Escherichia coli Contamination of Lettuce Grown in Soils Amended with Animal Slurry

Abstract

A pilot study was conducted to assess the transfer of Escherichia coli from animal slurry fertilizer to lettuce, with E. coli serving as an indicator of fecal contamination and as an indicator for potential bacterial enteric pathogens. Animal slurry was applied as fertilizer to three Danish agricultural fields prior to the planting of lettuce seedlings. At harvest, leaves (25 g) of 10 lettuce heads were pooled into one sample unit (n = 147). Soil samples (100 g) were collected from one field before slurry application and four times during the growth period (n = 75). E. coli was enumerated in slurry, soil, and lettuce on 3M Petrifilm Select E. coli Count Plates containing 16 mg/liter streptomycin, 16 mg/liter ampicillin, or no antimicrobial agent. Selected E. coli isolates (n = 83) originating from the slurry, soil, and lettuce were genotyped by pulsed-field gel electrophoresis (PFGE) to determine the similarity of isolates. The slurry applied to the fields contained 3.0 to 4.5 log CFU/g E. coli. E. coli was found in 36 to 54% of the lettuce samples, streptomycin-resistant E. coli was found in 10.0 to 18.0% of the lettuce samples, and ampicillin-resistant E. coli in 0 to 2.0% of the lettuce samples (the detection limit was 1 log CFU/g). The concentration of E. coli exceeded 2 log CFU/g in 19.0% of the lettuce samples. No E. coli was detected in the soil before the slurry was applied, but after, E. coli was present until the last sampling day (harvest), when 10 of 15 soil samples contained E. coli. A relatively higher frequency of E. coli in lettuce compared with the soil samples at harvest suggests environmental sources of fecal contamination, e.g., wildlife. The higher frequency was supported by the finding of 21 different PFGE types among the E. coli isolates, with only a few common PFGE types between slurry, soil, and lettuce. The frequent finding of fecal-contaminated lettuce indicates that human pathogens such as Salmonella and Campylobacter can be present and represent food safety hazards.

Multifactorial Effects of Ambient Temperature, Precipitation, Farm Management, and Environmental Factors Determine the Level of Generic Escherichia coli Contamination on Preharvested Spinach

“…One soil drag sample was positive on the same farm for Salmonella spp. These results support that detection of generic E. coli in spinach at high levels could be considered an indicator of a recent exposure to risk factors for Salmonella and potentially other foodborne pathogens (46), as it has been envisioned by the recent initiative of the EFSA to use E. coli as a hygiene criterion in the production of leafy greens (8).…”

New horizons in microbiological food safety: Photodynamic Decontamination based on a curcumin derivative

“…The contamination of plant food such as lettuce with E. coli has been shown to happen by the transfer from slurry fertilizer, with E. coli serving as an indicator of fecal contamination and potential bacterial enteric pathogens. 26 Coliform contaminations in fertilized soil generally persist for more than 100 days after the application of manure with a decrease of ∼1 log 10 per month from the initial levels of about 4 log 10 CFU g −1 . Photodynamic Decontamination based on a cationic derivate of curcumin induces an antibacterial effect (3 log 10 criterion) at concentrations of the photoactive compound of 10 µM or higher on samples with a "flat" geometry, i.e.…”

Fresh-Cut Produce Quality

“…The survival of food-borne pathogens is a potential threat to humans, and is far more important than any other quality aspect. Jensen et al (2013) reported the transfer of E. coli from animal slurry fertilizer to lettuce. This occurred in a pilot study in which animal slurry was applied as fertilizer in three Danish agricultural fields, prior to the planting of lettuce seedlings, with E. coli serving as an indicator of fecal contamination and of potential bacterial enteric pathogens.…”