Interactions Between Foodborne Pathogens and Protozoa Isolated from Lettuce and Spinach

Center for the Management, Utilization and Protection of Water Resources Tennessee Technological University, Cookeville, TN 38505; USDA-ARS, Produce Safety and Microbiology Research Unit, 800 Buchanan St., Albany, CA 94710

	Abstract

The survival of Salmonella enterica was recently shown to increase when the bacteria were sequestered in expelled food vacuoles (vesicles) of Tetrahymena. Because fresh produce is increasingly linked to outbreaks of enteric illness, the present investigation aimed to determine the prevalence of protozoa on spinach and lettuce, and to examine their interactions with S. enterica, E.coli O157:H7, and Listeria monocytogenes. Glaucoma sp., Colpoda steinii, and Acanthamoeba palestinensis were cultured from store-bought spinach and lettuce and used in our study. A strain of Tetrahymena pyriformis previously isolated from spinach, and a soilborne Tetrahymena sp. were also used. Washed protozoa were allowed to graze on GFP- or DsRed-labeled enteric pathogens. Significant differences in interactions among the various protist-enteric pathogen combinations were observed. Vesicles were produced by Glaucoma with all bacterial strains, although L. monocytogenes resulted in the smallest number per ciliate. Vesicle production was observed also during grazing of Tetrahymena on E. coli O157:H7 and S. enterica, but not with L. monocytogenes, in vitro and on leaves. All vesicles contained intact fluorescing bacteria. In contrast, Colpoda steinii and the amoeba did not produce vesicles from any of the enteric pathogens, nor were pathogens trapped within their cysts. Studies of the fate of E. coli O157:H7 in expelled vesicles revealed that by 4 h after addition of spinach extract, the bacteria multiplied and escaped the vesicles. The presence of protozoa on leafy vegetables and their sequestration of enteric bacteria in vesicles indicate that they may play an important role in the ecology of human pathogens on produce.