Effects of rhamnolipids from Pseudomonas aeruginosa DS10-129 on luminescent bacteria: toxicity and modulation of cadmium bioavailability

Antibacterial mechanism of Pseudomonas aeruginosa UKMP14T rhamnolipids against multidrug resistant Acinetobacter baumannii

Rhamnolipids, a major category of glycolipid biosurfactant, have recently gained enormous attention in medical field because of their relevance as effective antibacterial agents against a wide variety of pathogenic bacteria. Our previous studies have shown that rhamnolipids from an environmental isolate of Pseudomonas aeruginosa UKMP14T possess antibacterial, anti-adhesive and anti-biofilm activity against multidrug-resistant ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter sp.) pathogens. However, the mechanism of their antibacterial action remains unclear. Thus, this study aimed to elucidate the mechanism of the antibacterial action of P. aeruginosa UKMP14T rhamnolipids by studying the changes in cells of one of the ESKAPE pathogens, Acinetobacter baumannii, which is the most difficult strain to kill. Results revealed that rhamnolipid treatment rendered A. baumannii cells more hydrophobic as evaluated through contact angle measurements. It also induced the release of cellular proteins measuring 510 μg/mL at a rhamnolipid concentration of 1000 μg/mL. In addition, rhamnolipids were found to be bactericidal in their action as they could permeate the inner membranes, leading to a leak-out of nucleotides. More than 50 % of the cells were found to be killed upon 1000 μg/mL rhamnolipid treatment as observed through fluorescence microscopy. Other cellular changes such as irregular shape and size, membrane perturbations, clumping, shrinkage and physical damage were clearly visible in SEM, FESEM and laser micrographs. Furthermore, rhamnolipid treatment inhibited the levels of acyl-homoserine lactones (AHLs) in A. baumannii, which are vital for their biofilm formation and virulence. The obtained results indicate that P. aeruginosa UKMP14T rhamnolipids target outer and inner bacterial membranes through permeation, including physical damage to the cells, leading to cell leakage. Furthermore, AHL inhibition appears to be the mechanism behind their anti-biofilm action. All these observations can be correlated to rhamnolipids' antibacterial effect against A. baumannii.

Enhancement of cell migration and wound healing by nano-herb ointment formulated with biosurfactant, silver nanoparticles and Tridax procumbens

Introduction

Ointments are generally used as a therapeutic agent for topical medication or transdermal drug delivery, such as wound healing and skin lesions.

Methods

In this study, Tridax procumbens plant extract (0.7 g/mL) was used to prepare herbal-infused oil as the oil phase and gelatin-stabilized silver nanoparticle (G-AgNPs) (0.3 g/mL) as the aqueous phase. To blend the oil and aqueous phases, rhamnolipid biosurfactant with a critical micelle concentration of 55 mg/L from strain Pseudomonas aeruginosa PP4 has been used for herb ointment preparation. The average size of the synthesized G-AgNPs was observed between 10-30 nm and confirmed as spherical-shaped particles by TEM analysis. Subsequently, GC-MS and FTIR characterization are used to confirm herb ointment's chemical and functional characteristics.

Results

Based on the antibacterial studies, the highest microbial growth inhibition was observed for herb ointment, about 19.5 mm for the pathogen Staphylococcus aureus at the concentration of 100 μg/mL, whereas 15.5 mm was obtained for Escherichia coli, respectively. In addition, the minimum inhibitory concentration (MIC) assay showed negligible bacterial growth at 100 μg/mL for S. aureus and E. coli, respectively. Moreover, the cell viability assay for herb ointment exhibited low cytotoxic activity at higher concentrations (100 μg/mL) in Vero cell lines. In this study, wound scratch assay showed a significant cell migration rate (90 ± 2%) in 3 days of incubation than the control (62 ± 2%).

Discussion

As a result, the biosurfactant-based nano-topical herb ointment revealed a low cytotoxic and higher cell migration capacity. Altogether, these findings highlighted the utility of this herb ointment in therapeutic applications such as wound healing.

Limited Role of Rhamnolipids on Cadmium Resistance for an Endogenous-Secretion Bacterium

Rhamnolipids, a type of biosurfactant, represent a potential strategy for both enhancing organismic resistance and in situ remediation of heavy metals contaminations. In-depth study of the mechanism of rhamnolipids synthesis in response to heavy metals stress, is indispensable for a wide use of biosurfactant-secreting microbes in bioremediation. In this study, we employed the wild-type and the rhlAB deficient strain (ΔrhlAB) of Pseudomonas aeruginosa, a prototypal rhamnolipids-producing soil microorganism, to investigate its responses to cadmium resistance based on its physicochemical, and physiological properties. Compared with the wild-type strain, the ΔrhlAB were more sensitive to Cd-stress at low Cd concentration (<50 mg/L), whereas there was little difference in sensitivity at higher Cd concentrations, as shown by spot titers and cell viability assays. Secreted rhamnolipids reduced intracellular Cd²⁺ accumulation to alleviate Cd²⁺ stress, whereas endogenous rhamnolipids played a limited role in alleviating Cd²⁺ stress. Synthesized rhamnolipids exhibited a higher critical micelle concentration (CMC) (674.1 mg/L) and lower emulsification index (4.7%) under high Cd-stress, while these parameters showed no obvious changes. High Cd-stress resulted in high hydrophilic wild-type bacterial surface and lower bioremediation ability. This study could advance a deeper understanding of the mechanism of cadmium resistance and provide a theoretical foundation for the application of biosurfactant and biosurfactant-secreted bacterium in contaminant bioremediation.

A review on the application of chemical surfactant and surfactant foam for remediation of petroleum oil contaminated soil

Soil, exposed to petroleum oil contaminants (in the form of petrol, diesel, gasoline, crude oil, used motor oil), may cause potential damage to the environment, animal and human health. In this review article, mechanisms of the petroleum oil contaminant removal from soil by chemical surfactant systems such as surfactant solution, surfactant foam and nanoparticle stabilized surfactant foams are explained. Laboratory based research works, reported within the last decade on the application of similar systems towards the removal of petroleum oil contaminant from the soil, have been discussed. It is an important fact that the commercial implementation of the chemical surfactant based technology depends on the environmental properties (biodegradability and toxicity) of the surfactants. In recent times, surfactant foam and nanoparticle stabilized surfactant foam are becoming more popular and considered advantageous over the use of surfactant solution alone. However, more research works have to be conducted on nanoparticle stabilized foam. The impact of physicochemical properties of the nanoparticles on soil remediation has to be explored in depth.

Effect of using powdered biochar and surfactant on desorption and biodegradability of phenanthrene sorbed to biochar

The present study aimed to investigate the relationship between the desorption and biodegradability of phenanthrene sorbed to biochars by employing two approaches that may change the desorption and biodegradability: the use of powdered biochars and nonionic surfactants. Biochars derived from two feedstocks (rice husk and sewage sludge; pyrolyzed at 500 °C but showing different aromaticity) were used. When the biochars were powdered to obtain particles <250 μm the mass fractions of the desorbed phenanthrene at ∼80 days (f_des) increased from 0.303 to 0.431 for sewage sludge biochars. On the other hand, f_des for rice husk biochars remained virtually unchanged (from 0.264 to 0.255). The mass fractions of the biodegraded phenanthrene (f_bio) increased from 0.191 to 0.306 for rice husk biochars and from 0.077 to 0.168 for sewage sludge biochars. When a nonionic surfactant was added at the sub-critical micelle concentration (CMC), f_bio increased by 4.7 times and 8.3 times for rice husk and sewage sludge biochars. For both types of biochars, f_bio was larger than f_des when the surfactant was added. This study suggests that the addition of nonionic surfactants can be considered if the inhibition of microbial activity is of concern in soils and sediments treated by biochar.

Plasma membrane is the target of rapid antibacterial action of silver nanoparticles in Escherichia coli and Pseudomonas aeruginosa

Introduction

Silver nanoparticles (AgNP) are widely used in consumer products and in medicine, mostly due to their excellent antimicrobial properties. One of the generally accepted antibacterial mechanisms of AgNP is their efficient contact with cells and dissolution in the close vicinity of bacterial cell envelope. Yet, the primary mechanism of cell wall damage and the events essential for bactericidal action of AgNP are not elucidated.

Materials and methods

In this study we used a combination of various assays to differentiate the adverse effects of AgNP on bacterial cell envelope: outer membrane (OM) and plasma membrane (PM).

Results

We showed that PM was the main target of AgNP in gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa: AgNP depolarized PM, induced the leakage of the intracellular K⁺, and inhibited cellular respiration. The results of bacterial bioluminescence inhibition assay in combination with AgNP dissolution and oxidation assays demonstrated that the adverse effects of AgNP occurred at concentrations 7–160 µM. These toxic effects occurred already within the first few seconds of contact of bacteria and AgNP and were driven by dissolved Ag⁺ ions targeting bacterial PM. However, the irreversible inhibition of bacterial growth detected after 1-hour exposure occurred at 40 µM AgNP for P. aeruginosa and at 320 µM AgNP for E. coli. In contrast to effects on PM, AgNP and Ag⁺ ions had no significant effect on the permeability and integrity of bacterial OM, implying that AgNP indeed targeted mainly PM via dissolved Ag⁺ ions.

Conclusion

AgNP exhibited antibacterial properties via rapid release of Ag⁺ ions targeting the PM and not the OM of gram-negative bacteria.

Comparison of landfarming amendments to improve bioremediation of petroleum hydrocarbons in Niger Delta soils

Large scale landfarming experiments, using an extensive range of treatments, were conducted in the Niger-Delta, Nigeria to study the degradation of oil in contaminated soils. In this work the effect of nutrient addition, biosurfactant, Eisenia fetida (earthworm) enzyme extract, bulking and sorption agents and soil neutralization were tested. It was found that these treatments were successful in removing up to 53% of the total petroleum hydrocarbon in the soil within 16 weeks. A comparison between treatments demonstrated that most were no more effective than agricultural fertilizer addition alone. One strategy that did show better performance was a combination of nutrients, biochar and biosurfactant, which was found to remove 23% more Total Petroleum Hydrocarbons (TPH) than fertilizer alone. However, when performance normalized costs were considered, this treatment became less attractive as a remedial option. Based on this same analysis it was concluded that fertilizer only was the most cost effective treatment. As a consequence, it is recommended that fertilizer is used to enhance the landfarming of hydrocarbon contaminated soils in the Niger Delta. The attenuation rates of both bulk TPH and Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG) fractions are also provided. These values represent one of the first large scale and scientifically tested datasets for treatment of contaminated soil in the Niger Delta region. An inverse correlation between attenuation rates and hydrocarbon molecular weight was observed with heavy fractions showing much slower degradation rates than lighter fractions. Despite this difference, the bioremediation process resulted in significant removal of all TPH compounds independent of carbon number.

Aquatic toxicity and biodegradability of a surfactant produced by Bacillus subtilis ICA56

In this work, the environmental compatibility of a biosurfactant produced by a Bacillus subtilis strain isolated from the soil of a Brazilian mangrove was investigated. The biosurfactant, identified as surfactin, is able to reduce surface tension (ST) to 31.5 ± 0.1 mN m^-1 and exhibits a lowcritical micelle concentration (CMC) value (0.015 ± 0.003 g L^-1). The highest crude biosurfactant concentration (224.3 ± 1.9 mg L^-1) was reached at 72 h of fermentation. Acute toxicity tests, carried out with Daphnia magna, Vibrio fischeri and Selenastrum capricornutum indicated that the toxicity of the biosurfactant is lower than that of its chemically derived counterparts. The results of the biodegradability tests demonstrated that the crude surfactin extract was degraded by both Pseudomonas putida and a mixed population from a sewage-treatment plant, in both cases the biodegradation efficiency being dependent on the initial concentration of the biosurfactant. Finally, as the biodegradation percentages obtained fall within the acceptance limits established by the Organization for Economic Co-operation and Development (Guidelines for Testing of Chemicals, OECD 301E), crude surfactin can be classified as a "readily" biodegradable compound.

Saponaria officinalis L. extract: Surface active properties and impact on environmental bacterial strains

Plant-derived surfactants are characterised by low toxicity, high biodegradability and environmental compatibility. They therefore have many applications; for instance, they can be used in bioremediation to accelerate biodegradation processes, especially of hydrophobic pollutants. This paper analyses the properties of an extract from Saponaria officinalis L. containing saponins and its impact on bacterial strains isolated from soil, as well as its potential for application in hydrocarbon bioremediation. The tested extract from Saponaria officinalis L. contains gypsogenin, hederagenin, hydroxyhederagenin and quillaic acid aglycone structures and demonstrates good emulsification properties. Contact with the extract led to modification of bacterial cell surface properties. A decrease in cell surface hydrophobicity and an increase in membrane permeability were recorded in the experiments. An increase of up to 63% in diesel oil biodegradation was also recorded for Pseudomonas putida DA1 on addition of 1gL^-1 of saponins from Saponaria officinalis L. Saponaria extract showed no toxic impact on the tested environmental bacterial strains at the concentration used in the biodegradation process.

BioSurfDB: knowledge and algorithms to support biosurfactants and biodegradation studies

Crude oil extraction, transportation and use provoke the contamination of countless ecosystems. Therefore, bioremediation through surfactants mobilization or biodegradation is an important subject, both economically and environmentally. Bioremediation research had a great boost with the recent advances in Metagenomics, as it enabled the sequencing of uncultured microorganisms providing new insights on surfactant-producing and/or oil-degrading bacteria. Many research studies are making available genomic data from unknown organisms obtained from metagenomics analysis of oil-contaminated environmental samples. These new datasets are presently demanding the development of new tools and data repositories tailored for the biological analysis in a context of bioremediation data analysis. This work presents BioSurfDB, www.biosurfdb.org, a curated relational information system integrating data from: (i) metagenomes; (ii) organisms; (iii) biodegradation relevant genes; proteins and their metabolic pathways; (iv) bioremediation experiments results, with specific pollutants treatment efficiencies by surfactant producing organisms; and (v) a biosurfactant-curated list, grouped by producing organism, surfactant name, class and reference. The main goal of this repository is to gather information on the characterization of biological compounds and mechanisms involved in biosurfactant production and/or biodegradation and make it available in a curated way and associated with a number of computational tools to support studies of genomic and metagenomic data.

Database URL: www.biosurfdb.org

Evaluation of the ecotoxicity of pollutants with bioluminescent microorganisms

This chapter deals with the use of bioluminescent microorganisms in environmental monitoring, particularly in the assessment of the ecotoxicity of pollutants. Toxicity bioassays based on bioluminescent microorganisms are an interesting complement to classical toxicity assays, providing easiness of use, rapid response, mass production, and cost effectiveness. A description of the characteristics and main environmental applications in ecotoxicity testing of naturally bioluminescent microorganisms, covering bacteria and eukaryotes such as fungi and dinoglagellates, is reported in this chapter. The main features and applications of a wide variety of recombinant bioluminescent microorganisms, both prokaryotic and eukaryotic, are also summarized and critically considered. Quantitative structure-activity relationship models and hormesis are two important concepts in ecotoxicology; bioluminescent microorganisms have played a pivotal role in their development. As pollutants usually occur in complex mixtures in the environment, the use of both natural and recombinant bioluminescent microorganisms to assess mixture toxicity has been discussed. The main information has been summarized in tables, allowing quick consultation of the variety of luminescent organisms, bioluminescence gene systems, commercially available bioluminescent tests, environmental applications, and relevant references.

Surfactin restores and enhances swarming motility under heavy metal stress

The present work reports the importance of lipopeptide biosurfactant on swarming motility of multi-metal resistant (MMR) bacterium under heavy metal stress. The MMR bacteria strain CM100B, identified as Bacillus cereus, was isolated from the coal mine sample. The strain was able to grow and reduce several metals namely Cd(2+), Co(2+), Cu(2+), Ni(2+), Mn(2+) and Pb(2+) ions which are common environmental pollutants. Presence of toxic heavy metal ions in the swarming medium significantly altered the motility of CM100B. Presence of Cd(2+) and Pb(2+) ions inhibited development of peritrichous flagella, thus inhibiting swarming motility. However, the addition of anionic biosurfactant surfactin restored (in case of Cd(2+) and Pb(2+) ions) or enhanced (in case of Co(2+), Cu(2+), Ni(2+) and Mn(2+)) the swarming ability of CM100B. Zeta potential studies for determining bacterial cell surface charge indicated that surfactin provided a suitable swarming environment to bacteria even under metal stress by chelating to cationic metal ions. Non-ionic surfactant Triton X-100 was unable to restore swarming under Cd(2+) and Pb(2+) ion stress. Thus, suggesting that surfactin can aid in motility not only by reducing the surface tension of swarming medium but also by binding to metal ions in the presence of metal ions stress.

Removal of hydrocarbon from refinery tank bottom sludge employing microbial culture

Accumulation of oily sludge is becoming a serious environmental threat, and there has not been much work reported for the removal of hydrocarbon from refinery tank bottom sludge. Effort has been made in this study to investigate the removal of hydrocarbon from refinery sludge by isolated biosurfactant-producing Pseudomonas aeruginosa RS29 strain and explore the biosurfactant for its composition and stability. Laboratory investigation was carried out with this strain to observe its efficacy of removing hydrocarbon from refinery sludge employing whole bacterial culture and culture supernatant to various concentrations of sand-sludge mixture. Removal of hydrocarbon was recorded after 20 days. Analysis of the produced biosurfactant was carried out to get the idea about its stability and composition. The strain could remove up to 85 ± 3 and 55 ± 4.5 % of hydrocarbon from refinery sludge when whole bacterial culture and culture supernatant were used, respectively. Maximum surface tension reduction (26.3 mN m(-1)) was achieved with the strain in just 24 h of time. Emulsification index (E24) was recorded as 100 and 80 % with crude oil and n-hexadecane, respectively. The biosurfactant was confirmed as rhamnolipid containing C8 and C10 fatty acid components and having more mono-rhamnolipid congeners than the di-rhamnolipid ones. The biosurfactant was stable up to 121 °C, pH 2-10, and up to a salinity value of 2-10 % w/v. To our knowledge, this is the first report showing the potentiality of a native strain from the northeast region of India for the efficient removal of hydrocarbon from refinery sludge.

Contributions of biosurfactants to natural or induced bioremediation

The number of studies dedicated to evaluating the influence of biosurfactants on bioremediation efficiency is constantly growing. Although significant progress regarding the explanation of mechanisms behind biosurfactant-induced effects could be observed, there are still many factors which are not sufficiently elucidated. This corresponds to the fact that although positive influence of biosurfactants is often reported, there are also numerous cases where no or negative effect was observed. This review summarizes the recent finding in the field of biosurfactant-amended bioremediation, focusing mainly on a critical approach towards potential limitations and causes of failure while investigating the effects of biosurfactants on the efficiency of biodegradation and phytoextraction processes. It also provides a summary of successive steps, which should be taken into consideration when designing biosurfactant-related treatment processes.

LuxCDABE--transformed constitutively bioluminescent Escherichia coli for toxicity screening: comparison with naturally luminous Vibrio fischeri

We show that in vitro toxicity assay based on inhibition of the bioluminescence of recombinant Escherichia coli encoding thermostable luciferase from Photorhabdus luminescens is a versatile alternative to Vibrio fischeri Microtox™ test. Performance of two luxCDABE-transformed E. coli MC1061 constructs (pDNlux) and (pSLlux) otherwise identical, but having 100-fold different background luminescence was compared with the performance of V. fischeri. The microplate luminometer and a kinetic Flash-Assay test format was used that differently from Microtox test is also applicable for high throughput analysis. Toxic effects (30-s till 30-min EC₅₀) of four heavy metals (Zn, Cd, Hg, Cu) and three organic chemicals (aniline, 3,5-dichloroaniline and 3,5-dichlorophenol) were studied. Both E. coli strains had comparable sensitivity and the respective 30-min EC₅₀ values highly correlated (log-log R² = 0.99; p < 0.01) showing that the sensitivity of the recombinant bacteria towards chemicals analyzed did not depend on the bioluminescence level of the recombinant cells. The most toxic chemical for all used bacterial strains (E. coli, V. fischeri) was mercury whereas the lowest EC₅₀ values for Hg (0.04–0.05 mg/L) and highest EC₅₀ values for aniline (1,300–1,700 mg/L) were observed for E. coli strains. Despite of that, toxicity results obtained with both E. coli strains (pSLlux and pDNlux) significantly correlated with V. fischeri results (log-log R² = 0.70/0.75; p < 0.05/0.01). The use of amino acids (0.25%) and glucose (0.05%)-supplemented M9 medium instead of leucine-supplemented saline significantly (p < 0.05) reduced the apparent toxicity of heavy metals to both E. coli strains up to three orders of magnitude, but had little or no complexing effect on organic compounds. Thus, P. luminescens luxCDABE-transformed E. coli strains can be successfully used for the acute toxicity screening of various types of organic chemicals and heavy metals and can replace V. fischeri in certain cases where the thermostability of luciferase >30 °C is crucial. The kinetic Flash Assay test format of the bioluminescence inhibition assay facilitates high throughput analysis. The assay medium, especially in case of testing heavy metals should be a compromise: optimal for the viability/luminescence of the recombinant test strain and of minimum complexing potential.