MICROBIAL ECOLOGY AND SAFETY OF FRESH ON-FARM ORGANICALLY GROWN PRODUCE
Title: The effect of total organic carbon content and repeated irrigation on the persistence of E. coli O157:H7 on baby spinach
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: March 9, 2010
Publication Date: August 1, 2010
Citation: Ingram, D.T., Mudd, C.L., Ferguson, S.E., Hoover, D., Kniel, K.E., Sharma, M. 2010. The effect of total organic carbon content and repeated irrigation on the persistence of E. coli O157:H7 on baby spinach. [abstract]. IAFP Abstracat Book p. 41.
Introduction: Contaminated fresh-cut spinach and other leafy greens have caused foodborne illness in the United States. In response, growers are adopting recommendations stated in the California Leafy Greens Marketing Agreement (LGMA). The LGMA permits a maximum population of 126 Most Probable Number (MPN) generic E. coli per 100 ml irrigation water which directly contacts foliar surfaces. These recommendations do not involve other measurements of water quality (e.g. organic carbon content) which might enhance the survival of bacterial pathogens on foliar surfaces.
Purpose: We investigated the effect of repeated irrigation with water containing differing levels of total organic carbon (TOC) on the foliar survival of enterohemorrhagic E. coli O157:H7 (EHEC) on baby spinach plants.
Methods: Sterile, diluted bovine manure was prepared to contain TOC concentrations of 0ppm, 14ppm and 140ppm. An inoculum of three nalidixic acid-resistant EHEC strains, were introduced into each irrigation solution at either a high (5-6 log CFU/100 ml) or low (0-1 log CFU/100 ml) inoculum, and sprayed on to plants using a fine mist air-brush. Baby spinach plants were irrigated twice a week with each uninoculated TOC treatment. A 3-tube MPN was used to determine the persistence of EHEC on the aerial tissues, which were harvested daily for microbial analysis.
Results: A 3-log reduction was observed within the first 24 h on plants irrigated with high EHEC populations, regardless of TOC level. EHEC exhibited a low level of persistence for up to 16 days regardless of TOC level. The level of persistence ranged from 76.4 MPN / plant (day 1) to 0.40 MPN / plant (day 16). For inoculation events with low EHEC populations, no viable EHEC were detected after day 0. The TOC in irrigation solutions did not affect the persistence of EHEC on baby spinach tissues.
Significance: This study suggests that EHEC in irrigation water which complies with LGMA guidelines will not survive for more than 24 h on foliar surfaces. TOC content of irrigation water did not influence the survival of EHEC on foliar surfaces of spinach plants.