Fate of Escherichia coli O157:H7 in Manure Compost–Amended Soil and on Carrots and Onions Grown in an Environmentally Controlled Growth Chamber
Studies were done to determine the fate of Escherichia coli O157:H7 in manure compost–amended soil and on carrots and green onions grown in an environmentally controlled growth chamber. Commercial dairy cattle manure compost was inoculated with a five-strain mixture of green fluorescent protein–labeled E. coli O157:H7 at 107 CFU g−1 and mixed with unsterilized Tifton sandy loam soil at a ratio of 1:5. Baby carrot or green onion seedlings were planted into the manure compost–amended soil in pots, and soil samples surrounding the plant, edible carrot roots and onion bulb samples, and soil immediately beneath the roots were assayed for E. coli O157:H7 in triplicate at weekly intervals for the first 4 weeks, and every 2 weeks for the remainder of the plant growth cycle (up to 3 months). E. coli O157:H7 cell numbers decreased within 64 days by 3 log CFU/g in soil and soil beneath the roots of green onions and by more than 2 log CFU/g on onions. E. coli O157:H7 survived better during the production of carrots, with a 2.3-log CFU/g reduction in soil and a 1.7-log CFU/g reduction on carrots within 84 days. These results indicate that the type of plant grown is an important factor influencing the survival of E. coli O157:H7 both on the vegetable and in the soil in which the vegetable is grown.