Genetic Regulations of the Biosynthesis of Microbial Surfactants: An Overview

Abstract

Microbial biosurfactants are surface active metabolites synthesized by microbes growing on a variety of substrates. In spite of having great potential for commercial, therapeutic and environmental applications, industrial level production has not been realized for their low yields and productivities. One vital factor determining their biosynthesis is the genetic makeup of the producer organisms. Studies on molecular genetics and biochemistry of the synthesis of several biosurfactants have revealed the operons, the enzymes and the metabolic pathways required for their extracellular production. Surfactin, a cyclic lipopeptide biosurfactant is a potent antimicrobial agent and is produced as a result of non-ribosomal biosynthesis catalyzed by a large multienzyme peptide synthetase complex called the surfactin synthetase. Pathways for the synthesis of other lipopeptides such as iturin, lichenysin and arthrofactin are also mediated by similar enzyme complexes. These non-ribosomal peptide synthetases (NRPSs) responsible for lipopeptide biosynthesis display a high degree of structural similarity among themselves even from distant microbial species. Plasmid-encoded- rhlA, B, R and I genes of rhl quorum sensing system are required for production of glycolipid biosurfactants by Pseudomonas species. Molecular genetics of biosynthesis of alasan and emulsan by Acinetobacter species and of the fungal biosurfactants such as mannosylerythritol lipids (MEL) and hydrophobins have been deciphered. However, limited genetic information is available about biosynthesis of other biosurfactants such as viscosin, amphisin and synthesized counterparts. As these molecules have emerged as potential agents in many industrial and environmental processes as well as in biomedical and therapeutic applications, it is essential to make them cost competitive. Genetically engineered hyper producing organisms giving high yields can bring the real breakthrough in the production process. This is possible only if the genetics of the microbial surfactant production is known in details. It is therefore desirable that the future research on biosurfactants be focused on the development and use of hyperproducers. The detailed knowledge of the genetics of microbial surfactant production should be used to produce organisms giving higher production with better product characteristics. With a better knowledge of the genes involved in this process, biosurfactant production can be realized in non-pathogenic industrial strains. The knowledge of expression of genes of a particular biosurfactant producer in a particular habitat will also throw light upon substrate dependence of production and preference for a particular substrate. Detailed description of the genetics of production of the newly identified biosurfactants like flavolipids, tensin and lokisin is not available. Efforts should be made by investigators to develop high yielding strains of microorganisms producing strong biosurfactants such as arthrofactin and lichenysin. A few marine biosurfactants have been discovered (Kalinovskaya et al. 2004) and there is high possibility of finding many other novel surface active compounds from the marine sources. Many of these are expected to possess interesting properties as pharmaceuticals and biomedical agents. The knowledge of molecular genetics of microbial surfactant production and its subsequent use to produce hyperproducers will determine the fate of biosurfactant industry.