Enhanced rhamnolipid production by Pseudomonas aeruginosa under phosphate limitation

Bioconversion of waste glycerol into viscosinamide by Pseudomonas fluorescens DR54 and its activity evaluation

Lipopeptides, derived from microorganisms, are promising surface-active compounds known as biosurfactants. However, the high production costs of biosurfactants, associated with expensive culture media and purification processes, limit widespread industrial application. To enhance the sustainability of biosurfactant production, researchers have explored cost-effective substrates. In this study, crude glycerol was evaluated as a promising and economical carbon source in viscosinamide production by Pseudomonas fluorescens DR54. Optimization studies using the Box - Behnken design and response surface methodology were performed. Optimal conditions for viscosinamide production including glycerol 70.8 g/L, leucine 2.7 g/L, phosphate 3.7 g/L, and urea 9.3 g/L were identified. Yield of viscosinamide production, performed under optimal conditions, reached 7.18 ± 0.17 g/L. Preliminary characterization of viscosinamide involved the measurement of surface tension. The critical micelle concentration of lipopeptide was determined to be 5 mg/L. Furthermore, the interactions between the viscosinamide and lipase from Candida rugosa (CRL) were investigated by evaluating the impact of viscosinamide on lipase activity and measuring circular dichroism. It was observed that the α-helicity of CRL increases with increasing viscosinamide concentration, while the random coil structure decreases.

Nutrient-Limited Operational Strategies for the Microbial Production of Biochemicals

Limiting an essential nutrient has a profound impact on microbial growth. The notion of growth under limited conditions was first described using simple Monod kinetics proposed in the 1940s. Different operational modes (chemostat, fed-batch processes) were soon developed to address questions related to microbial physiology and cell maintenance and to enhance product formation. With more recent developments of metabolic engineering and systems biology, as well as high-throughput approaches, the focus of current engineers and applied microbiologists has shifted from these fundamental biochemical processes. This review draws attention again to nutrient-limited processes. Indeed, the sophisticated gene editing tools not available to pioneers offer the prospect of metabolic engineering strategies which leverage nutrient limited processes. Thus, nutrient- limited processes continue to be very relevant to generate microbially derived biochemicals.

Interfacial and Solution Aggregation Behavior of a Series of Bioinspired Rhamnolipid Congeners Rha-C14-Cx (x = 6, 8, 10, 12, 14)

Rhamnolipids are glycolipids produced by microorganisms with outstanding surfactant properties. They are a class of biosurfactants that are potential candidates for biodegradable and nontoxic replacements of current specialty synthetic surfactants. Building on our previous efforts in developing an efficient and practical chemical methodology to synthesize rhamnolipids allows us to now explore the tunability of rhamnolipid properties. Here, we explore the impact on solution self-assembly and adsorption at the air/water interface of symmetry of the two lipid tails for diastereomeric mixtures of a series of monorhamnolipids of the generic structure Rha-C14-Cx. Surface activity of the anionic forms of these molecules at pH 8 is described by surface tensiometry. Characteristics of their aggregation behavior in aqueous solutions including hydrodynamic radius, aggregation number, and aggregate morphology are determined using dynamic light scattering and time-resolved fluorescence quenching spectroscopy. The solution aggregation behavior of this series is found to unexpectedly vary in a nonmonotonic fashion. This is explained by molecular structural attributes of each series member that result in differences in the respective intermolecular interactions of various parts of these surfactants.

Recent advancements in the production of rhamnolipid biosurfactants by Pseudomonas aeruginosa

Rhamnolipid (RL) biosurfactant which is produced by Pseudomonas species is one of the most effective surface-active agents investigated in the literature. Over the years, many efforts have been made and an array of techniques has been developed for the isolation of RL produced strains as well as RL homolog characterization. Reports show that RL productivity by the best-known producer, Pseudomonas aeruginosa, is very diverse, from less than 1 gr/l to more than 200 g L-1. There are some major parameters that can affect RL productivity. These are culture conditions, medium composition, the mode of operation (batch, fed-batch and continuous), bioengineering/gene manipulation and finally extraction methods. The present paper seeks to provide a comprehensive overview on the production of rhamnolipid biosurfactant by different species of Pseudomonas bacteria. In addition, we have extensively reviewed their potential for possible future applications.

Rhamnolipids stabilize quorum sensing mediated cooperation in Pseudomonas aeruginosa

Pseudomonas aeruginosa is one of the main models to study social behaviors in bacteria since it synthesizes several exoproducts, including exoproteases and siderophores and release them to the environment. Exoproteases and siderophores are public goods that can be utilized by the individuals that produce them but also by non-producers, that are considered social cheaters. Molecularly exoprotease cheaters are mutants in regulatory genes such as lasR, and are commonly isolated from chronic infections and selected in the laboratory upon serial cultivation in media with protein as a sole carbon source. Despite that the production of exoproteases is exploitable, cooperators have also ways to restrict the growth and selection of social cheaters, for instance by producing toxic metabolites like pyocyanin. In this work, using bacterial competitions, serial cultivation and growth assays, we demonstrated that rhamnolipids which production is regulated by quorum sensing, selectively affect the growth of lasR mutants and are able to restrict social cheating, hence contributing to the maintenance of cooperation in Pseudomonas aeruginosa populations.

Advances in applications of rhamnolipids biosurfactant in environmental remediation: A review

The objective of this review is to provide a comprehensive overview of the advances in the applications of rhamnolipids biosurfactants in soil and ground water remediation for removal of petroleum hydrocarbon and heavy metal contaminants. The properties of rhamnolipids associated with the contaminant removal, that is, solubilization, emulsification, dispersion, foaming, wetting, complexation, and the ability to modify bacterial cell surface properties, were reviewed in the first place. Then current remediation technologies with integration of rhamnolipid were summarized, and the effects and mechanisms for rhamnolipid to facilitate contaminant removal for these technologies were discussed. Finally rhamnolipid-based methods for remediation of the sites co-contaminated by petroleum hydrocarbons and heavy metals were presented and discussed. The review is expected to enhance our understanding on environmental aspects of rhamnolipid and provide some important information to guide the extending use of this fascinating chemical in remediation applications.

Effects of rhamnolipids on microorganism characteristics and applications in composting: A review

Biosurfactant rhmnolipids have been applied in many fields, especially in environmental bioremediation. According to previous researches, many research groups have studied the influence of rhamnolipids on microorganism characteristics and/or its application in composting. In this review, the effects of rhamnolipids on the cell surface properties of microorganisms was discussed firstly, such as cell surface hydrophobicity (CSH), electrical, surface compounds, etc. Moreover, the deeper mechanisms were also discussed, such as the effects of rhamnolipids on the structural characteristics and functional characteristics of the cell membrane, and the effects of rhamnolipids on the related enzymes and genes. Additionally, the application of rhamnolipids in composting was discussed, which is an important way for pollutant biodegradation and resource reutilization. It is believed that rhamnolipids will play more and more important role in composting.

Current status in biotechnological production and applications of glycolipid biosurfactants

Biosurfactants are natural compounds with surface activity and emulsifying properties produced by several types of microorganisms and have been considered an interesting alternative to synthetic surfactants. Glycolipids are promising biosurfactants, due to low toxicity, biodegradability, and chemical stability in different conditions and also because they have many biological activities, allowing wide applications in different fields. In this review, we addressed general information about families of glycolipids, rhamnolipids, sophorolipids, mannosylerythritol lipids, and trehalose lipids, describing their chemical and surface characteristics, recent studies using alternative substrates, and new strategies to improve of production, beyond their specificities. We focus in providing recent developments and trends in biotechnological process and medical and industrial applications.

Strategies for improved rhamnolipid production by Pseudomonas aeruginosa PA1

Rhamnolipids are biosurfactants with potential for diversified industrial and environmental uses. The present study evaluated three strategies for increasing the production of rhamnolipid-type biosurfactants produced by Pseudomonas aeruginosa strain PA1. The influence of pH, the addition of P. aeruginosa spent culture medium and the use of a fed-batch process were examined. The culture medium adjusted to pH 7.0 was the most productive. Furthermore, the pH of the culture medium had a measurable effect on the ratio of synthesized mono- and dirhamnolipids. At pH values below 7.3, the proportion of monorhamnolipids decreased from 45 to 24%. The recycling of 20% of the spent culture medium in where P. aeruginosa was grown up to the later stationary phase was responsible for a 100% increase in rhamnolipid volumetric productivity in the new culture medium. Finally, the use of fed-batch operation under conditions of limited nitrogen resulted in a 3.8-fold increase in the amount of rhamnolipids produced (2.9 g L(-1)-10.9 g L(-1)). These results offer promising pathways for the optimization of processes for the production of rhamnolipids.

Enhanced production of antifungal lipopeptides by Bacillus amyloliquefaciens for biocontrol of postharvest disease

Food security to sustain increasing populations is a global concern. A major factor threatening food security is crop spoilage during postharvest storage. Reduction of postharvest spoilage has mainly been addressed by the application of synthetic chemicals. Bacillus lipopeptides, specifically lipopeptide homologues exhibiting antifungal efficacy, offer an alternative environmentally benign protocol for reduction of postharvest phytopathogens. This work is directed towards Bacillus lipopeptide production for biocontrol of postharvest phytopathogens in general and fungal phytopathogens in particular. Bacillus amyloliquefaciens DSM 23117 was identified as an organism with superior potential for lipopeptide production, via screening of 4 Bacillus candidates, in terms of antifungal lipopeptide concentration, yield, productivity and preferred homologue ratio. Efficacy of B. amyloliquefaciens lipopeptides against Botrytis cinerea substantiated appropriateness of this Bacillus species. Subsequent process modification of B. amyloliquefaciens cultures demonstrated that the concentration and ratio of the lipopeptides were significantly influenced by process conditions and further, distinguished nitrate and oxygen availability as key parameters defining optimal lipopeptide production. Discrete B. amyloliquefaciens cultures supplied with 4, 8, 10 and 12 g/L NH4NO3 demonstrated optimal lipopeptide concentration, yield and productivity, with respect to both total and antifungal lipopeptides, in the culture containing 8 g/L NH4NO3. Enhancement of total and antifungal lipopeptide kinetics similar to those quantified on increasing the nitrate from 4 to 8 g/L NH4NO3 were exhibited in B. amyloliquefaciens cultures when the oxygen in the sparge gas was increased from 21 to 30 mol%. The enhancement of lipopeptide production under conditions of increased nitrate and increased oxygen supply is explained in terms of increased availability of nitrogen for synthesis. This work has highlighted key parameters for maximisation of Bacillus lipopeptide production and manipulation of antifungal/surfactin ratios for optimum efficacy and informs on future development of process strategies towards production optimisation of antifungal lipopeptides as a green alternative to synthetic chemicals.

Statistical screening of medium components for recombinant production of Pseudomonas aeruginosa ATCC 9027 rhamnolipids by nonpathogenic cell factory Pseudomonas putida KT2440

Rhamnolipids (RLs) produced by the opportunistic human pathogen Pseudomonas aeruginosa are considered as potential candidates for the next generation of surfactants. Large-scale production of RLs depends on progress in strain engineering, medium design, operating strategies, and purification procedures. In this work, the rhlAB genes extracted from a mono_RLs_producing strain of P. aeruginosa (ATCC 9027) were introduced to an appropriate safety host Pseudomonas putida KT2440. The capability of the recombinant strain was evaluated in various media. As a prerequisite for optimal medium design, a set of 32 experiments was performed in two steps for screening a number of macro-nutritional compounds. In the experiments, a two-level fractional factorial design resolution IV was followed by a two-level full factorial one. By means of this approach, it was observed that glycerol, yeast extract, and peptone have significant positive influence on recombinant RLs production while the yeast extract/peptone two-factor and glycerol/yeast extract/peptone three-factor interactions have considerable negative effects. A wide range of variation from 0 to 570 mg/l was obtained for RLs production during the screening experiments indicating the importance of medium optimization. The results point out the opportunity for possible higher yields of RLs through further screening, mixture/combined mixture designs, and high-cell-density cultivations.

NrsZ: a novel, processed, nitrogen-dependent, small non-coding RNA that regulates Pseudomonas aeruginosa PAO1 virulence

The opportunistic pathogen Pseudomonas aeruginosa PAO1 has a remarkable capacity to adapt to various environments and to survive with limited nutrients. Here, we report the discovery and characterization of a novel small non-coding RNA: NrsZ (nitrogen-regulated sRNA). We show that under nitrogen limitation, NrsZ is induced by the NtrB/C two component system, an important regulator of nitrogen assimilation and P. aeruginosa's swarming motility, in concert with the alternative sigma factor RpoN. Furthermore, we demonstrate that NrsZ modulates P. aeruginosa motility by controlling the production of rhamnolipid surfactants, virulence factors notably needed for swarming motility. This regulation takes place through the post-transcriptional control of rhlA, a gene essential for rhamnolipids synthesis. Interestingly, we also observed that NrsZ is processed in three similar short modules, and that the first short module encompassing the first 60 nucleotides is sufficient for NrsZ regulatory functions.

Phosphate limited fed-batch processes: impact on carbon usage and energy metabolism in Escherichia coli

Phosphate starvation is often applied as a tool to limit cell growth in microbial production processes without hampering carbon and/or nitrogen supply alternatively. This contribution focuses on the interplay of process induced phosphate starvation and microbial performance studying an l-tryptophan overproducing Escherichia coli strain as a model for highly ATP demanding processes in comparison with an E. coli wildtype strain. To enable a time-resolved analysis, constant phosphate feeding strategies were applied to elongate the transition from phosphate saturated to phosphate limited cell growth. With increasing phosphate limitation, a reduced cellular efficiency of ATP formation via respiratory chain activity and the ATP synthase complex was found for both strains. Process balancing, transcriptome analysis and flux balance analysis are pointing toward a multi-stage decoupling scenario, which in essence deteriorates the stoichiometric ratio of ATP formation to proton translocation, thereby affecting ATP availability from respiration and carbon usage. Starting off with a potential influence on ATP-synthase efficiency (stage 1), decoupling is further increased by modified respiratory activity (stage 2) and byproduct overflow (stage 3) finally resulting in a metabolic breakdown entering complete phosphate depletion (stage 4). The decoupling is initiated by phosphate limitation; further effects are mainly mediated on metabolic level through ATP availability and energy charge, additionally affected by ATP demanding product synthesis.

Enhanced rhamnolipids production by Pseudomonas aeruginosa based on a pH stage-controlled fed-batch fermentation process

Rhamnolipids find broad applications as natural surfactants, emulsifiers, and antibiotics because of their low toxicity, high biodegradability and environmental soundness. In this study, a pH stage-controlled process of fermentation of rhamnolipids by Pseudomonas aeruginosa O-2-2 was established. A yield of 24.06 g/L in batch fermentation was achieved in a 5-L fermentor via the optimization of stirring speed. By controlling pH, rhamnolipid production was increased to 28.8 g/L, an improvement of 19.7%, and more substrate was converted to rhamnolipids rather than to biomass. Fermentation kinetics models for cell growth, product synthesis and substrate consumption based on the pH stage-controlled fermentation indicated that rhamnolipid production could be further improved by fed-batch fermentation. Rhamnolipid production reached 70.56 g/L, an improvement of 193%, in the pH stage-controlled fed-batch fermentation when the stirring speeds was controlled at 500 rpm and the fermentation temperature was maintained at 30 °C.