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 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. 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 and TDS 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% (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 Salmonella or Listeria 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 Salmonella and Listeria spp. scanning electron microscopy and transmission electron microscopy images showed that SLs and TDS damaged Listeria 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.

Reductionin Listeria monocytogenes, Salmonella enterica and Escherichia coli O157:H7 in vitro and on tomato by sophorolipid and sanitiser as affected by temperature and storage time

“…In this study, the reductions in Salmonella and E. coli O157:H7 populations by SL-p were at much lower concentrations of sophorolipid than those were reported previously in which 5-6 log reductions were recorded (Zhang et al, 2016). In this study, the reductions in Salmonella and E. coli O157:H7 populations by SL-p were at much lower concentrations of sophorolipid than those were reported previously in which 5-6 log reductions were recorded (Zhang et al, 2016).…”

“…Our results substantially differ from that of Zhang et al, in that we recorded 5-7 log reductions on pathogenic strains of L. monocytogenes such as 008, 2634 and 2625 after 24 h of storage and with much higher SL-p concentrations of 0.02%, 0.08% and 0.12%. The difference in our results with that of Zhang et al (2016) may be due to differences in stains of L. monocytogenes, while L. monocytogenes Scott A724 and nonpathogenic L. innocua surrogate strains 33090, 33091 and 51742 were evaluated by Zhang et al (2016). The population counts of Salmonella treated with SLp also differed significantly (P < 0.05) with storage time (Fig.…”

“…In previous studies, Zhang et al (2016) reported about 7 log reductions in Listeria spp. In previous studies, Zhang et al (2016) reported about 7 log reductions in Listeria spp.…”

“…In previous studies, Zhang et al (2016) reported about 7 log reductions in Listeria spp. The difference in our results with that of Zhang et al (2016) may be due to differences in stains of L. monocytogenes, while L. monocytogenes Scott A724 and nonpathogenic L. innocua surrogate strains 33090, 33091 and 51742 were evaluated by Zhang et al (2016). Our results substantially differ from that of Zhang et al, in that we recorded 5-7 log reductions on pathogenic strains of L. monocytogenes such as 008, 2634 and 2625 after 24 h of storage and with much higher SL-p concentrations of 0.02%, 0.08% and 0.12%.…”

“…The duration of bacterial storage following treatment with SL-p significantly (P < 0.05) affected populations of L. monocytogenes based on in vitro assays, as bacterial cell numbers differed with SL-p treatments of 30 min and 24 h. This suggests that the efficacy of SL-p for reductions in L. monocytogenes populations was substantially greater at 24 h than at 30 min of storage, due to exposure duration. In previous studies, Zhang et al (2016) reported about 7 log reductions in Listeria spp. after 1 min of treatment.…”