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Title: The effect of repeated irrigation with varying total organic carbon content on the persistence of E. coli O157:H7 on baby spinach

Author
item Ingram, David
item East, Cheryl - Roberts
item Ferguson, Sean
item Sharma, Manan

Submitted to: BARC Poster Day
Publication Type: Abstract Only
Publication Acceptance Date: 3/15/2010
Publication Date: 4/21/2010
Citation: Ingram, D.T., Mudd, C.L., Ferguson, S.E., Sharma, M. 2010. The effect of repeated irrigation with varying total organic carbon content on the persistence of E. coli O157:H7 on baby spinach. BARC Poster Day Abstract Book. p. 21.

Interpretive Summary:

Technical Abstract: In response to U.S. foodborne illnesses caused by contaminated spinach, growers have adopted regulations stated in the California Leafy Greens Marketing Agreement (LGMA). The LGMA permits a maximum population mean of 126 Most Probable Number (MPN) generic E. coli per 100 ml irrigation water. These metrics, however, do not include other measurements of water quality that may influence the epiphytic survival of bacterial pathogens. We investigated the effect of repeated irrigation of baby spinach plants with water containing differing levels of total organic carbon (TOC) on the epiphytic survival of enterohemorrhagic E. coli O157:H7 (EHEC). Sterile, diluted bovine manure was prepared to contain TOC concentrations of 0ppm, 12-15ppm and 120-150ppm. An inoculum containing three nalidixic acid-resistant EHEC strains was introduced into each irrigation solution at either a high (5-6 log CFU/100 ml) or low (0-1 log CFU/100 ml) population, and sprayed twice a week onto baby spinach plants using a fine mist air-brush. The persistence of EHEC on the aerial tissues, harvested daily, was determined using a 3-tube MPN. Plants irrigated with high EHEC populations, regardless of TOC levels, showed a 3-log reduction within the first 24 h. EHEC then exhibited low levels of persistence for up to 16 days on all treatments, ranging from 76.4 MPN per plant (day 1) to 0.40 MPN per plant (day 16). For the irrigation events containing low EHEC populations, no viable EHEC were detected after 24 h. The TOC content of irrigation water did not affect the persistence of EHEC on baby spinach tissues. This study suggests that EHEC populations in irrigation water which comply with LGMA guidelines will not survive for more than 24 h on foliar surfaces of spinach plants. The TOC content of irrigation water did not influence EHEC survival.