Author: Dan Shelton, USDA Agricultural Research Service Scientist, sheltond@ba.ars.usda.gov There is no information indicating that separate analyses were conducted of the internal tissues versus the external surfaces of contaminated spinach leaves. Consequently, reports suggesting that E. coli O157:H7 cells were taken up through the spinach roots and sequestered within the leaves are speculative. Laboratory studies have clearly demonstrated that E. coli O157:H7 cells can colonize the vascular tissue of lettuce plants and remain viable for some period of time (although how long is unclear). However, the likelihood of uptake of human pathogens by produce plants and subsequent colonization of internal leaf tissues under actual growing conditions remains very controversial. Although some studies suggest that seedling roots are vulnerable to internal colonization, most research suggests that the roots of mature produce plants are not readily colonized by human pathogens. Unlike plant pathogens, E. coli O157:H7 and related bacteria have no mechanisms for independently invading plant tissues. However, disruptions of the plant surface, due to mechanical injury, insect damage, or other pests, may provide an entry point for internal colonization. For example, internal contamination of spinach leaves is theoretically feasible, due to the nature of spinach cultivation. Spinach leaves are harvested using a band saw-like blade that spans the width of the spinach bed. If the blade were to inadvertently contact fecal material in/on the soil surface, fecal-borne pathogens could be transferred to the cut ends of both the harvested leaf and intact stem. Since spinach plants continue to grow and are re-harvested, internal contamination of the new leaf growth could, theoretically, occur. At present, however, there is too little information to assess the actual risk associated with internal colonization of produce by E. coli O157:H7 or any other zoonotic pathogens. Assuming that the internal tissues of spinach, or other produce, do become contaminated, relatively stringent disinfection procedures are required that can penetrate the plant tissue, such as irradiation or gaseous chlorine dioxide.

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