Purpose: To compare bacterial persistence on lettuce and spinach grown in the mid-Atlantic region following a one-time contamination event with water contaminated with varying amounts of coliforms originating from bovine manure.
Methods: Parris Island Romaine lettuce and Melody hybrid spinach plants (n = 388) were irrigated overhead with water containing varying levels of coliforms (0, 100, 1000 and 10000 CFU/ml each in 3 plots). Samples were randomly collected at days 0, 0+ (immediately following inoculation), 1, 3, 5, 7 and 10. One sample was composed of three randomly chosen 28-day old plants for each leafy green. Total coliforms (37°C), fecal coliforms (44.5°C), and Escherichia coli were detected using bacterial enumeration on TBX media and MPN using colilert methods.
Results: Log MPN counts were significantly lower on Days 0 and 0+ compared to all other days (P < 0.0001) for both produce types. After day 7 counts were significantly higher (P < 0.0001) in relation to all other days and the highest on day 10 (log MPN/g = 6.56+/- 0.11), specifically associated with a spinach sample from 10000 CFU/ml. By day 10, the fields had received > 6 cm of water during a hurricane, which showed an increase in total coliform count but had a diminishing effect on fecal coliform count. Coliform levels from TBX, incubated at 44.5°C, indicated higher levels from days 1-5 at 20,000 CFU/g on lettuce and 1500 CFU/g on spinach associated with higher inoculation levels. E. coli levels did not differ significantly on lettuce or spinach.
Significance: These data suggest that monitoring irrigation water and the use of field trials to simulate pre-harvest contamination effects will enhance our understanding of irrigation water metrics and meteorological effects.