Elucidation of the Natural Function of Sophorolipids Produced by Starmerella bombicola

Abstract

The yeast Starmerella bombicola distinguishes itself from other yeasts by its potential of producing copious amounts of the secondary metabolites sophorolipids (SLs): these are glycolipid biosurfactants composed out of a(n) (acetylated) sophorose moiety and a lipid tail. Although SLs are the subject of numerous research papers and have been commercialized, e.g., in eco-friendly cleaning solutions, the natural function of SLs still remains elusive. This research article investigates several hypotheses for why S. bombicola invests that much energy in the production of SLs, and we conclude that the main natural function of SLs in S. bombicola is niche protection: (1) the extracellular storage of an energy-rich, yet metabolically less accessible carbon source that can be utilized by S. bombicola upon conditions of starvation with (2) antimicrobial properties. In this way, S. bombicola creates a dual advantage in competition with other microorganisms. Additionally, SLs can expedite growth on rapeseed oil, composed of triacylglycerols which are hydrophobic substrates present in the yeasts’ environment, for a non-SL producing strain (Δcyp52M1). It was also found that—at least under lab conditions—SLs do not provide protection against high osmotic pressure prevalent in sugar-rich environments such as honey or nectar present in the natural habitat of S. bombicola.

A honey authenticity test by interphase emulsion reveals biosurfactant activity and biotechnology in the stingless bee nest ofScaptotrigonasp. ‘Catiana’ from Ecuador

“…Extracellular storage of usable energy in starvation emergency, and 2. Antimicrobial activity (De Clercq et al, 2021). Proteomic and genetic research on S. bombicola is recent, such as the study of its exoproteome (Ciesielska et al, 2014) and its transportome (Claus et al, 2022), demonstrating the potential of omics sciences for the functional design of molecular biosynthesis in industrial biotechnology.…”

“…Extracellular storage of usable energy in starvation emergency, and 2. Antimicrobial activity(De Clercq et al, 2021).…”

Lyotropic Liquid-Crystalline Phases of Sophorolipid Biosurfactants

“…27 In addition, this work might provide a new perspective on the biological function of sophorolipids. Indeed, a recent study has shown that these compounds could represent an extracellular energy storage form in yeasts, 60 and the highly dense and efficient molecular packing of liquid-crystalline phases may be an important advantage in this respect.…”

Oleaginous yeasts: Time to rethink the definition?

“…Although based on TAG levels S. bombicola might not qualify as an oleaginous yeast, the production of sophorolipids and similar metabolic adaptations require a reconsideration of the oleaginicity requirements. Furthermore, it has been defended that the natural purpose of sophorolipids for the cell is to create extracellular storage of energy‐rich molecules that can be consumed while at the same time presenting antimicrobial properties (De Clercq et al, 2021). If this storage is indeed the case, we would encounter a case in which S. bombicola falls out of the oleaginous definition only because it is accumulating lipids extracellularly instead of intracellularly.…”