Liquid chromatography/mass spectrometry for the identification and quantification of rhamnolipids

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.

Simple method for quantification of anionic biosurfactants in aqueous solutions

Biosurfactants are microbial products that have applications as cleaning agents, emulsifiers, and dispersants. Detection and quantification of biosurfactants can be done by various methods, including colorimetric tests, high performance liquid chromatography (HPLC) coupled to several types of detectors, and tests that take advantage of biosurfactants reducing surface tension of aqueous liquids, allowing for spreading and droplet formation of oils. We present a new and simple method for quantifying biosurfactants by their ability, on paper, to reduce surface tension of aqueous solutions, causing droplet dispersion on an oiled surface in correlation with biosurfactant content. We validated this method with rhamnolipids, surfactin, sophorolipids, and ananatoside B; all are anionic microbial surfactants. Linear ranges for quantification in aqueous solutions for all tested biosurfactants were between 10 and 500 µM. Our method showed time-dependent biosurfactant accumulation in cultures of Pseudomonas aeruginosa strains PA14 and PAO1, and Burkholderia thailandensis E264. Mutants in genes responsible for surfactant production showed negligible activity on oiled paper. In summary, our simple assay provides the opportunity to quantify biosurfactant contents of aqueous solutions, for a diversity of surfactants, by means readily available in any laboratory.

Biodegradation of hazardous naphthalene and cleaner production of rhamnolipids - Green approaches of pollution mitigation

Toxic and hazardous waste poses a serious threat to human health and the environment. Green remediation technologies are required to manage such waste materials, which is a demanding and difficult task. Here, effort was made to explore the role of Pseudomonas aeruginosa SR17 in alleviating naphthalene via catabolism and simultaneously producing biosurfactant. The results showed up to 89.2% naphthalene degradation at 35 °C and pH 7. The GC/MS analysis revealed the generation of naphthalene degradation intermediates. Biosurfactant production led to the reduction of surface tension of the culture medium to 34.5 mN/m. The biosurfactant was further characterized as rhamnolipids. LC-MS of the column purified biosurfactant revealed the presence of both mono and di rhamnolipid congeners. Rhamnolipid find tremendous application in medical field and as well as in detergent industry and since they are of biological origin, they can be used as favorable alternative against their chemical counterparts. The study demonstrated that catabolism of naphthalene and concurrent formation of rhamnolipid can result in a dual activity process, namely environmental cleanup and production of a valuable microbial metabolite. Additionally, the present-day application of rhamnolipids is highlighted.

Altered Pseudomonas Strategies to Inhibit Surface Aspergillus Colonies

Pseudomonas aeruginosa and Aspergillus fumigatus infections frequently co-localize in lungs of immunocompromised patients and individuals with cystic fibrosis (CF). The antifungal activity of P. aeruginosa has been described for its filtrates. Pyoverdine and pyocyanin are the principal antifungal P. aeruginosa molecules active against A. fumigatus biofilm metabolism present in iron-limited or iron-replete planktonic P. aeruginosa culture filtrates, respectively. Using various P. aeruginosa laboratory wild-type strains (PA14, PAO1, PAK), we found antifungal activity against Aspergillus colonies on agar. Comparing 36 PA14 and 7 PAO1 mutants, we found that mutants lacking both major siderophores, pyoverdine and pyochelin, display higher antifungal activity on agar than their wild types, while quorum sensing mutants lost antifungal activity. Addition of ferric iron, but not calcium or magnesium, reduced the antifungal effects of P. aeruginosa on agar, whereas iron-poor agar enhanced antifungal effects. Antifungal activity on agar was mediated by PQS and HHQ, via MvfR. Among the MvfR downstream factors, rhamnolipids and elastase were produced in larger quantities by pyoverdine–pyochelin double mutants and showed antifungal activity on agar. In summary, antifungal factors produced by P. aeruginosa on agar differ from those produced by bacteria grown in liquid cultures, are dependent on quorum sensing, and are downregulated by the availability of ferric iron. Rhamnolipids and elastase seem to be major mediators of Pseudomonas’ antifungal activity on a solid surface.

Microbial biosurfactant research: time to improve the rigour in the reporting of synthesis, functional characterization and process development

The demand for microbially produced surface-active compounds for use in industrial processes and products is increasing. As such, there has been a comparable increase in the number of publications relating to the characterization of novel surface-active compounds: novel producers of already characterized surface-active compounds and production processes for the generation of these compounds. Leading researchers in the field have identified that many of these studies utilize techniques are not precise and accurate enough, so some published conclusions might not be justified. Such studies lacking robust experimental evidence generated by validated techniques and standard operating procedures are detrimental to the field of microbially produced surface-active compound research. In this publication, we have critically reviewed a wide range of techniques utilized in the characterization of surface-active compounds from microbial sources: identification of surface-active compound producing microorganisms and functional testing of resultant surface-active compounds. We have also reviewed the experimental evidence required for process development to take these compounds out of the laboratory and into industrial application. We devised this review as a guide to both researchers and the peer-reviewed process to improve the stringency of future studies and publications within this field of science.

Recent progress and trends in the analysis and identification of rhamnolipids

Rhamnolipids have extensive potential applications and are the most promising biosurfactants for commercialization. The efficient and accurate identification and analysis of these are important to their production, application and commercialization. Accordingly, significant efforts have been made to identify and analyse rhamnolipids during screening of producing strains, fermentation and application processes. Cationic cetyltrimethylammonium bromide-methylene blue (CTAB-MB) test combines a series of indirect assays to efficiently assist in the primary screening of rhamnolipids-producing strains, while the secretion of rhamnolipids by these strains can be identified through TLC, FTIR, NMR, electrospray ionization mass spectrometry (ESI-MS) and HPLC-MS analysis. Rhamnolipids can be quantified by colorimetric methods requiring the use of concentrated acid, and this approach has the advantages of reliability, simplicity, low-cost and excellent reproducibility with very low technological requirements. HPLC-MS can also be employed as required as a more accurate quantification method. In addition, HPLC-ELSD has been established as the internationally acceptable measure of rhamnolipids for commercial purposes. The preparation of well-accepted rhamnolipids standards and modifications of analysis operations are essential to further enhance the accuracy and improve the simplicity of rhamnolipid analysis.Key points• Current status of R&D works on determination of rhamnolipids is listed• Advantages and disadvantages of various types analysis are summarized• Limitations of current rhamnolipid quantification are discussed Graphical abstract.

Semi-rational evolution of the 3-(3-hydroxyalkanoyloxy)alkanoate (HAA) synthase RhlA to improve rhamnolipid production in Pseudomonas aeruginosa and Burkholderia glumae

The 3-(3-hydroxyalkanoyloxy)alkanoate (HAA) synthase RhlA is an essential enzyme involved in the biosynthesis of HAAs in Pseudomonas and Burkholderia species. RhlA modulates the aliphatic chain length in rhamnolipids, conferring distinct physicochemical properties to these biosurfactants exhibiting promising industrial and pharmaceutical value. A detailed molecular understanding of substrate specificity and catalytic performance in RhlA could offer protein engineering tools to develop designer variants involved in the synthesis of novel rhamnolipid mixtures for tailored eco-friendly products. However, current directed evolution progress remains limited due to the absence of high-throughput screening methodologies and lack of an experimentally resolved RhlA structure. In the present work, we used comparative modeling and chimeric-based approaches to perform a comprehensive semi-rational mutagenesis of RhlA from Pseudomonas aeruginosa. Our extensive RhlA mutational variants and chimeric hybrids between the Pseudomonas and Burkholderia homologs illustrate selective modulation of rhamnolipid alkyl chain length in both Pseudomonas aeruginosa and Burkholderia glumae. Our results also demonstrate the implication of a putative cap-domain motif that covers the catalytic site of the enzyme and provides substrate specificity to RhlA. This semi-rational mutant-based survey reveals promising 'hot-spots' for the modulation of RL congener patterns and potential control of enzyme activity, in addition to uncovering residue positions that modulate substrate selectivity between the Pseudomonas and Burkholderia functional homologs. DATABASE: Model data are available in the PMDB database under the accession number PM0081867.

Culture Medium Optimization for Production of Rhamnolipids by Burkholderia glumae

Burkholderia glumae is a biosafety level 1 bacterium capable of producing rhamnolipid biosurfactant with longer 3-hydroxy fatty acid chains moieties than those produced by the prototypal producer, the opportunistic pathogen Pseudomonas aeruginosa. Although the capacity of production of rhamnolipid, and the parameters affecting this production, are well established for P. aeruginosa, little is known about the factors that may affect their production in B. glumae. Hence, to evaluate and enhance the production of rhamnolipids in B. glumae, following the selection of best carbon and nitrogen sources, a two-level fractional factorial design experiment was performed to identify the limiting factors significantly affecting the production of rhamnolipids in this bacterial species. Effects of six inorganic nutrients and two physical parameters were studied, and mannitol, urea, CaCl2, and potassium phosphate buffer were selected for further optimization by applying a response surface methodology (RSM). Under the identified optimized conditions, a rhamnolipid production of 1.66 g/L was obtained, about five times higher than that of the initial non-optimized conditions. This represents a key step in the development of large-scale production processes.

Simple glycolipids of microbes: Chemistry, biological activity and metabolic engineering

Glycosylated lipids (GLs) are added-value lipid derivatives of great potential. Besides their interesting surface activities that qualify many of them to act as excellent ecological detergents, they have diverse biological activities with promising biomedical and cosmeceutical applications. Glycolipids, especially those of microbial origin, have interesting antimicrobial, anticancer, antiparasitic as well as immunomodulatory activities. Nonetheless, GLs are hardly accessing the market because of their high cost of production. We believe that experience of metabolic engineering (ME) of microbial lipids for biofuel production can now be harnessed towards a successful synthesis of microbial GLs for biomedical and other applications. This review presents chemical groups of bacterial and fungal GLs, their biological activities, their general biosynthetic pathways and an insight on ME strategies for their production.

Broth versus Surface-Grown Cells: Differential Regulation of RsmY/Z Small RNAs in Pseudomonas aeruginosa by the Gac/HptB System

Two-component systems are capable of profoundly affecting genetic regulation in bacteria by detecting environmental stimuli, allowing them to quickly adapt. In Pseudomonas aeruginosa, the small RNAs (sRNAs) RsmY and RsmZ are under the control of the GacS/A system. They have been described as ones of the major key players in the control of planktonic and surface-associated behaviors. Genetic regulation by these sRNAs is achieved by the titration of the negative post-transcriptional regulator RsmA which affects the expression of over 500 genes. There is increasing evidence pinpointing the importance of RsmY and RsmZ in the planktonic-sessile P. aeruginosa lifestyles switch control. Using swarming motility as a model, we show here that these sRNA are differentially regulated depending on the selected growth conditions (i.e., planktonic versus surface grown-cells). Also, we report that opposite to planktonically grown cells, rsmZ regulation does not implicate the response regulator GacA in swarming cells. Furthermore, we present data indicating that RsmY/Z expression influence swarming motility via the protein HptB which acts as a negative regulator of these sRNAs and that they do not strictly converge to RsmA as previously reported.

Adaptive Significance of Quorum Sensing-Dependent Regulation of Rhamnolipids by Integration of Growth Rate in Burkholderia glumae: A Trade-Off between Survival and Efficiency

Quorum sensing (QS) is a cell density-dependent mechanism which enables a population of bacteria to coordinate cooperative behaviors in response to the accumulation of self-produced autoinducer signals in their local environment. An emerging framework is that the adaptive significance of QS in the regulation of production of costly extracellular metabolites ("public goods") is to maintain the homeostasis of cooperation. We investigated this model using the phytopathogenic bacterium Burkholderia glumae, which we have previously demonstrated uses QS to regulate the production of rhamnolipids, extracellular surface-active glycolipids promoting the social behavior called "swarming motility." Using mass spectrometric quantification and chromosomal lux-based gene expression, we made the unexpected finding that when unrestricted nutrient resources are provided, production of rhamnolipids is carried out completely independently of QS regulation. This is a unique observation among known QS-controlled factors in bacteria. On the other hand, under nutrient-limited conditions, QS then becomes the main regulating mechanism, significantly enhancing the specific rhamnolipids yield. Accordingly, decreasing nutrient concentrations amplifies rhamnolipid biosynthesis gene expression, revealing a system where QS-dependent regulation is specifically triggered by the growth rate of the population, rather than by its cell density. Furthermore, a gradual increase in QS signal specific concentration upon decrease of specific growth rate suggests a reduction in quorum threshold, which reflects an increase in cellular demand for production of QS-dependent target gene product at low density populations. Integration of growth rate with QS as a decision-making mechanism for biosynthesis of costly metabolites, such as rhamnolipids, could serve to assess the demand and timing for expanding the carrying capacity of a population through spatial expansion mechanisms, such as swarming motility, thus promoting the chances of survival, even if the cell density might not be high enough for an otherwise efficient production of rhamnolipids. In conclusion, we propose that the adaptive significance of growth rate-dependent functionality of QS in biosynthesis of costly public goods lies within providing a regulatory mechanism for selecting the optimal trade-off between survival and efficiency.

Characterization of rhamnolipids by liquid chromatography/mass spectrometry after solid-phase extraction

Rhamnolipids are surface-active agents with a broad application potential that are produced in complex mixtures by bacteria of the genus Pseudomonas. Analysis from fermentation broth is often characterized by laborious sample preparation and requires hyphenated analytical techniques like liquid chromatography coupled to mass spectrometry (LC-MS) to obtain detailed information about sample composition. In this study, an analytical procedure based on chromatographic method development and characterization of rhamnolipid sample material by LC-MS as well as a comparison of two sample preparation methods, i.e., liquid-liquid extraction and solid-phase extraction, is presented. Efficient separation was achieved under reversed-phase conditions using a mixed propylphenyl and octadecylsilyl-modified silica gel stationary phase. LC-MS/MS analysis of a supernatant from Pseudomonas putida strain KT2440 pVLT33_rhlABC grown on glucose as sole carbon source and purified by solid-phase extraction revealed a total of 20 congeners of di-rhamnolipids, mono-rhamnolipids, and their biosynthetic precursors 3-(3-hydroxyalkanoyloxy)alkanoic acids (HAAs) with different carbon chain lengths from C8 to C14, including three rhamnolipids with uncommon C9 and C11 fatty acid residues. LC-MS and the orcinol assay were used to evaluate the developed solid-phase extraction method in comparison with the established liquid-liquid extraction. Solid-phase extraction exhibited higher yields and reproducibility as well as lower experimental effort.

A Novel Glycolipid Biosurfactant Confers Grazing Resistance upon Pantoea ananatis BRT175 against the Social Amoeba Dictyostelium discoideum

Pantoea is a versatile genus of bacteria with both plant- and animal-pathogenic strains, some of which have been suggested to cause human infections. There is, however, limited knowledge on the potential determinants used for host association and pathogenesis in animal systems. In this study, we used the model host Dictyostelium discoideum to show that isolates of Pantoea ananatis exhibit differential grazing susceptibility, with some being resistant to grazing by the amoebae. We carried out a high-throughput genetic screen of one grazing-resistant isolate, P. ananatis BRT175, using the D. discoideum pathosystem to identify genes responsible for the resistance phenotype. Among the 26 candidate genes involved in grazing resistance, we identified rhlA and rhlB, which we show are involved in the biosynthesis of a biosurfactant that enables swarming motility in P. ananatis BRT175. Using liquid chromatography-mass spectrometry (LC-MS), the biosurfactant was shown to be a glycolipid with monohexose-C10-C10 as the primary congener. We show that this novel glycolipid biosurfactant is cytotoxic to the amoebae and is capable of compromising cellular integrity, leading to cell lysis. The production of this biosurfactant may be important for bacterial survival in the environment and could contribute to the establishment of opportunistic infections. IMPORTANCE The genetic factors used for host interaction by the opportunistic human pathogen Pantoea ananatis are largely unknown. We identified two genes that are important for the production of a biosurfactant that confers grazing resistance against the social amoeba Dictyostelium discoideum. We show that the biosurfactant, which exhibits cytotoxicity toward the amoebae, is a glycolipid that incorporates a hexose rather than rhamnose. The production of this biosurfactant may confer a competitive advantage in the environment and could potentially contribute to the establishment of opportunistic infections.

Quorum Sensing Controls Swarming Motility of Burkholderia glumae through Regulation of Rhamnolipids

Burkholderia glumae is a plant pathogenic bacterium that uses an acyl-homoserine lactone-mediated quorum sensing system to regulate protein secretion, oxalate production and major virulence determinants such as toxoflavin and flagella. B. glumae also releases surface-active rhamnolipids. In Pseudomonas aeruginosa and Burkholderia thailandensis, rhamnolipids, along with flagella, are required for the social behavior called swarming motility. In the present study, we demonstrate that quorum sensing positively regulates the production of rhamnolipids in B. glumae and that rhamnolipids are necessary for swarming motility also in this species. We show that a rhlA- mutant, which is unable to produce rhamnolipids, loses its ability to swarm, and that this can be complemented by providing exogenous rhamnolipids. Impaired rhamnolipid production in a quorum sensing-deficient B. glumae mutant is the main factor responsible for its defective swarming motility behaviour.