Submitted to: Biocatalysis and Agricultural Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2017
Publication Date: 7/11/2017
Citation: Zhang, X., Ashby, R.D., Solaiman, D., Liu, Y., Fan, X. 2017. Antimicrobial activity and inactivation mechanism of lactonic and free acid sophorolipids against Escherichia coli O157:H7. Biocatalysis and Agricultural Biotechnology. 11(C):176-182. doi: 10.1016/j.bcab.2017.07.002. Interpretive Summary: In recent years, there has been an increasingly interest in natural antimicrobials due to concerns over long term impact of synthetic chemicals on the environment and human health. In this study, we evaluated the antimicrobial activity of six different sophorolipids, natural compounds produced by yeasts from sugars and fatty acids, on five strains of pathogenic E. coli O157:H7. Results show that the effect of sophorolipids on E. coli O157:H7 depends on the form of sophorolipids (free-acid vs cyclic), the type of fatty acids, presence of ethanol, treatment time, and sophorolipids concentration. It appears that E. coli inactivation elicited by sophorolipids is attributable to cell membrane damages. The information would be useful for food and other industries to apply natural antimicrobials to mitigate the risk of the pathogen.
Technical Abstract: In this study, we synthesized sophorolipids (SLs) from glucose and either palmitic, stearic or oleic acid via fermentation using the osmophilic yeast Starmerella bombicola, purified and separated the SLs into lactonic and free-acid forms and assessed their antimicrobial efficacy against 5 strains of pathogenic Escherichia coli O157:H7. Results showed that the antimicrobial activities of individual SLs depended on the type of SL, treatment time and concentration. Lactonic stearic and oleic SLs were more effective in reducing E. coli O157:H7 populations than the free-acid counterparts. Compared to ethanol (20%), 0.5 and 1% lactonic stearic acid SLs in the presence of 20% ethanol achieved more than 5-log reductions of E. coli after 1 h of treatment. The five strains of E. coli O157:H7 had different susceptibilities to SLs. Analysis using the ethidium monoazide-PCR amplification demonstrated that SLs inactivated bacteria through cell membrane damage in E. coli O157:H7. Overall, our results suggest that SLs have the potential to inactivate pathogenic E. coli O157:H7 in combination with low levels of ethanol.