Lactonic sophorolipids by <i>Saccharomyces cerevisiae</i> : production, characterization and their evaluation for combating clinically important microbial pathogens

The purpose of this study was to produce sophorolipids (SLs) through microbial conversion of <i>Lepidium sativum</i> (LS) oil cake, an agro-industrial waste, using locally isolated yeast (<i>Saccharomyces cerevisiae</i>), and to examine their physicochemical characteristics. Furthermore, anti-proliferative activity was investigated against many drug-resistant microbial pathogens. Some mechanistic parameters of SLs on the treated bacterial cells were also evaluated <i>via</i> oxidative stress activities. The SLs yield was 7.2 g per 100 g of substrate using methanol extraction and 28.6 g per 100 g of substrate through ethyl acetate re-extraction from the same medium, resulting in a total yield of 35.8 g per 100 g of substrate. Our findings indicated that the SLs have an effectiveness in lowering the surface tension (ST) to 48 mN m^-1 at a critical micelle concentration (CMC) level of 30 mg L^-1. The produced SLs were identified by FTIR, LC-MS/MS, ¹H NMR spectroscopy, and GC/MS chromatography, confirming the existence of a lactonic form of SLs with the majority of total unsaturated fatty acids (81.98%). The produced SLs demonstrated strong antimicrobial properties, exhibiting MIC values lower than 100 μg mL^-1 against various clinically important pathogens. Efficiently, SLs have shown significant activity in inducing oxidative stress through lipid peroxidation in bacterial cell membranes and increasing the release of reactive oxygen species within some treated bacterial pathogens, such as <i>Bacillus cereus</i> and <i>Klebsiella pneumonia</i>.