Effect of Repeated Irrigation with Water Containing Varying Levels of Total Organic Carbon on the Persistence of Escherichia coli O157:H7 on Baby Spinach

What is it about?

Outbreaks of Escherichia coli O157:H7 on leafy greens have been associated with irrigation water. E. coli is widely used as an indicator for the presence of bacterial foodborne pathogens in the Leafy Greens Marketing Agreement (LGMA), being implemented by numerous leafy green handlers. The LGMA state that irrigation water for leafy greens contain a maximum E. coli population of 126 Most Probable Number (MPN) per 100 ml irrigation water. This study was conducted to determine the fate of the foodborne pathogen E. coli O157:H7 on baby spinach leaves after repeated introduction at low levels through irrigation water containing different concentrations of total organic carbon (TOC), another water quality indicator. Baby spinach plants were repeatedly irrigated by overhead spray with water containing very high (5 – 6 log CFU / 100 ml) or low populations of E. coli O157:H7 (0 -1 log CFU / 100 ml) in a growth chamber. Plants irrigated with high E. coli O157:H7 populations, regardless of TOC levels, showed a 3-log reduction within the first 24 h. Low levels of E. coli O157:H7 persistence were observed for up to 16 days on all TOC treatments, ranging from 76.4 MPN per plant (day 1) to 0.40 MPN per plant (day 16). No viable cells were detected on spinach tissue 24 h after irrigation with water containing < 126 CFU / 100 ml E. coli O157:H7. The fate of pathogens on foliar surfaces should be considered when evaluating irrigation water quality.

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