Biosurfactants and emulisfiers are not just helpful when washing up! What are their natural roles?

Eliora Z. Ron and Eugene Rosenberg, 2001, The natural role of biosurfactants, Environmental Microbiology, Vol 23 229 - 236

Natural roles of biosurfactants

Biosurfactants, also known as surface- active agents, are produced by a wide variety of microorganisms. Each has a very different chemical structure but they all stabilize the dispersal of one substance into another; a common example is the effect of washing up liquid on the dispersal of oil in water. Emulsifiers are one type of biosurfactant often categorized into two different groups, low molecular weight molecules, that lower surface interfacial (boundary) tensions between substances and high molecular weight polymers which bind to the surfaces of these substances. Biosurfactants have the ability to get ‘in between’ two substances that do not usually disperse amongst one another as they are amphipathic, that is, they have both polar and hydrophobic regions and this gives them their stabilizing abilities. Due to the variability in chemical structure between each biosurfactant it is hypothesized that they have individual roles in the growth and productivity of each microorganism. Different surfactants could therefore convey advantages in separate niches by benefiting the each microorganism differently. This review therefore focuses on the natural benefits of biosurfactants on microorganisms.

Low molecular weight biosurfactants are usually glycolipids or lipoproteins. Glycolipids are often found in microorganisms such as Rhodococcus erythropolis and the yeast Torulopsis. The best studied glycolipid is rhamnolipid produced by several species of Pseudomonads, however the most potent biosurfactant glycolipid, named surfactin, is a cyclic lipopeptide and has been found to be produced by the bacteria Bacilus subtilis.

High molecular weight biosurfactants produce exocellular surfactants made of polysaccharides, proteins, lipopolysaccharides lipoproteins or a mixture of them all. Acinetobactor produces the best studied bioemulsans, which are effective emulsifiers even at low concentrations.

There are numerous ways in which the production of emulsifiers can benefit a microorganism. Firstly, they could increase the bioavailability of hydrophobic water – insoluble substrates i.e these substrates would become available to those microorganisms that produce emulsifiers only, conveying an advantage over other microorganisms. Organic molecules become bound to surfaces making them unavailable to biodegradation; however surfactants can desorb and lower surface tension between surfaces, mobilizing bound hydrophobic substances making them available for such degradation.

Rhamnolipids have also been shown to bind heavy metals, removing cadmium, lead and zincs from soil. As well as this rhamnolipids can reduce the toxicity of cadmium metal by interacting with its surface and reducing uptake.

Biosurfactants are involved in pathogenesis, that is, the development of a disease; again rhamnolipid is used as an example. It is associated with the production of other virulence factors from the microorganism P. aeruginosa and is mediated by the same protein, the AlgC protein. This means that rhamnolipids are involved in the infection or colonization of a host by P. aeruginosa, a helpful trait.

B. subtilis’ cyclic lipopeptide, mentioned earlier and other biosurfactants like streptofactin produced by Streptomyces tendae have antibiotic properties. Bacillus species produce peptide biosurfactants during the early stages of sporulation and they are therefore speculated to play a role here.

Secretion of biosurfactants can lead to the formation of a film around the interface of a microorganism. This can either increase or decrease hydrophobicity and therefore the secretion of surfactants helps to regulate what a microorganism will attach to and can therefore inhibit attachment if need be.

Throughout the report the production of biosurfactants is linked with cell density and it is suggested that this has a selective advantage. Being virulence factors the production of biosurfactants is therefore linked to the volume of bacteria needed to create an attack on the host, once there are enough cells the virulence factor is produced.

Finally and most interestingly, horizontal transfer of biosurfactants has been shown to exist. Alasan, the exocellular polymeric emulsifier produced by A. radioresistens was seen to bind to two other heterologous bacteria changing their surface properties! Such transfer is thought to have implications on microbial communities and the biofilms that they produce.