A longitudinal study of microbiological quality of pre-harvest produce: Comparison between organic and conventional farms in Minnesota and Wisconsin

Microbiological analyses of fruits and vegetables produced by farms in Minnesota and Wisconsin were conducted to determine coliform and Escherichia coli counts, and the presence of Salmonella and E. coli O157:H7. In 2003 and 2004, fourteen organic (certified by accredited organic agencies), 30 semi-organic (used organic practices but not certified) and 19 conventional farms were sampled during the harvest season to collect and analyze 2,029 pre-harvest produce samples (473 organic, 911 semi-organic, 645 conventional). The E. coli prevalence was linked to certain farm management practices, collected through farmers' surveys at the beginning of each of the two seasons. The effect of the harvest months on E. coli prevalence was determined using the generalized estimated equation model of logistic regression. The E. coli isolates were identified using pulsed filed gel electrophoresis, and the clonal diversity among isolates obtained from the entire testing was determined. None of the produce samples had Salmonella or E. coli O157:H7 contamination in the two years of this study. E. coli contamination was detected in 8% of the samples. Leafy greens, lettuces and cabbages had significantly greater E. coli prevalence compared to all the other produce types in both years for the three farm types. Using animal manure for fertilization of produce increased the risk of fecal contamination significantly in both organic and semi-organic produce. Organic farms that aged their animal manure for less than 6 months had 4- times greater risk of fecal contamination in their produce, compared to those that used older cattle manure. The risk of contamination with E. coli was significantly greater in June and July compared to August and September, irrespective of year of sampling, produce types and farm types. A wide diversity was observed among the E. coli isolates from the produce collected from different farms, and among the isolates obtained from the same farm.