Date of Award

2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Goktepe, Ipek

Abstract

The 2006 multi-state Escherichia coli O157:H7 (EHEC) outbreak linked to bagged spinach raised concerns about the safety of fresh produce. One novel approach to control foodborne pathogens is lytic bacteriophages, which does not affect the produce microflora. The objectives of this study were; 1) to test and verify the efficacy of a phage cocktail against EHEC in a laboratory medium, 2) to determine the effectiveness of the phage cocktail on fresh-cut leafy greens, and 3) to test and compare the effectiveness of the phage cocktail against EHEC on fresh-cut green leafy vegetables stored under air and modified air (low O2/high CO2) conditions. The efficacy of the phage cocktail was determined against a nalidixic acid resistant (NalR) EHEC strain in Tryptic Soy Broth (TSB). Next, pieces (~2x2 cm2) of leafy greens inoculated with 4.50 log EHEC NalR/cm2 and air-dried were sprayed with 6.50 log phage cocktail/cm2. Samples were stored at 4 or 10°C for 15 days under atmospheric or modified (5% O2/35% CO2/60% N2) air. The recovered EHECs were enumerated on MacConkey agar supplemented with 25 μg/ml Nal. Phage treatment resulted in 6 log reduction (P<0.05) of EHEC NalR in TSB. At 4°C under air, the phage cocktail significantly (P<0.05) lowered the EHEC NalR counts in one day by 1.19, 3.21, and 3.25 log units on spinach, green leaf, and romaine lettuce, respectively. When stored under modified atmosphere, the reductions in EHEC NalR concentrations were 2.18, 3.50, and 3.13 logs. At 10°C, EHEC reductions under air were 1.99, 3.90, and 3.99 logs, and under modified air were 3.08, 3.89, and 4.34 logs on spinach, green leaf, and romaine lettuce, respectively. The results of this study suggest that bacteriophages may be used successfully for controlling/reducing pathogenic bacterial presence and/or growth on fresh produce stored under modified atmosphere packaging.

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