Surfactants tailored by the class Actinobacteria

Abstract

Globally the change towards the establishment of a bio-based economy has resulted in an increased need for bio-based applications. This, in turn, has served as a driving force for the discovery and application of novel biosurfactants. The class Actinobacteria represents a vast group of microorganisms with the ability to produce a diverse range of secondary metabolites, including surfactants. Understanding the extensive nature of the biosurfactants produced by actinobacterial strains can assist in finding novel biosurfactants with new potential applications. This review therefore presents a comprehensive overview of the knowledge available on actinobacterial surfactants, the chemical structures that have been completely or partly elucidated, as well as the identity of the biosurfactant-producing strains. Producer strains of not yet elucidated compounds are discussed, as well as the original habitats of all the producer strains, which seems to indicate that biosurfactant production is environmentally driven. Methodology applied in the isolation, purification and structural elucidation of the different types of surface active compounds, as well as surfactant activity tests, are also discussed. Overall, actinobacterial surfactants can be summarized to include the dominantly occurring trehalose-comprising surfactants, other non-trehalose containing glycolipids, lipopeptides and the more rare actinobacterial surfactants. The lack of structural information on a large proportion of actinobacterial surfactants should be considered as a driving force to further explore the abundance and diversity of these compounds. This would allow for a better understanding of actinobacterial surface active compounds and their potential for biotechnological application.

Keywords: Rhodococcus; biosurfactant; emulsifier; glycolipid; lipopeptide; rhamnolipid; trehalose lipid.

Figure 1

Systematic classification of the class Actinobacteria including subclasses and orders. Suborder, families and genera examined for the production of biosurfactants and bioemulsifying compounds are displayed in numbers. Thirty six surfactant-producing genera are reported, all belonging to the largest order within the Actinobacteria: Actinomycetales.

Figure 2

Frequency distribution of solvents used for the enrichment of surfactants by two-phase extraction from the culture broth or cell free supernatant of 47 “rare” actinobacteria.