Inactivation of Salmonella spp. and Listeria spp. by palmitic, stearic and oleic acid sophorolipids and thiamine dilauryl sulfate

Authors: ZHANG, XUEJIE - Chinese Academy Of Agricultural Sciences; Ashby, Richard - Rick; Solaiman, Daniel; Uknalis, Joseph; Fan, Xuetong

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2016
Publication Date: 12/22/2016
Citation: Zhang, X., Ashby, R.D., Solaiman, D., Uknalis, J., Fan, X. 2016. Inactivation of Salmonella spp. and Listeria spp. by palmitic, stearic and oleic acid sophorolipids and thiamine dilauryl sulfate. Frontiers in Microbiology. doi: 10.3389/fmicb.2016.02076.

Interpretive Summary: Consumer concerns over chemically synthesized additives have stimulated research into natural antimicrobials to improve the microbial safety of food. Sophorolipids are a class of natural compounds that are composed of sugar and fat, and are produced by a number of yeasts. In this study we evaluated the antimicrobial properties of six different sophorolipids purified from various culture media fermented by a highly productive yeast strain. Our results demonstrated that sophorolipids in the cyclic configuration were more effective against Salmonella and Listeria spp. than those in the free acid form. The cyclic sophorolipids, when dissolved in low, non-toxic concentrations of ethanol, inactivated both foodborne pathogens. Therefore, these newly purified natural antimicrobials may be used to enhance food safety.

Technical Abstract: Food contaminated with human pathogens, such as Salmonella spp. and Listeria monocytogenes, frequently causes outbreaks of foodborne illness. Consumer concern over the use of synthesized antimicrobials to enhance microbial food safety has led to a search of natural alternatives. The objectives of this study were to evaluate the antimicrobial activity of various types of sophorolipids (SLs) and thiamine dilauryl sulfate (TDS) against pathogenic Salmonella spp. and Listeria spp. and to investigate inactivation mechanisms of the compounds. Both free and lactonic forms of SLs were synthesized from Candida bombicola using palmitic, stearic and oleic acids as co-feedstocks. TDS and purified SLs were used to treat cocktails of Salmonella spp. and Listeria spp. Results showed that lactonic SLs had higher antimicrobial activity than the free-acid form, and Gram-positive Listeria spp. were more susceptible to SLs than Gram-negative Salmonella spp. Listeria populations were reduced from an initial concentration of 7.2 log CFU/mL to a non-detectible level within a 1 min treatment of 0.1% lactonic SLs in the presence of 20% ethanol, which itself did not significantly reduce the populations. There were no significant differences in the antimicrobial efficacy among palmitic, stearic and oleic acid-based SLs against Salmonella or Listeria spp. Ethanol was utilized to improve the antimicrobial activity of free-acid SLs against Gram-negative bacteria. SEM and TEM images showed that SLs damaged cell membranes and resulted in cell lysis. Overall, our results demonstrated that SLs in the presence of ethanol can be used to inactivate foodborne pathogens, especially Gram-positive bacteria.