A new study has revealed that protozoa found on lettuce and spinach may sequester harmful food-borne pathogens, eventually contributing to their survival on produce surfaces.
Various cases of food-borne illnesses attributed to Escherichia coli O157:H7 and Salmonella enterica have raised national concerns in recent years.
According to a report by the Centres for Disease Control and Prevention, fresh produce was the most significant source of food-borne illness in 2005.
Protozoa are single-celled organisms whose main function is bacterial consumption and a are commonly found in the natural microflora of plants and several species of amoebae associated with fresh salad vegetables.
For the study, researchers cultivated protozoa Glaucoma sp., Colpoda steinii, and Acanthamoeba palestinensis as well as the soil-borne strain, Tetrahymena pyriformis from store-bought spinach and lettuce
After washing it they were allowed to graze on green fluorescent protein- or red fluorescent protein-labelled enteric pathogens including E. coli O157:H7, S. enterica, and Listeria monocytogenes.
The researchers then monitored their ability to impound the bacteria and for vesicle production, food vacuoles released by protozoa offering a means of protection to some bacteria.
The findings revealed that Glaucoma produced vesicles with all bacterial strains and Tetrahymena also displayed vesicle production, but only of E. coli O157:H7 and S. enterica.
Further studies of E. coli O157:H7 following vesicle production showed that 4 hours after the addition of spinach extract, the bacteria had multiplied and escaped the vesicles, but C. steinii did not produce any vesicles from any of the pathogens.
"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," said researchers.
The study appears in April 2008 issue of the journal Applied and Environmental Microbiology.