Katherine Metcalf '98

The Presence of Escherichia coli on Fresh Produce from Mexico and America


The objectives of this project were to determine if E.coli was present on local fresh produce using simple culturing methods (Petrifim Plates), and if E.coli was present, determine whether there was a significant difference between American and Mexican produce contamination.

E.coli is present in all mammal feces, and the O157:H7 strain may cause severe sickness and death in humans. The number of E.coli illness outbreaks associated with produce is increasing in the U.S. for several possible reasons. Americans are urged to increase fresh fruit and vegetable consumption, and the per capita intake is increasing. When feces are used as fertilizer and produce is improperly washed, the risk of E.coli contamination is possible.

Foreign imports of fresh produce into the U.S. are also increasing, and it is not known if produce sanitizing methods of other countries meet U.S. standards. There are also new changes in the produce shipping and processing methods which have many opportunities for E.coli contamination. Finally, consumers often relate E.coli outbreaks with under-cooked meat, and may overlook the possibility of fruit and vegetable contamination.

A total of 104 Mexican and American fruits and vegetables were tested for E.coli using 3-M Petrifilm Plates. These are AOAC approved, sample-ready culturing plates designed specifically for general E.coli (except O157:H7) detection. Fruits and vegetables growing close to the ground and root vegetables were chosen, since they have the highest risk for fecal, soil, or irrigation water contamination. Fifty-three Mexican and 51 U.S. samples were used from four different shipments. Produce managers from two grocery stores granted permission to swab fruits/vegetables directly upon arrival, before placement out for sales. Clinically sterile swabs were used to apply culture samples to Petrifilm Plates at the store. The plates were incubated for 48 hours at 35 degrees Celsius to determine results. Since Petrifilm Plates are new, certain samples were cultured on conventional blood agar and EMB petri dishes, to Adouble check@ the Petrifilm results. Waste disposal followed proper biohazard precautions.

No E.coli was detected on any of the samples. This suggests the produce was grown, handled, processed and shipped in reasonably sanitary conditions. Unidentified organisms grew on both the blood agar and EMB petri dishes. These were presumably normal microbes present in soil, including fungi and bacteria, proving the fruits and vegetables were not sterile. There is the possibility that some of these organisms were harmful pathogens, such as Salmonella. This growth also confirmed culturing methods and techniques were effective. Therefore, if E.coli were present, it would have been detected. Proper handling and washing of fruits and vegetables is necessary to minimize bacterial contamination.

Faculty Mentor: Amy Olson, PhD, RD