Antibacterial Properties Of Sophorolipids Against Staphylococcus Aureus And Their Effects Of Application In Milk

Sophorolipids is a kind of glycolipid biosurfactant produced by non pathogenic yeast.Sophorolipids not only have excellent physicochemical properties,but also have good antibacterial activity.Investigating the antibacterial activity of SLs on food pathogenic bacteria Staphylococcus aureus and its effect on the application of milk for food additives to widen the application range of sophorolipids is of great significance.The main works and conclusions are as follows:1.The effects of L-SLs alone and L-SLs combined with nisin on S.aureus were studied by cylinder plate method,tube broth method,liquid cuiture inhibition assay and checker-board titration.S.aureus was inhibited in a concentration dependent manner,antimicrobial diameters were 10.50 mm and 21.30 mm after the treatment of L-SLs and nisin respectively at the same concentration of 10 mg/mL.The minimum inhibitory concentrations were 32 ?g/mL and 0.5 ?g/mL,and the minimum bactericidal concentrations were 64 ?g/mL and 1 ?g/mL.L-SLs and nisin showed stronger inhibitory activity in an hour,then antibacterial activities decreased and stabilized.The value of fractional inhibitory concentration(FIC)was 1.5,which showed independent effect of combination of L-SLs and nisin.These results demonstrated that L-SLs alone and L-SLs combined with nisin have strong inhibitory effects on S.aureus.Furthermore,L-SLs combined with nisin showed no antagonism inhibition,expecting to develop into food additive and compound food additives.2.The effects of L-SLs and nisin on cell wall and cytomembrane of S.aureus were investigated.The consequences showed that L-SLs and nisin could increase hydrophobicity of cell surface and the permeability of cell wall.Furthermore,L-SLs and nisin enhance the relative release of ?-galactosidase as an important enzyme in metabolism process,and cause a leakage of intracellular substances.During this period,the activity of the respiratory-chain dehydrogenase decreased.These results demonstrated that L-SLs and nisin damaged the integrity of cell wall and cell memberane,inhibit S.aureus growth remarkably.3.The effects of application of L-SLs in milk during storage were researched.Comparing with the control group,the data showed that sophorolipids could inhibit the protein content decrease and acidity increase and the total number of bacterial count,also delay the clotting time of milk.These results indicated that sophorolipids could inhibit the growth of bacteria in milk.So,physical and chemical indexes of milk maintained normal levels,which prolonged the shelf life of milk.Therefore,this study provides a possibility for the application of sophorolipids as a preservative in milk.