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Title: Persistence of enterohemorrhagic and non-pathogenic E. coli on spinach leaves and in rhizosphere soil

Author
item Patel, Jitu
item Millner, Patricia
item Nou, Xiangwu
item Sharma, Manan

Submitted to: Journal of Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2009
Publication Date: 5/1/2010
Citation: Patel, J.R., Millner, P.D., Nou, X., Sharma, M. 2010. Persistence of enterohemorrhagic and non-pathogenic E. coli on spinach leaves and in rhizosphere soil. Journal of Applied Microbiology. 108:1789-1796.

Interpretive Summary: The number of foodborne illness outbreaks linked to fresh produce and reported to the United States Centers for Disease Control and Prevention (CDC) has increased in the last years. Microbial contamination of produce may occur anytime during the production, harvesting, and processing/packing. Since there is no “kill-step” during packing of raw produce, identifying the source of pre-harvest contamination of fresh produce is necessary to control foodborne illnesses. We compared survival of E. coli O157:H7 and non-pathogenic E. coli on spinach plants and in organic soil in a growth chamber. Five separate inocula, each containing one strain of E. coli O157:H7 and one non-pathogenic E. coli isolate (from plants or soil) were used to contaminate growing spinach leaves and soil. Four replicates of each plant shoot and soil sample per inoculum were analyzed on day 0 and weekly for 28 days for E. coli O157:H7 and non-pathogenic E. coli (by MPN), and for heterotrophic plate counts (HPC). E. coli O157:H7 was not detected on plant shoots after 7 days but did survive in soil for up to 28 days. Non-pathogenic E. coli survived up to 14 days on shoots, and at low levels for up to 28 days. HPC in soil at day 7 were significantly greater than those at day 0, but populations on day 0 and day 28 in soil and on plants were similar. This information will allow for improved risk assessments from produce contaminated in the field.

Technical Abstract: Foodborne illness outbreaks associated with leafy greens have raised concerns about the persistence of Escherichia coli O157:H7 on fresh produce and in the cropping environment. The set of characteristics that enable the enteric bacterium E. coli O157:H7 to survive on undamaged spinach leaves, roots and in soils remain to be elucidated. We compared survival of E. coli O157:H7 and non-pathogenic E. coli on spinach plants and in organic soil in a growth chamber. Five separate inocula, each containing one strain of E. coli O157:H7 and one non-pathogenic E. coli isolate (from plants or soil) were grown in filter-sterilized water extracts of dairy manure solids to condition inocula to environmental nutrient sources and simulate contaminated water. Spinach cultivar ‘Whale’ was grown in sandy loam soil (3 percent organic matter). Leaf and soil inocula consisted of: 100uL in 5.0 microliter droplets on the upper side of leaves and 1.0mL in soil used to inoculate individual 4-week old plants with 6.5 log CFU/plant and 7.5 log CFU/200 g soil per plant. Four replicates of each plant shoot and soil sample per inoculum were analyzed on day 0 and weekly for 28 days for E. coli O157:H7 and non-pathogenic E. coli (by MPN), and for heterotrophic plate counts (HPC). E. coli O157:H7 was not detected on plant shoots after 7 days but did survive in soil for up to 28 days. Non-pathogenic E. coli survived up to 14 days on shoots, and at low levels for up to 28 days. HPC in soil at day 7 were significantly greater than those at day 0, but populations on day 0 and day 28 in soil and on plants were similar.