A study of U.S. orchards to identify potential sources of Escherichia coli O157:H7

The association of unpasteurized apple cider with Escherichia coli O157:H7 foodborne illness has led to increased interest in potential reservoirs of this pathogen in the orchard. Fourteen U.S. orchards were surveyed in autumn 1999 to determine the incidence and prevalence of E. coli O157:H7, E. coli, total aerobic microflora, and yeasts and molds. Fruit samples (n = 63) (eight apple and two pear varieties) and soil, water, and fecal samples were collected. Samples were plated on (i) tryptic soy agar for total mesophilic aerobic count, (ii) E. coli and coliform Petrifilm for total coliforms and E. coli, and (iii) yeast and mold Petrifilm. Samples positive for coliforms and E. coli were enriched and tested for E. coli O157:H7. Fruit was also tested for internalization of microflora by aseptically removing the core, stem, and calyx areas, and the individual sections were assessed for the categories of microflora listed above. E. coli was detected in soil and water and in 6% of fruit samples (three pear samples and one apple sample), generally collected from areas previously designated as high risk in this study. However, no E. coli O157:H7 was found. Coliforms were found in 74% of fruit samples and were internalized in the cores of 40% of fruit tested. Yeasts and molds were internalized in 96.7% of samples and aerobic bacteria in 89.6%. E. coli was not found to be internalized. Total aerobic counts and total coliforms were higher in dropped and damaged fruit (P < 0.05). Findings suggest that dropped or damaged fruit should not be included in fruit designated for the production of unpasteurized juice or for the fresh or fresh-cut market. In addition, orchards should be located away from potential sources of contamination, such as pastures.