherichia coli IEM-1, Proteus vulgaris PА-12, Pseudomonas sp. MI-2, L. rhamnosus 13/2) and gram-positive (Bacillus (B.) subtilis BТ-2, Staphylococcus aureus BМС-1) bacteria, as well as yeast (Candida (C.) albicans D-6, C. tropicalis PE-2, C. utilis BVS-65) were used as test cultures.

Results

The MIC for the studied bacterial test cultures after application of L. rhamnosus DV bacterial lysates was from 1.0 ± 0.05 mg/mL to 12.5 ± 0.63 mg/mL, which was significantly less than that of the thermally inactivated control (MIC from 125.0 ± 6.25 mg/mL to 250.0 ± 12.5 mg/mL). B. subtilis BT-2 culture was the least sensitive to the action of the bacterial lysate (MIC-12.5 ± 0.63 mg/mL). It showed the best antibacterial and antifungal effect bacterial lysate with the phytonutrient blackcurrant.

Conclusions

It was demonstrated that bacterial lysate of lactic acid bacteria L. rhamnosus DV exhibits antibacterial and antifungal properties during direct contact with pathogenic agents.

Antifouling Potential of Ethyl Acetate Extract of Marine Bacteria Pseudomonas aeruginosa Strain RLimb

Biofouling is defined as the excessive colonization process of epibiotic organisms, ranging from microfoulers to macrofoulers, on any submerged surface in water. Previous research has attempted to explore the antifouling activity of bacterial isolates due to the biofouling problems occurring worldwide. One solution is to inhibit the early stage of fouling using secondary metabolites produced by marine bacteria. This study aims to determine the antifouling activities of the marine microorganism P. aeruginosa and to characterize the bacteria isolated as a potential anti-biofouling agent. The bacterial isolate was cultured and isolated on a media culture. The bacteria culture extract was extracted using ethyl acetate and concentrated prior to the bioassay method. It was screened for antibacterial activities against Gram-positive and Gram-negative bacteria, such as Bacillus cereus, Streptococcus uberis, Pseudomonas sp., and Vibrio parahaemolyticus, using the disk diffusion technique. The extract was investigated to verify its bioactivity in the prevention of biofilm formation following the crystal violet assay and aquarium test. The results indicated the inhibition of activity through biofilm formation, with the highest percentage at 83% of biofilm inhibition at a concentration of 0.1563 mg/mL. The bacterial isolate at a concentration of 5% showed the highest reduction in bacteria colonies in the aquarium test (161.8 × 103 CFU/mL compared to 722.5 × 103 CFU/mL for the blank sample). The bacterial isolate was characterized through phenotypic and genotypic tests for species identification. It was identified as a Gram-stain-negative, aerobic, and long-rod-shaped bacteria, designated as RLimb. Based on the 16S rDNA gene sequencing analysis, RLimb was identified as Pseudomonas aeruginosa (accession number: OP522351), exhibiting a similarity of 100% to the described neighbor P. aeruginosa strain DSM 50071. These results indicated that these isolated bacteria can potentially be used as a substitute for toxic antifoulants to prevent the formation of microfoulers.

Biofouling in Membrane Bioreactors: Mechanism, Interactions and Possible Mitigation Using Biosurfactants

Biofouling roots damage to membrane bioreactors (MBRs), such as physical, functional and organisational changes and even therefore clogging of the membrane pores and successive microbial degradation. Further, it blocks the pores, results into a biomass cake and in due course reduces the membrane flux and leads to an increase in the operational costs. MBR fouling contributed to the rise in transmembrane pressure (TMP) and decrease in permeate flux (in case of constant pressure operation mode). Chemical surfactants adopted for the cleaning of membrane surfaces have certain disadvantages such as toxicity manifestations, damage to the membranes and high CMC concentrations. Biosurfactant surfactants have attained increasing interest due to their low toxicity, biodegradability, stability to extreme environmental conditions such as temperatures, pH and tolerance to salinity. The biosurfactants trapped the foulants via micelle formation, which distresses hydrophobic interactions amongst bacteria and the surface. Rhamnolipids as an anionic biosurfactant pose a significant interfacial potential and have affinity to bind organic matter. The present review discusses the problem of biofouling in MBRs, type and interactions of foulants involved and also highlights the mechanisms of biosurfactant cleaning, effect of different parameters, effect of concentration, TMP, flux recovery, permeability, mitigation practices and challenges.

Insights into rhamnolipid amendment towards enhancing microbial electrochemical treatment of petroleum hydrocarbon contaminated soil

Environmental pollution by hydrophobic hydrocarbons is increasing, notably nowadays due to a large amount of industrial activity. Microbial electrochemical technologies (MET) are promising bio-based systems which can oxidize hydrophobic hydrocarbon pollutants and produce bioelectricity simultaneously. However, MET faces some issues in terms of soil remediation, including low mass transfer, limited electro-activity of anodes as electron acceptors, low bioavailability of hydrocarbons, and the limited activity of beneficial bacteria and inefficient electron transport. This study aims to investigate the role of the addition of rhamnolipid as an analyte solution to the MET to enhance the efficacy and concurrently solve the abovementioned issues. In this regard, a novel long chain of RL was produced by using low-cost carbon winery waste through non-pathogenic Burkholderia thailandensis E264 strains. Different doses of RL were tested, including 10, 50, and 100 mg/L. A maximum enhancement in the oxidation of hydrophobic hydrocarbons was found to be up to 72.5%, while the current density reached 9.5 Am-2 for the MET reactor having a dose of 100 mg/L. The biosurfactants induced a unique microbial enrichment associated with Geobacter, Desulfovibrio, Klebsiella, and Comamona on the anode surface, as well as Pseudomonas, Acinetobacter, and Franconibacter in soil MET, indicating the occurrence of a metabolic pathway in microbes working with the anode and soil bioelectrochemical remediation system. According to cyclic voltammetry analysis, redox peaks appeared, showing a minor shift in redox MET-biosurfactant compared to the bare MET system. Furthermore, the phytotoxicity of polluted soil to L. sativum seeds after and before MET remediation shows a decrease in phytotoxicity of 77.5% and 5% for MET-biosurfactant system and MET only, respectively. With MET as a tool, this study confirmed for the first time that novel long-chain RL produced from non-Pseudomonas bacteria could remarkably facilitate the degradation of petroleum hydrocarbon via extracellular electron transfer, which provides novel insights to understand the mechanisms of RL regulating petroleum hydrocarbon degradation.

Bioprospecting the Antibiofilm and Antimicrobial Activity of Soil and Insect Gut Bacteria

Antimicrobial resistance is a growing concern in public health and current research shows an important role for bacterial biofilms in recurrent or chronic infections. New strategies, therefore, are necessary to overcome antimicrobial resistance, through the development of new therapies that could alter or inhibit biofilm formation. In this sense, antibiofilm natural products are very promising. In this work, a bioprospection of antimicrobial and antibiofilm extracts from Uruguayan soil bacteria and insect gut bacteria was carried out. Extracts from extracellular broths were tested for their ability to inhibit planktonic cell growth and biofilm formation. Genomic analysis of Bacillus cereus ILBB55 was carried out. All extracts were able to inhibit the growth of, at least, one microorganism and several extracts showed MICs lower than 500 µg mL⁻¹ against microorganisms of clinical relevance (Staphylococcus aureus, Pseudomonas aeruginosa, and Enterobacter cloacae). Among the extracts evaluated for biofilm inhibition only ILBB55, from B. cereus, was able to inhibit, S. aureus (99%) and P. aeruginosa (62%) biofilms. Genomic analysis of this strain showed gene clusters similar to other clusters that code for known antimicrobial compounds. Our study revealed that extracts from soil bacteria and insect gut bacteria, especially from B. cereus ILBB55, could be potential candidates for drug discovery to treat infectious diseases and inhibit S. aureus and P. aeruginosa biofilms.

Recent Strategies to Combat Biofilms Using Antimicrobial Agents and Therapeutic Approaches

Biofilms are intricate bacterial assemblages that attach to diverse surfaces using an extracellular polymeric substance that protects them from the host immune system and conventional antibiotics. Biofilms cause chronic infections that result in millions of deaths around the world every year. Since the antibiotic tolerance mechanism in biofilm is different than that of the planktonic cells due to its multicellular structure, the currently available antibiotics are inadequate to treat biofilm-associated infections which have led to an immense need to find newer treatment options. Over the years, various novel antibiofilm compounds able to fight biofilms have been discovered. In this review, we have focused on the recent and intensively researched therapeutic techniques and antibiofilm agents used for biofilm treatment and grouped them according to their type and mode of action. We also discuss some therapeutic approaches that have the potential for future advancement.

Seeking faster, alternative methods for glycolipid biosurfactant characterization and purification

Biosurfactants have been investigated as potential alternatives for synthetic surfactants in several areas, for example, in environmental and pharmaceutical fields. In that regard, extensive research has been carried out with sophorolipids and rhamnolipids that also present various biological properties with therapeutic significance. These biosurfactants are obtained as complex mixtures of slightly different molecules, and thus when studying these microbial glycolipids, the ability to identify and purify the produced compounds is of extreme importance. This study aimed to develop improved methodologies for the identification, separation, and purification of sophorolipids and rhamnolipids. Therefore, an ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method was modified to ensure faster characterization of both sophorolipids and rhamnolipids, enabling the identification and fragmentation pattern description of 10 and 13 congeners, respectively. The separation and purification of these biosurfactants was achieved with novel reversed-phase solid-phase extraction methods guaranteeing the isolation of different glycolipids, including those considered for their significant biological activity (e.g. antimicrobial, anticancer). It was possible to isolate sophorolipids and rhamnolipids with purity of 94% and 99%, respectively. The methods presented herein can be easily implemented and are expected to make purification of these biosurfactants easier, facilitating the study of their individual properties in further works.

Burkholderia thailandensis E264 as a promising safe rhamnolipids' producer towards a sustainable valorization of grape marcs and olive mill pomace

Within the circular economy framework, our study aims to assess the rhamnolipid production from winery and olive oil residues as low-cost carbon sources by nonpathogenic strains. After evaluating various agricultural residues from those two sectors, Burkholderia thailandensis E264 was found to use the raw soluble fraction of nonfermented (white) grape marcs (NF), as the sole carbon and energy source, and simultaneously, reducing the surface tension to around 35 mN/m. Interestingly, this strain showed a rhamnolipid production up to 1070 mg/L (13.37 mg/g of NF), with a higher purity, on those grape marcs, predominately Rha-Rha C14-C14, in MSM medium. On olive oil residues, the rhamnolipid yield of using olive mill pomace (OMP) at 2% (w/v) was around 300 mg/L (15 mg/g of OMP) with a similar CMC of 500 mg/L. To the best of our knowledge, our study indicated for the first time that a nonpathogenic bacterium is able to produce long-chain rhamnolipids in MSM medium supplemented with winery residues, as sole carbon and energy source. KEY POINTS: • Winery and olive oil residues are used for producing long-chain rhamnolipids (RLs). • Both higher RL yields and purity were obtained on nonfermented grape marcs as substrates. • Long-chain RLs revealed stabilities over a wide range of pH, temperatures, and salinities.

Rhamnolipids Application for the Removal of Vanadium from Contaminated Sediment

The use of biosurfactants in bioremediation of hydrocarbons and in the removal of heavy metals in crude oils is considered an attractive subject. The vanadium pollution in soil and sediments had attracted research interest in exploring eco-friendly methods of remediation. The present study was conducted to evaluate the potential of a biosurfactant to remove vanadium from artificially contaminated sand. The biosurfactant producer's strain selection process was carried out from 23 strains in two steps. In the primary screening, four preliminary tests were carried out: the emulsification index (24 and 72 h), the surface tension, and the rate of bacterial adhesion to hydrocarbons. In the secondary screening, the surface tension and rhamnolipids concentration were determined, also critical micellar concentration and dilution were calculated. The RNA 16s of selected strain was sequence and the strain was identified as Pseudomonas sp. By chromatographic and spectroscopic assays, the structure of the rhamnolipids was determined. The maximal vanadium removal efficiency (85.5%) was achieved with a rhamnolipids' concentration of 240 mg l^-1. The vanadium concentration was determined by spectroscopic technique. Rhamnolipids produced by this strain can potentially be used in the removal of vanadium.

Isolation and characterization of a novel rhamnolipid producer Pseudomonas sp. LGMS7 from a highly contaminated site in Ain El Arbaa region of Ain Temouchent, Algeria

This study aims to isolate and characterize a novel rhamnolipid producer within the recent bioremediation approaches for treating hydrocarbon-contaminated soils in Algeria. In this context, from a hydrocarbon-contaminated soil, a newly bacterium designated LGMS7 was screened and identified, belonged to the Pseudomonas genus, and was closely related to Pseudomonas mucidolens, with a 16S rRNA sequence similarity of 99.05%. This strain was found to use different hydrocarbons and oils as a sole carbon and energy source for growth. It showed a stable emulsification index E₂₄ (%) of 66.66% ± 3.46 when growing in mineral salts medium (MSM) supplemented with 2% (v/v) glycerol after incubation for 6 days at 30 °C. Interestingly, it was also able to reduce the surface tension of the cell-free supernatant to around 30 ± 0.65 mN m^-1 with a critical micelle concentration (CMC) of 800 mg l^-1. It was found to be able to produce around 1260 ± 0.57 mg l^-1 as the yield of rhamnolipid production. Its biosurfactant has demonstrated excellent stability against pH (pH 2.0-12.0), salinity (0-150 g l^-1), and temperature (-20 to 121 °C). Based on various chromatographic and spectroscopic techniques (i.e., TLC, FTIR, ¹H-NMR), it was found to belong to the glycolipid class (i.e., rhamnolipids). Taken altogether, the strain LGMS7 and its biosurfactant display interesting biotechnological capabilities for the bioremediation of hydrocarbon-contaminated sites. To the best of our knowledge, this is the first study that described the production of biosurfactants by Pseudomonas mucidolens species.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02751-6.

Lipopeptides production by a newly Halomonas venusta strain: Characterization and biotechnological properties

A halotolerant marine strain PHKT of Halomonas venusta was isolated from contaminated seawater as an efficient biosurfactant producer candidate, on low-value substrate (glycerol). The produced biosurfactants (Bios-PHKT) were characterized as lipopeptides molecules, belonging to surfactin and pumilacidin families, by using Thin Layer Chromatography (TLC), Fourier Transform Infrared Spectroscopy (FT-IR) and Tandem Mass Spectrometry (MALDI-TOF/MS-MS). Bios-PHKT has a critical micelle concentration (CMC) of 125 mg/L, and showed a high steadiness against a wide spectrum of salinity (0-120 g/L NaCl), temperature (4-121 °C) and pH (2-12), supporting its powerful tensioactive properties under various environmental conditions. Likewise, the cytotoxic test revealed that the biosurfactant Bios-PHKT, at concentrations lower than 125 µg/mL, was not cytotoxic for human HEK-293 cells since the cell survival is over than 80%. Furthermore, Bios-PHKT lipopeptides showed excellent anti-adhesive and anti-biofilm activities, being able to avoid and disrupt the biofilm formation by certain pathogenic microorganisms. In addition, the biosurfactant Bios-PHKT showed a remarkable anti-proliferative activity towards tumor B16 melanoma cell line. Besides, Bios-PHKT exhibited an excellent in vitro and in vivo wound healing process. In light of these promising findings, Bios-PHKT could be successfully used in different biotechnological applications.

Investigation of halotolerant marine Staphylococcus sp. CO100, as a promising hydrocarbon-degrading and biosurfactant-producing bacterium, under saline conditions

A halotolerant strain CO100 of Staphylococcus sp. was isolated from contaminated sediments taken from the fishing harbour of Sfax, Tunisia, as an efficient hydrocarbonoclastic candidate. Strain CO100 exhibited a high capacity to break down almost 72% of the aliphatic hydrocarbons contained in crude oil (1%, v/v), used as the sole carbon and energy source, after 20 days of culture, at 100 g/l NaCl, 37 °C and 180 rpm. The isolate CO100 displayed also its ability to grow on phenanthrene, fluoranthene and pyrene (100 mg/l), at 100 g/l NaCl. Moreover, the isolate CO100 showed a notable aptitude to synthesize an efficient tensioactive agent namely BS-CO100, on low-value substrates including residual frying oil and expired milk powder, thus reducing the high cost of biosurfactant production. The ESI/MS analysis designated that BS-CO100 belonged to lipopeptide class, in particular lichenysin and iturine members. Critical micelle concentrations of BS-CO100 were varying between 65 and 750 mg/l, depending on of the purity of the biosurfactant and the used carbon sources. BS-CO100 showed a high steadiness against a wide spectrum of pH (4.3-12), temperature (4-121 °C) and salinity (0-300 g/l NaCl), supporting its powerful tensioactive properties under various environmental conditions. Likewise, BS-CO100 exhibited no cytotoxic effect toward human HEK293 cells, at concentrations within 125 and 1000 μg/ml. Furthermore, the biosurfactant BS-CO100 exhibited remarkable anti-adhesive and anti-biofilm activities, being able to avoid and disrupt the biofilm formation by certain pathogenic microorganisms. In addition, BS-CO100 was found to have more potential to remove hydrocarbons from contaminated soils, compared to some chemical surfactants. In light of these promising findings, strain CO100, as well as its biosurfactant, could be successfully used in different biotechnological applications including the bioremediation of oil-polluted areas, even under saline conditions.

The use of low-cost brewery waste product for the production of surfactin as a natural microbial biocide

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For the first time Bacillus subtilis was able to grow in a culture medium containing Brewery waste (Trub) and produced surfactin.

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Surfactin showed bactericidal effect against Pseudomonas aeruginosa.

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P. aeruginosa biofilm was inhibited (79.8%) when co-incubated with surfactin.

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Surfactin showed anti-adhesive activity on polystyrene surfaces.

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P. aeruginosa biofilm was disrupted (44.9%) when treated with surfactin (700 μg/mL).

Surfactin has potential as next generation antibiofilm agent to combat antimicrobial resistance against emerging pathogens. However, the widespread industrial applications of surfactin is hampered by its high production cost. In this work, surfactin was produced from Bacillus subtilis using a low-cost brewery waste as a carbon source. The strain produced 210.11 mg L−1 after 28 h. The antimicrobial activity was observed against all tested strains, achieving complete inhibition for Pseudomonas aeruginosa, at 500 μg mL−1. A growth log reduction of 3.91 was achieved for P. aeruginosa while, Staphylococcus aureus and Staphylococcus epidermidis showed between 1 and 2 log reductions. In the anti-biofilm assays against P. aeruginosa, the co-incubation, anti-adhesive and disruption showed inhibition, where the greatest inhibition was observed in the co-incubation assay (79.80%). This study provides evidence that surfactin produced from a low-cost substrate can be a promising biocide due to its antimicrobial and anti-biofilm abilities against pathogens.

Rhamnolipids and surfactin inhibit the growth or formation of oral bacterial biofilm

Background

Bacteria survive in various environments by forming biofilms. Bacterial biofilms often cause significant problems to medical instruments and industrial processes. Techniques to inhibit biofilm formation are essential and have wide applications. In this study, we evaluated the ability of two types of biosurfactants (rhamnolipids and surfactin) to inhibit growth and biofilm formation ability of oral pathogenic bacteria such as Aggregatibacter actinomycetemcomitans, Streptococcus mutans, and Streptococcus sanguinis.

Results

Rhamnolipids inhibited the growth and biofilm formation ability of all examined oral bacteria. Surfactin showed effective inhibition against S. sanguinis ATCC10556, but lower effects toward A. actinomycetemcomitans Y4 and S. mutans UA159. To corroborate these results, biofilms were observed by scanning electron microscopy (SEM) and confocal microscopy. The observations were largely in concordance with the biofilm assay results. We also attempted to determine the step in the biofilm formation process that was inhibited by biosurfactants. The results clearly demonstrated that rhamnolipids inhibit biofilm formation after the initiation process, however, they do not affect attachment or maturation.

Conclusions

Rhamnolipids inhibit oral bacterial growth and biofilm formation by A. actinomycetemcomitans Y4, and may serve as novel oral drug against localized invasive periodontitis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-020-02034-9.

The effect of co-cultivation of Rhodococcus erythropolis with other bacterial strains on biological activity of synthesized surface-active substances

Surface-active substances synthesized by Rhodococcus erythropolis ІMВ Ас-5017 during co-cultivation with inducing bacteria either Bacillus subtilis BT-2 or Escherichia coli ІЕM-1 (SASI) had the higher antimicrobial and antiadhesive activities in comparison with surface-active substances synthesized in the medium without cells of inducing bacteria (SAS). Minimum inhibitory concentrations of SASI ranged from 3 to12 μg/mL and were in 4-32 times lower than the same parameter for SAS. Treatment of abiotic surfaces (ceramic, steel, and glass) with SASI decreased adhesion of bacteria Staphylococcus aureus BMC-1 or yeasts Candida albicans D-6 to the level of 10-32 % in comparison with 32-87 % after treatment of surfaces with SAS. Destruction of bacterial and yeast biofilms treated with SASI was in the range from 40 to 94 %, while it was 32-65 % in the case when SAS was used. Increasing of biological activity of surface-active substances synthesized by R. erythropolis in co-culture with inducing bacteria E. coli or B. subtilis was shown for the first time.

Diversity of Bacteria and Bacterial Products as Antibiofilm and Antiquorum Sensing Drugs Against Pathogenic Bacteria

The increase in antibiotic resistance of pathogenic bacteria has led to the development of new therapeutic approaches to inhibit biofilm formation as well as interfere quorum sensing (QS) signaling systems. The QS system is a phenomenon in which pathogenic bacteria produce signaling molecules that are involved in cell to cell communication, production of virulence factors, biofilm maturation, and several other functions. In the natural environment, several non-pathogenic bacteria are present as mixed population along with pathogenic bacteria and they control the behavior of microbial community by producing secondary metabolites. Similarly, non-pathogenic bacteria also take advantages of the QS signaling molecule as a sole carbon source for their growth through catabolism with enzymes. Several enzymes are produced by bacteria which disrupt the biofilm architecture by degrading the composition of extracellular polymeric substances (EPS) such as exopolysaccharide, extracellular- DNA and protein. Thus, the interference of QS system by bacterial metabolic products and enzymatic catalysis, modification of the QS signaling molecules as well as enzymatic disruption of biofilm architecture have been considered as the alternative therapeutic approaches. This review article elaborates on the diversity of different bacterial species with respect to their metabolic products as well as enzymes and their molecular modes of action. The bacterial enzymes and metabolic products will open new and promising perspectives for the development of strategies against the pathogenic bacterial infections.

Trehalose Lipid Biosurfactant Reduces Adhesion of Microbial Pathogens to Polystyrene and Silicone Surfaces: An Experimental and Computational Approach

Rhodococcus fascians BD8, isolated from Arctic soil, was found to produce biosurfactant when grown on n-hexadecane as the sole carbon source. The glycolipid product was identified as the trehalose lipid with a molecular mass of 848 g mol^-1. The purified biosurfactant reduced the surface tension of water from 72 to 34 mN m^-1. The critical micelle concentration of trehalose lipid was 0.140 mg mL^-1. To examine its potential for biomedical applications, the antimicrobial and antiadhesive activity of the biosurfactant was evaluated against several pathogenic microorganisms. Trehalose lipid showed antimicrobial activity against resistant pathogens. The largest antimicrobial activities of trehalose lipid were observed against Vibrio harveyi and Proteus vulgaris. The highest concentration tested (0.5 mg mL^-1) caused a partial (11-34%) inhibition of other Gram-positive and Gram-negative bacteria and 30% inhibition of Candida albicans growth. The trehalose lipid also showed significant antiadhesive properties against all of the tested microorganisms to polystyrene surface and silicone urethral catheters. The biosurfactant showed 95 and 70% antiadhesive activity against C. albicans and Escherichia coli, respectively. Finally, the role and application of trehalose lipid as an antiadhesive compound was investigated by the modification of the polystyrene and silicone surfaces. The intermolecular interaction energy calculations were performed for investigated complexes at the density functional level of theory. The results indicate that the presence of aromatic moieties can be substantial in the stabilization of trehalose lipid-surface complexes. The antimicrobial and antiadhesive activities of trehalose lipid make them promising alternatives to synthetic surfactants in a wide range of medical applications. Based on our findings, we propose that, because of its ability to inhibit microbial colonization of polystyrene and silicone surfaces, trehalose lipid can be used as a surface coating agent.

Anti-biofilm Properties of Bacterial Di-Rhamnolipids and Their Semi-Synthetic Amide Derivatives

A new strain, namely Lysinibacillus sp. BV152.1 was isolated from the rhizosphere of ground ivy (Glechoma hederacea L.) producing metabolites with potent ability to inhibit biofilm formation of an important human pathogens Pseudomonas aeruginosa PAO1, Staphylococcus aureus, and Serratia marcescens. Structural characterization revealed di-rhamnolipids mixture containing rhamnose (Rha)-Rha-C10-C10, Rha-Rha-C8-C10, and Rha-Rha-C10-C12 in the ratio 7:2:1 as the active principle. Purified di-rhamnolipids, as well as commercially available di-rhamnolipids (Rha-Rha-C10-C10, 93%) were used as the substrate for the chemical derivatization for the first time, yielding three semi-synthetic amide derivatives, benzyl-, piperidine-, and morpholine. A comparative study of the anti-biofilm, antibacterial and cytotoxic properties revealed that di-Rha from Lysinibacillus sp. BV152.1 were more potent in biofilm inhibition, both cell adhesion and biofilm maturation, than commercial di-rhamnolipids inhibiting 50% of P. aeruginosa PAO1 biofilm formation at 50 μg mL^-1 and 75 μg mL^-1, respectively. None of the di-rhamnolipids exhibited antimicrobial properties at concentrations of up to 500 μg mL^-1. Amide derivatization improved inhibition of biofilm formation and dispersion activities of di-rhamnolipids from both sources, with morpholine derivative being the most active causing more than 80% biofilm inhibition at concentrations 100 μg mL^-1. Semi-synthetic amide derivatives showed increased antibacterial activity against S. aureus, and also showed higher cytotoxicity. Therefore, described di-rhamnolipids are potent anti-biofilm agents and the described approach can be seen as viable approach in reaching new rhamnolipid based derivatives with tailored biological properties.

Antibiofilm agents: A new perspective for antimicrobial strategy

Biofilms are complex microbial architectures that attach to surfaces and encase microorganisms in a matrix composed of self-produced hydrated extracellular polymeric substances (EPSs). In biofilms, microorganisms become much more resistant to antimicrobial treatments, harsh environmental conditions, and host immunity. Biofilm formation by microbial pathogens greatly enhances survival in hosts and causes chronic infections that result in persistent inflammation and tissue damages. Currently, it is believed over 80% of chronic infectious diseases are mediated by biofilms, and it is known that conventional antibiotic medications are inadequate at eradicating these biofilm-mediated infections. This situation demands new strategies for biofilm-associated infections, and currently, researchers focus on the development of antibiofilm agents that are specific to biofilms, but are nontoxic, because it is believed that this prevents the development of drug resistance. Here, we review the most promising antibiofilm agents undergoing intensive research and development.