title: 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.
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 (10(3) and 10(7) 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 10(7) 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.
authors.affiliation: Department of Food Science, Louisiana State University Agricultural Center, Baton Rouge, Louisiana 70803, USA.
authors.forename: Shuaihua
authors.initials: S
authors.lastname: Pu
authors.forename: John C
authors.initials: JC
authors.lastname: Beaulieu
authors.forename: Witoon
authors.initials: W
authors.lastname: Prinyawiwatkul
authors.forename: Beilei
authors.initials: B
authors.lastname: Ge
mesh_terms: D015169:Colony Count, Microbial; D003257:Consumer Product Safety; D003470:Culture Media / Q000737:chemistry; D004196:Disease Outbreaks / Q000517:prevention & control; D019453:Escherichia coli O157 / Q000254:growth & development / Q000502:physiology*; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D049452:Green Fluorescent Proteins; D006801:Humans; D018515:Plant Leaves / Q000382:microbiology; D036226:Seedlings / Q000382:microbiology; D018724:Spinacia oleracea / Q000254:growth & development / Q000382:microbiology*
chemical_list: D003470:Culture Media; D049452:Green Fluorescent Proteins