Purpose: To evaluate the transfer of Escherichia coli O157:H7 and Listeria monocytogenes: (i) from inoculated knives to three fresh leafy greens and (ii) from inoculated vegetables to knife and then to uninoculated vegetables.
Methods: Sterilized knives were inoculated with pathogen (6 log CFU/ml) and after 10 min (25 °C), knives were used to cut three vegetables (spinach, lettuce, cabbage). Three cuts were performed for every sample (20 g), with totally 10 samples (200 g) analyzed per knife. Inoculated (6 or 4 log CFU/g, 5 °C, 30 min) leafy greens (200 g) were cut with sterile knife 10 times, and the contaminated knife was then used to cut 600 g of uninoculated vegetable of the same type (30 cuts).
Results: Cell counts transferred from knives to vegetables were highly dependent on the type of salad and less on pathogen. During cut of the vegetables with a contaminated knife, 4.5-5 log CFU/g per cut for 30 cuts were observed for spinach, 2-4.5 log CFU/g per cut for lettuce and 0.7-3 log CFU/g per cut for cabbage, regardless of the pathogen. Contamination transferred from an inoculated lettuce to a sterile knife resulted in further transfer of cells to fresh uninoculated lettuce; 4 log CFU/g per cut of E. coli O157:H7 for high inoculum and 2 log CFU/g for low inoculum were observed during 30 cuttings, while the corresponding populations of L. monocytogenes were 2 log CFU/g and 0.5 log CFU/g. In case of cabbage, transfer occurred for high inoculum level with 2 log CFU/g per cut, but not for low inoculum level, for both pathogens.
Significance: The type of fresh produce has great impact on the bacterial transfer to knives, leading to high likelihood of cross-contamination in the industry of the preparation of salads.