Rhamnolipids: diversity of structures, microbial origins and roles

Rhamnolipids are glycolipidic biosurfactants produced by various bacterial species. They were initially found as exoproducts of the opportunistic pathogen Pseudomonas aeruginosa and described as a mixture of four congeners: α-L-rhamnopyranosyl-α-L-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-Rha-C10-C10), α-L-rhamnopyranosyl-α-L-rhamnopyranosyl-β-hydroxydecanoate (Rha-Rha-C10), as well as their mono-rhamnolipid congeners Rha-C10-C10 and Rha-C10. The development of more sensitive analytical techniques has lead to the further discovery of a wide diversity of rhamnolipid congeners and homologues (about 60) that are produced at different concentrations by various Pseudomonas species and by bacteria belonging to other families, classes, or even phyla. For example, various Burkholderia species have been shown to produce rhamnolipids that have longer alkyl chains than those produced by P. aeruginosa. In P. aeruginosa, three genes, carried on two distinct operons, code for the enzymes responsible for the final steps of rhamnolipid synthesis: one operon carries the rhlAB genes and the other rhlC. Genes highly similar to rhlA, rhlB, and rhlC have also been found in various Burkholderia species but grouped within one putative operon, and they have been shown to be required for rhamnolipid production as well. The exact physiological function of these secondary metabolites is still unclear. Most identified activities are derived from the surface activity, wetting ability, detergency, and other amphipathic-related properties of these molecules. Indeed, rhamnolipids promote the uptake and biodegradation of poorly soluble substrates, act as immune modulators and virulence factors, have antimicrobial activities, and are involved in surface motility and in bacterial biofilm development

Quorum sensing:Implications on rhamnolipid biosurfactant production

Quorum sensing (QS) has received significant attention in the past few decades. QS describes population density dependent cell to cell communication in bacteria using diffusible signal molecules. These signal molecules produced by bacterial cells, regulate various physiological processes important for social behavior and pathogenesis. One such process regulated by quorum sensing molecules is the production of a biosurfactant, rhamnolipid. Rhamnolipids are important microbially derived surface active agents produced by Pseudomonas spp. under the control of two interrelated quorum sensing systems; namely las and rhl. Rhamnolipids possess antibacterial, antifungal and antiviral properties. They are important in motility, cell to cell interactions, cellular differentiation and formation of water channels that Currently, biosurfactants are unable to compete economically with chemically synthesized compounds in the market due to high production costs. Once the genes required for biosurfactant production have been identified, they can be placed under the regulation of strong promoters in nonpathogenic, heterologous hosts to enhance production. The production of rhamnolipids could be increased by cloning both the rhlAB rhamnosyltransferase genes and the rhlRI quorum sensing system into a suitable bacterium such as E. coli or P. putida and facilitate rhamnolipid production. Biosurfactants can also be genetically engineered for different industrial applications assuming there is a strong understanding of both the genetics and the structure-function relationships of each component of the molecule. Genetic engineering of surfactin has already been reported, with recent papers describing the creation of novel peptide structures from the genetic recombination of several peptide synthetases. Recent application of dynamic metabolic engineering strategies for controlled gene expression could lower the cost of fermentation processes by increasing the product formation. Therefore, by integrating a genetic circuit into applications of metabolic engineering the biochemical production can be optimized. Furthermore, novel strategies could be designed on the basis of information obtained from the studies of quorum sensing and biosurfactants produced suggesting enormous practical applications.</p