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United States Department of Agriculture

Agricultural Research Service

Research Project: IMPROVING THE SENSORY QUALITY AND SHELF LIFE OF FRESH-CUT FRUIT PRODUCTS Title: Effects of plant maturity and bacterial inoculum level on the colonization and internalization of escherichia coli 0157:H7 in growing spinach leaves.

Authors
item Pu, Shuaihua -
item Beaulieu, John
item Prinyawiwatkul, Witoon -
item Ge, Belei -

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 26, 2009
Publication Date: November 12, 2009
Citation: Pu, S., Beaulieu, J.C., Prinyawiwatkul, W., Ge, B. 2009. Effects of plant maturity and bacterial inoculum level on the colonization and internalization of escherichia coli 0157:H7 in growing spinach leaves. Journal of Food Protection. 72:2313-2320.

Interpretive Summary: Contaminated produce eaten raw has been increasingly implicated in foodborne outbreaks in the United States. In September 2006, tainted pre-packaged baby spinach triggered an Escherichia coli O157:H7 outbreak which resulted in 205 confirmed illnesses, and 3 deaths in 26 states, as well as, an estimated $37-74 million loss to the California produce industry. Once harvested, fresh produce undergoes minimum processing (no lethal kill step), mainly, cleaning and sanitizing using chlorine or alternative sanitizers. Post-harvest sanitizer wash is intended to reduce microorganisms on the produce surface, but will not be effective if microbes are inside the tissues. There is currently contradiction regarding if internalization of E. coli O157:H7 in growing lettuce or spinach occurs. It remains controversial whether E. coli O157:H7 is capable of contaminating the edible part of a mature plant when introduced through soil or irrigation water. The main objective of this study was to determine the effects of plan maturity and bacterial inoculum level on the colonization and contamination of E. coli O157:H7 in the aerial leaf tissue of growing spinach plants in a greenhouse setting. Spinach seeds of a commercial variety were sown in 8-inch pots. After seed germination, two levels (103 and 107 CFU) of an E. coli O157:H7 green fluorescent protein (GFP)-expressing strain were introduced into the plant growth media weekly for a total of five times. Inoculated spinach plants were examined weekly for the presence of E. coli O157:H7 on leaves, and in surrounding growth media. Among 120 spinach plant samples examined for internal leaf contamination, only one yielded positive result. On the other hand, when inoculated at the 107 CFU level, the E. coli O157:H7 GFP strain survived the entire cultivation period in the soil, although, with gradually reduced levels. The experiments demonstrated that internalization of E. coli O157:H7 of growing spinach plant leaves under greenhouse conditions was a rare event, but surface contamination did occur, primarily when the plants reached 3 weeks of age. The study provided important data to further assess the association between spinach age and potential contamination of E. coli O157:H7.

Technical Abstract: The incidence of foodborne outbreaks linked to fresh produce has increased in the United States. Particularly noteworthy, was the 2006 Escherichia coli O157:H7 outbreak associated with pre-packaged baby spinach. The study aimed to determine whether E. coli O157:H7 would be present in the aerial leaf tissue of a growing spinach plant when introduced at various plant maturities and different inoculum levels in a greenhouse setting. Spinach seeds of a commercial variety were sown in 8-inch pots. After seed germination, two levels (103 and 107 CFU) of an E. coli O157:H7 green fluorescent protein (GFP)-expressing strain were introduced into the plant growth media weekly for a total of five times. Inoculated spinach plants were examined weekly for the presence of E. coli O157:H7 on leaves and in surrounding growth media. Among 120 spinach plant samples examined for internal leaf contamination, only one yielded positive result. Surface leaf contamination occurred occasionally, and clustered between 3 to 5 weeks of age, but not among leaves younger than 3 weeks of age. On the other hand, when inoculated at the 107 CFU level, the E. coli O157:H7 GFP strain survived the entire cultivation period, although, with gradually reduced levels. The experiments demonstrated that internalization of E. coli O157:H7 of growing spinach plant leaves under greenhouse conditions was a rare event, but surface contamination did occur, primarily when the plants reached 3 weeks of age. The study provided important data to further assess the association between spinach age and potential contamination of E. coli O157:H7.

Last Modified: 10/30/2014
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