Protozoa Contain Harmful Foodborne Pathogens

Commonly found in the microflora of plants

Protozoa on lettuce and spinach may contain hidden, harmful foodborne pathogens that aid in their survival on produce surfaces, a new study reports.
Protozoa, single-celled organisms whose main function is bacterial consumption, are commonly found in the microflora of plants, and several species of amoebae have been associated with salad vegetables, according to researchers at Tennessee Technological University (TTU) at Cookeville.

Most protozoa normally feed on bacteria and can control their populations and activities, as well as their potential virulence. -Sharon Berk, PhD, Tennessee Technological University at Cookeville.

Recent outbreaks of Escherichia coli O157:H7 and Salmonella enterica prompted the team to study the interaction between foodborne pathogens and protozoa (Gourabathini P, Brandl MT, Redding KS, Gunderson JH, Berk SG. Interactions between Foodborne Pathogens and Protozoa Isolated from Lettuce and Spinach. Appl Environ Microbiol . 2008;74(8):2518-2525).
     Two species of ciliated protozoa and one species of amoeba were isolated from store-bought spinach and lettuce, says study author Sharon Berk, PhD.
     "They were allowed to feed on E. coli O157:H7, Salmonella enterica , and Listeria monocytogenes that were labeled with a fluorescent red or green marker protein," Dr. Berk, a professor of biology at TTU, says. "In addition, two other ciliates, one from soil, and one from a commercial culture collection"-and originally isolated from spinach-"were also fed the red or green pathogens."
     Dr. Berk adds that in the past, research into food safety and pathogens looked at many aspects relating to the ecology of the bacteria and the environment of the bacteria and the produce, "but failed to recognize fully that protozoa, also unicellular and microscopic, are not only present in the same environment, but may interact with the bacteria because most protozoa normally feed on bacteria and can control their populations and activities, as well as their potential virulence."
     Because some protozoa can package concentrated pathogenic bacteria into pellets (vesicles) on leaves of produce, Dr. Berk says, it could affect the foodborne disease risk if the pellets help the bacteria survive, and possibly cause an underestimate of the bacterial load if colony forming units are used to assess the load. "Passage through protozoa may also increase their virulence, as has been shown for other pathogens not related to foodborne illnesses," she says.
     Also participating in the study were researchers at the USDA's Produce Safety and Microbiology Research Unit in Albany, Calif.