FACTORS CONTRIBUTING TO THE TRANSFER OF Escherichia coli O157:H7 BETWEEN CUTTING SURFACES AND FRESH PRODUCE; CROSS CONTAMINATION SENARIOS

摘要

Bacterial transfer events between biotic and abiotic surfaces are considered to be the major cause of cross contamination and consequently a risk for food safety. This study aimed to quantitatively determine the transfer of the pathogen Escherichia coli O157:H7 between leafy greens and equipment surfaces (cutting knives). Populations of the bacterium were evaluated during transfer: (i) from inoculated knives to fresh leafy greens, (ii) from inoculated greens to uninoculated knife, and (iii) from inoculated greens to knife and then to uninoculated greens. The number of leaves per cut, the types of vegetables (i.e, spinach, lettuce, cabbage) and the way that fresh produce was cut (vertically movement of the knife; V vs sliding the knive 5) were also evaluated for their impact on the pathogen's transfer. Sterilized knives were inoculated with a cocktail of three strains of f. coli O157:H7 (10~6 CFU/ml) and after 10 min (25°C) to allow the cells attach, knives were used for greens cutting. Three cuts were performed for every greens sample (20 g) and totally 10 samples (200 g) per knife were analyzed. Inoculated leafy green (6 log CFU/g, 5°C, 1h) was cut by sterilized knife 7 times, and the same contaminated knife was then used to cut 30 samples (600 g totally) of uninoculated product of the same type (7 cuts per sample). Spinach was the most favorable environment for pathogen's transfer from knives, resulting in a transfer population of 4.5-5 log CFU/g per cut for 30 cuts (200 g in total). For lettuce, there was a difference between the cutting method, since for V cutting the transfer rate of population was 3.7-4.7 log CFU/g per cut, while for S at 27-30 cuttings this population reduced to 2 log CFU/g. The lowest transfer was observed in cabbage leaves. Specifically, transfer rate was increased from 2 to 4 log CFU/g per cut and decreased from 3 to 0 log CFU/g per cut during V and S cutting, respectively. After 12 cuts, the levels of population transferred on samples were reduced to undetectable levels, with sporadic re-occurrence (1.7 log CFU/g) at 19-21 cuts. As the number of leaves increased (1-9), the transferred population of E. coli O157:H7 was constant for spinach (4.8-5.4 log CFU/g) and lettuce (4 log CFU/g), while an increasing rate (1-3.8 log CFU/g per cut) was observed in cabbage. The contamination of a sterilized knife by inoculated leaf (20 g, 6 log CFU/g) was capable of transferring the contamination to 600 g (210 cuttings) of fresh lettuce. The transferred population gradually reduced from 4 to 2 log CFU/g, during 140 cuttings (400 g). Results of the study revealed the significant impact of the type of fresh produce on the intensity of cross-contamination by E. coli O157:H7, with the greatest overall transfer generally seen on spinach. Once a contaminated sample enters a processing line, it can lead to an important cross-contamination of fresh-cut leafy greens. Further investigation should be performed to determine the factors contributing to cross-contamination, in order to ensure the safety of fresh-cut salads.