Optimizing carbon-to-nitrogen ratios for enhanced antimicrobial sophorolipid production in Starmerella riodocensis GT-SL1R

Sophorolipids (SLs) are promising biosurfactants with diverse industrial and biomedical applications; however, optimizing their production efficiency remains a challenge. This study investigates the effect of carbon-to-nitrogen (C/N) ratios on SL production by Starmerella riodocensis GT-SL1R and evaluates the antimicrobial and anti-inflammatory properties of partial purified SLs. Among the tested C/N ratios (25-200), the highest SL production (26.71 ± 2.01 g·l⁻¹) was achieved at a C/N of 100, with a yield of 0.27 ± 0.0 g·g⁻¹ after 7 days. High C/N conditions (≥100) upregulated key SL biosynthesis genes (CYP52M1, UgtA1, UgtB1, At, and Sble), supporting enhanced SL production. Scale-up fermentation in a 5 l bioreactor further improved SL production to (55.94 ± 1.17 g·l⁻¹), outperforming Starmerellabombicola BCC5426 (50.49 ± 1.97 g·l⁻¹). Structural characterization revealed a predominance of lactonic SLs. SLs exhibited strong antimicrobial activity against Cutibacterium acnes, Staphylococcus epidermidis and Staphylococcus aureus and demonstrated potent anti-inflammatory effects by inhibiting nitric oxide production (IC50 = 21.93 ± 5.95 µg ml⁻¹), with lower cytotoxicity than indomethacin. These findings highlight S. riodocensis GT-SL1R as a promising candidate for industrial SL production and biomedical applications.