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Trehalose lipids are the basic component of the cell wall glycolipids in Mycobacteria and Corynebacteria and are known to be produced by Gram-positive bacteria, as Actinomycetales such as Mycobacterium, Nocardia or Corynebacterium and they differ in the structure, size and degree of saturation (Cappelletti et al. 2020; Franzetti et al. 2010).

Rhodococcus erythropolis DSM43215 was reported for the first time as a producer of trehalose lipids with chain length (C20–C90) of the esterified fatty acids in 1982 (Kretschmer et al. 1982) and in 1983. These trehalose lipids were characterized as trehalose-6-monocorynomycolates, trehalose 6,6´-diacylates and trehalose-6-acylates (Kretschmer et al. 1982). A non-ionic trehalose lipid, consisting of one major and ten minor components was produced using Rhodococcus strain H13-A (Bryant 1990). Other types of trehalose lipids, including mono-, di- and tri-corynomycolates, mono-, di-, tetra-, hexa- and octa-acylated derivatives of trehalose, trehalose tetraesters and succinoyl trehalose lipids were produced, in the following years, using R. erythropolis and R. ruber (Esders and Light 1972; Uchida et al. 1989; White et al. 2013). The large-scale production of trehalose lipids is very challenging. The effective use of biosurfactants is limited by the high cost of production and complex downstream processing (Franzetti et al. 2010). In addition, when Rhodococcus strains are used for this purpose, the major problem is the fact that trehalose lipids are associated with the cell walls leading to an increase in the costs of downstream processing and recovery (Espuny et al. 1996). Furthermore, several studies have shown that the production of trehalose lipids can either be extracellular, or cell-bound, depending on the growth conditions. Experiments presented that R. erythropolis ATCC 4277 was able to produce extracellular trehalose lipids, which were all released into the medium, using glycerol as the sole carbon source, while the production was partially cell-bound when cells were grown on n-hexadecane (Ciapina et al. 2006; Franzetti et al. 2010).

The yields of trehalose lipids appear to be very low compared to sophorolipids, rhamnolipids and mannosylerythritol lipids. They are often bound to cell surfaces, which reduces the production yield and increases downstream costs. Three basic strategies have been adopted to make the fermentation process cost-competitive: (i) using cheap and waste substrates, (ii) development of efficient bioprocesses, such as optimization of fermentative conditions, (iii) development of overproducing mutant or recombinant strains (Franzetti et al. 2010; Uchida et al. 1989). One study has shown that a high phosphate buffer concentration and neutral pH conditions optimize the production of succinoyl trehalose lipids in R. erythropols SD-74 up to 40 g L^-1 (Franzetti et al. 2010; Uchida et al. 1989).

In the bioproduction of trehalose lipids many factors need to be considered. Firstly, the microorganism must be carefully chosen to produce the trehalose lipid required, with special attention to the purpose of its application. The microbial production can be influenced by different factors such as media composition (e.g., carbon and nitrogen sources, salt composition or use of extract in culture broth), bioreactor and environmental conditions (e.g. temperature, pH, oxygen, speed). As well as sophorolipids, for trehalose lipids, there is the possibility of further chemical modifications to obtain novel analogues with diverse properties.