Inactivation of Salmonella spp. and Listeria spp. by Palmitic, Stearic, and Oleic Acid Sophorolipids and Thiamine Dilauryl Sulfate

Abstract

Food contaminated with human pathogens, such as <i>Salmonella</i> spp. and <i>Listeria monocytogenes</i>, 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 <i>Salmonella</i> spp. and <i>Listeria</i> spp. Both free and lactonic forms of SLs were synthesized from <i>Candida bombicola</i> using palmitic, stearic, and oleic acids as co-feedstocks. TDS and purified SLs were used to treat cocktails of <i>Salmonella</i> spp. and <i>Listeria</i> spp. Results showed that lactonic SLs had higher antimicrobial activity than the free-acid form, and Gram-positive <i>Listeria</i> spp. were more susceptible to SLs and TDS than Gram-negative <i>Salmonella</i> spp. <i>Listeria</i> 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% (w/v) lactonic SLs and TDS 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 <i>Salmonella</i> or <i>Listeria</i> spp. Ethanol was utilized to improve the antimicrobial activity of free-acid SLs against Gram-negative bacteria. In general, TDS was more effective than the SLs against <i>Salmonella</i> and <i>Listeria</i> spp. scanning electron microscopy and transmission electron microscopy images showed that SLs and TDS damaged <i>Listeria</i> cell membranes and resulted in cell lysis. Overall, our results demonstrated that SLs and TDS in the presence of ethanol can be used to inactivate foodborne pathogens, especially Gram-positive bacteria.