Abstract

The influence of organic nitrogen sources on the establishment of E. coil 0157:H7 and the occurrence of other potentially human pathogenic bacteria on baby leaf salads was evaluated. Greenhouse-grown rocket and Swiss chard were spray-inoculated with gfp-tagged E. coil 0157:H7 twice a week from when their first true leaves reached a length of 2 cm until three days before harvest. Analysis of nitrogen content in leaves revealed differences between treatments. Untreated plants had the lowest values, followed by plants fertilized with pig hair pellets and chicken manure. The same pattern was seen for the growth medium at the day of harvest. The applied strain showed similar establishment (measured using culture-dependent methods) irrespective of treatment, but Swiss chard hosted significantly more E. coli 0157:H7 than rocket. Differences in the risk of infection were found for the crops, with plants fertilized with pig hair pellets showing a slightly higher risk. No relationship was found for total nitrogen content in leaves and colonization with E. coli 0157:H7 gfp+. Isolating dominant Enterobacteriaceae from leaves, some strains showed conformity to bacterial species that have previously been identified to influence the establishment of E. coli 0157:H7 in the phyllosphere. In this study, no inhibitory effects were observed in vitro.Colonies showing matching characteristics for Listeria spp. and Salmonella spp. were detected up to 5.5 and 5 log CFU g(-1) fresh weight, respectively. However, presumptive Listeria spp. and Salmonella spp. could not be confirmed by sequencing of the 16S rRNA gene. (C) 2017 Elsevier Ltd. All rights reserved.

Keywords

Culture-dependent methods; Enterobacteriaceae; Food safety; Human pathogenic bacteria; Organic fertilizers; Phylogenetic analysis; Risk assessment

Published in

Food Control
2017, volume: 81, pages: 200-210

Authors' information