Biosurfactants: Potential Agents for Controlling Cellular Communication, Motility, and Antagonism

Abstract

Biosurfactants are surface-active molecules produced by microorganisms, either on the cell surface or secreted extracellularly. They form a thin film on the surface of microorganisms and help in their detachment or attachment to other cell surfaces. They are involved in regulating the motility of bacteria and quorum sensing. Here, we describe the various types of biosurfactants produced by microorganisms and their role in controlling motility, antagonism, virulence, and cellular communication.

Keywords: bacteria; biofilm; biosurfactant; motility; quorum sensing; virulence.

FIGURE 1

Role of biosurfactants in quorum sensing, motility, biofilm adhesion, biofilm detachement, virulence factors, and antagonism.

FIGURE 2

Main classes of biosurfactants and their structures.

FIGURE 3

Quorum sensing system and biosurfactant biosynthesis regulation in Pseudomonas aeruginosa: increasing cell density activates the three major quorum sensing mechanisms, the las, rhl, and PQS. The las system has transcriptional activators, LasR and LasI (produces autoinducer 3-oxo-C₁₂HSL). The 3-oxo-C₁₂HSL activates LasR by complex formation leading to the induction of various genes. The 3-oxo-C₁₂HSL–LasR complex also activates PQS system, where PQS binds to its receptor, PqsR, thus regulating the expression of various virulence factors and biofilm formation. The rhl system has rhlA and rhlB that produces rhamnosyltransferase, arranged with transcriptional activators, rhlR and rhlI (produce autoinducer C₄–HSL). The autoinducer C₄–HSL acts and forms complex with RhlR, which induces gene expression of various factors. The C₄–HSL–RhlR complex also activates the rhlA promoter, leading to the transcription of the rhlAB genes to produce rhamnolipids. Thick blue lines depict the sequential reactions for the formation of mono- and di-rhamnolipids, also showing the biosynthesis routes to produce their dTDP–rhamnose and fatty acid precursors. Green dashed lines depict the transport of various gene factors outside the cell membrane.

FIGURE 4

Correlation between motility and HAA production: a bacterial cell is shown with the flagella required for motility and with the four-tiered flagellar regulatory hierarchy and HAA production. FleQ transcriptionally activates FleSR, which is required for site selection and formation of hook and basal body complex formation. After the complete formation of hook and basal body complex, an anti-sigma factor is secreted through it (indicated in red dashed box and line), releases the FliA inhibition which directs the genetic expression of FliC (flagellin) and chemotaxis proteins, and controls the flagellar motor rotation. FleQ also controls the HAA formation from the acylated precursors by the transcriptional activity of rhlA. HAA is transported outside through cell membrane (shown with orange dashed line). Flagella synthesis depicts that the early flagellar assembly activates HAA production, but the late stages including fliC activation and flagellar maturation restrict the HAA production (Adapted from (Xu et al., 2012)).