Microbial production of rhamnolipids: opportunities, challenges and strategies

Abstract

Rhamnolipids are a class of biosurfactants which contain rhamnose as the sugar moiety linked to β-hydroxylated fatty acid chains. Rhamnolipids can be widely applied in many industries including petroleum, food, agriculture and bioremediation etc. Pseudomonas aeruginosa is still the most competent producer of rhamnolipids, but its pathogenicity may cause safety and health concerns during large-scale production and applications. Therefore, extensive studies have been carried out to explore safe and economical methods to produce rhamnolipids. Various metabolic engineering efforts have also been applied to either P. aeruginosa for improving its rhamnolipid production and diminishing its pathogenicity, or to other non-pathogenic strains by introducing the key genes for safe production of rhamnolipids. The three key enzymes for rhamnolipid biosynthesis, RhlA, RhlB and RhlC, are found almost exclusively in Pseudomonas sp. and Burkholderia sp., but have been successfully expressed in several non-pathogenic host bacteria to produce rhamnolipids in large scales. The composition of mono- and di-rhamnolipids can also be modified through altering the expression levels of RhlB and RhlC. In addition, cell-free rhamnolipid synthesis by using the key enzymes and precursors from non-pathogenic sources is thought to not only eliminate pathogenic effects and simplify the downstream purification processes, but also to circumvent the complexity of quorum sensing system that regulates rhamnolipid biosynthesis. The pathogenicity of P. aeruginosa can also be reduced or eliminated through in vivo or in vitro enzymatic degradation of the toxins such as pyocyanin during rhamnolipid production. The rhamnolipid production cost can also be significantly reduced if rhamnolipid purification step can be bypassed, such as utilizing the fermentation broth or the rhamnolipid-producing strains directly in the industrial applications of rhamnolipids.

Keywords: Application; Metabolic engineering; Pseudomonas, fermentation; Rhamnolipids.

Fig. 1

Rhamnolipid production in Pseudomonas. The synthesis pathway for rhamnolipids is indicated while the details for metabolic pathway of glucose and synthesis of Acyl-CoA are not provided. RhlA, RhlB and RhlC are three enzymes essential for rhamnolipid synthesis

Fig. 2

A flow chart of rhamnolipids production. This flow chart shows the procedures for rhamnolipid production. A large scale rhamnolipid production contains the following steps: seed preparation, fermentation, production purification. For a given strain, the costs are focused on the fermentation and purification steps. Seed preparation for fermentation is a common step which does not require extra cost. Fermentation cost includes carbon and nitrogen sources, fermentation styles, and other additional steps or chemicals required for rhamnolipid production. Cost for production purification depends on the required production purity. Some applications may not require rhamnolipid purification

Fig. 3

Strategies to reduce rhamnolipid production cost. a Strain screening. A good strain can reduce the production cost effectively; b strain modification by mutagenesis and metabolic engineering; c fermentation. Fermentation is an important step which requires a lot of efforts to optimize. The possible factors that affect fermentation cost are listed