LE3 .A278

2011

Bell, Colin

Acadia University

Bachelor of Science

Honours

Biology

Biology

Increasingly, fresh and minimally processed vegetables and fruit are being implicated as vectors for gastrointestinal disease in human consumers. There are various points on the field to fork continuum at which bacterial contamination can occur, many of which currently are under intensive study. However, little is known of the behaviour of bacterial pathogens once contact is made with plant tissue. The survival and migration dynamics of two toxin-negative variants of E. coli 0157:H7 (Strains NCTC 12900 & DM4) on packaged retail-level baby spinach was examined in order to expand this information base. Changes in the counts of E. coli deliberately introduced to the upper surface of individual leaves were monitored during their subsequent storage at 6C in the dark. In addition, the capacity of contaminated leaves to serve as a source of contamination (donor of E. coli) to other spinach leaves held in close proximity with them was assessed. By varying the moisture in the containers as well as the strain of E. coli, the effects of both ambient moisture (Rh) and bacterial flagella on survival and cross contamination rates were examined. Both E. coli strains survived in a culturable and transferable condition on the donor leaves for up to 12 days, regardless of moisture content or the presence of a defective flagella-coding gene. Transfer to "clean" spinach leaves did occur in mixing containers but at low migration rates (<1.0% migration when compared to initial inoculum of donor). Newly acquired E. coli from donor leaves declined in number during the first three days post-mixing but they tended to persist at low levels. The total aerobic counts collected from the same leaf wash liquid as the E coli. revealed some interesting trends. As the E. coli counts began to drop throughout the trial, the numbers of total aerobes usually increased. This indicates that the decline in numbers observed of the E. coli are not indicative of what is occurring to all bacteria on the leaf. E. coli survives in a culturable form on both directly and cross-contaminated spinach leaves under conditions simulating retail storage for up to 12 days post inoculation and is able to move to adjacent non-contaminated leaves for all of this period.

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https://scholar.acadiau.ca/islandora/object/theses:800