Effects of Plant Maturity and Growth Media Bacterial Inoculum Level on the Surface Contamination and Internalization of Escherichia coli O157:H7 in Growing Spinach Leaves

Authors: Pu, Shuaihua¹; Beaulieu, John C.²; Prinyawiwatkul, Witoon¹; Ge, Beilei³

Source: Journal of Food Protection®, Volume 72, Number 11, November 2009 , pp. 2313-2320(8)

Publisher: International Association for Food Protection

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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 prepackaged baby spinach. This 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-in. (20.32-cm) pots. After seed germination, two levels (10³ and 10⁷ CFU/ml) of an E. coli O157:H7 green fluorescent protein-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 a positive result. Surface leaf contamination occurred occasionally and clustered between 3 and 5 weeks of age, but not among leaves younger than 3 weeks of age. On the other hand, when inoculated at the 10⁷ CFU/ml level, the E. coli O157:H7 green fluorescent protein 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.

Document Type: Research article

Affiliations: 1: Department of Food Science, 111 Food Science Building, Louisiana State University Agricultural Center, Baton Rouge, Louisiana 70803 2: Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, New Orleans, Louisiana 70124, USA 3: Department of Food Science, 111 Food Science Building, Louisiana State University Agricultural Center, Baton Rouge, Louisiana 70803;, Email: bge@lsu.edu