Examination of the internalization of the human pathogenic bacteria, Escherichia coli O157:H7 and Salmonella s.v. Typhimurium, in plants

Amanda J Deering, Purdue University

Abstract

The number of outbreaks that have occurred involving human pathogenic bacteria and fresh produce has dramatically increased in recent years. One factor that has contributed to this increase is the ability of the bacteria to be internalized within plant tissue. This protects the bacteria from surface sanitizing treatments that are performed during processing to make fresh produce safe for consumers. In this work, the process of internalization was examined in mung bean (Vigna radiata) following contamination with E. coli O157:H7 and peanut (Arachis hypogaea) after seed contamination with Salmonella s.v. Typhimurium using immunocytochemical techniques. The cells and corresponding tissue types that the internalized bacteria occupied within the plant were determined. Plants were subjected to a variety of growth conditions that are known to influence the number of internalized bacteria within the plant, such as wounding, to determine if differences in the localization patterns were observed. The treatments did influence the number of internalized bacteria within the plant, but regardless of treatment, bacteria were localized to every major tissue (epidermis, cortex, vascular, and pith) and corresponding cell type. Differences were also observed in the cellular localization patterns between the bacteria. E. coli O157:H7 localized primarily to the spaces between the cells (apoplast), while S. Typhimurium was observed in both the apoplast and the spaces within the cells (symplast) suggesting the modes of internalization within the plant may be different for each bacterial species. The internalization of the bacteria was also examined in various cultivars of tomato plants by determining if bacteria were able to migrate to the ripe fruit following various contamination scenarios. E. coli O157:H7 was found in the ripe fruit of seed and leaf contaminated plants, but not from plants that were soil contaminated with the bacteria. Conversely, S. Typhimurium was not detected in any of the ripe tomatoes sampled indicating that the bacterium is not able to be internalized within the cultivars tested. This suggests tomato plants that were seed or leaf contaminated with E. coli O157:H7 could harbor internalized bacteria in the mature fruit and therefore could be a possible food safety risk to consumers. Together these studies demonstrate the ability of E. coli O157:H7 and S. Typhimurium to be internalized within plants, which may contribute to the increase of food related illness associated with fresh produce.

Degree

Ph.D.

Advisors

Reuhs, Purdue University.

Subject Area

Plant biology|Food Science|Microbiology

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