Submitted to: International Association for Food Protection
Publication Type: Abstract Only
Publication Acceptance Date: March 10, 2009
Publication Date: July 13, 2009
Citation: Ingram, D.T., Mudd, C.L., Ferguson, S.E., Sharma, M., Kniel, K. 2009. Survival of enterohemorrhagic and avian pathogenic Escherichia coli from spinach plants after overhead irrigation with (currently acceptable) contamination levels [abstract]. International Association for Food Protection 2009 Abstract Book. Paper No. P3-09. p. 151.
Introduction: In response to the growing food safety issues surrounding fresh cut lettuce and other leafy green commodities in the U.S., the Leafy Green Marketing Agreement (LGMA) proposes new commodity-specific guidelines that encompass all steps on the production-to-distribution continuum to ensure the safe on-farm production, processing and distribution of these commodities. Many of the proposed guidelines are based on metrics from current scientific literature, while some defer to standards not intended for agricultural use. For example, the current guidelines for E. coli levels in irrigation water was set by the EPA for recreational water use at less than 126 MPN/100ml (rolling geometric mean n=5, and any single sample must be less than 235 MPN/100ml).
Purpose: This study examined the irrigation water standards stated in LGMA.
Methods: Avian Pathogenic (APEC) and Enterohemorrhagic E. coli O157:H7 (EHEC) isolates, resistant to nalidixic acid, were adapted for growth in bovine manure and used to prepare two irrigation solutions in sterile water. Three irrigation solutions (APEC: 156 cfu/100ml; EHEC: 77 cfu/100ml; water control) were sprayed for 25 seconds each (1.5ml delivered) onto individual mature spinach plants using an air-brush to simulate overhead irrigation. Plant tissue was harvested daily for 3 days and enrichment techniques and real-time PCR were used to detect virulence factors of EHEC (stx2) and APEC (espA).
Results: Real-time PCR results suggest that very low levels of EHEC were, in fact, delivered to the surfaces of the spinach plants. No enrichments were able to recover viable E. coli cells.
Significance: Low levels of E. coli in irrigation water can be recovered from spinach plants by real time PCR methods. It is unclear how long E. coli cells are viable on spinach leaves. This study provides preliminary data to support the current LGMA guidelines for acceptable E. coli levels in irrigation water that contacts edible portions of the crop.