Antimicrobial effect of lauroyl arginate ethyl on Escherichia coli O157:H7 and Listeria monocytogenes on red oak leaf lettuce

The objective of the present study was to determine the antimicrobial activity of lauroyl arginate ethyl (LAE) against enterohemorrhagic Escherichia coli (EHEC) O157:H7 and Listeria monocytogenes on oak leaf lettuce. Thus, bacterial suspensions of both pathogens containing 9.0 log 10 CFU/mL were spot inoculated on the upper surfaces of lettuce leaves and the leaves were washed with water containing LAE in a final concentration of 100 mg/L. The viable counts of L. monocytogenes DSM 20600 T and Escherichia coli O157:H7 strain EDL933 were reduced with this treatment by ~3.5 log 10 CFU/g and ~2.5 log 10 CFU/g, respectively. The microbial load of the wash water was reduced by more than 4.1 log 10 CFU/mL and was below the detection limit. The second objective was to study whether adherence factors of EHEC O157:H7 strain EDL933 influence the ability to adhere on the lettuce surface, as well as the effectivity of the washing processes. Therefore, the flagellin gene fliC and the pilin subunit encoding gene hcpA of the hemorrhagic coli pilus were deleted. Based on the initial inoculation level, and without any washing step, the hcpA mutant was recovered 18 % less than the wild type from the leaf surface, the recovery of the fliC mutant was approximately 30 % higher as observed for the wild type. Both mutants could be washed from the lettuce leaves to a similar level as the wild type (~2.7 log 10 CFU/g with LAE treatment and ~1.0 log 10 CFU/g without LAE treatment). The findings of this study help to develop novel intervention strategies for fresh produce processing and washing treatments.