Interactions between Food-Borne Pathogens and Protozoa Isolated from Lettuce and Spinach

<jats:title>ABSTRACT</jats:title> <jats:p> The survival of <jats:italic>Salmonella enterica</jats:italic> was recently shown to increase when the bacteria were sequestered in expelled food vacuoles (vesicles) of <jats:italic>Tetrahymena</jats:italic> . 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 <jats:italic>S. enterica, Escherichia coli</jats:italic> O157:H7, and <jats:italic>Listeria monocytogenes. Glaucoma</jats:italic> sp., <jats:italic>Colpoda steinii</jats:italic> , and <jats:italic>Acanthamoeba palestinensis</jats:italic> were cultured from store-bought spinach and lettuce and used in our study. A strain of <jats:italic>Tetrahymena pyriformis</jats:italic> previously isolated from spinach and a soil-borne <jats:italic>Tetrahymena</jats:italic> sp. were also used. Washed protozoa were allowed to graze on green fluorescent protein- or red fluorescent protein-labeled enteric pathogens. Significant differences in interactions among the various protist-enteric pathogen combinations were observed. Vesicles were produced by <jats:italic>Glaucoma</jats:italic> with all of the bacterial strains, although <jats:italic>L. monocytogenes</jats:italic> resulted in the smallest number per ciliate. Vesicle production was observed also during grazing of <jats:italic>Tetrahymena</jats:italic> on <jats:italic>E. coli</jats:italic> O157:H7 and <jats:italic>S. enterica</jats:italic> but not during grazing on <jats:italic>L. monocytogenes</jats:italic> , in vitro and on leaves. All vesicles contained intact fluorescing bacteria. In contrast, <jats:italic>C. steinii</jats:italic> 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 <jats:italic>E. coli</jats:italic> 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. </jats:p>