Submitted to: International Association for Food Protection
Publication Type: Abstract Only
Publication Acceptance Date: March 5, 2008
Publication Date: August 3, 2008
Citation: Laycock, A., Sharma, M., Kniel, K. 2008. Addressing Potential Contaminants in Soil for the Study of Pathogenic E. coli O157 and O8 Strains. International Association for Food Protection 2008 Abstract Book, p. 109
Introduction: Foodborne illness associated with leafy greens has illustrated the need for information describing the transmission of pathogens via contaminated soil-amendments. A realistic means of studying the fate of pathogens simulating natural conditions proved difficult due to high levels of bacterial and fungal soil organisms.
Purpose: The purpose of this study was to establish methods to assess the fate of pathogenic E. coli strains (avian pathogenic (APEC) O8, APEC O157, 2006 spinach and lettuce outbreak isolates) in biosolids, manure and soil. APEC strains caused illness for the DELMARVA poultry industry.
Methods: Five-gram samples of poultry litter, dairy manure, or treated biosolids were inoculated with a cocktail of E. coli (106 CFU/g) and mixed into 45g of soil onto which spinach seedlings were sprouted. Initial counts were plated on tryptic soy agar (TSA) containing nalidixic acid (NA); however, this media proved unsuitable due to growth of contaminating organisms. In order to enumerate E. coli several media types were explored, xylose-lysine-deoxycholate, Levine’s eosin methylene blue (L-EMB), MacConkey with sorbitol (SMAC), hektoen enteric and SMAC containing NA. The efficacy of E. coli O157 antibody-coated magnetic Pathatrix beads to recover APEC strains from soil was also evaluated.
Results: L-EMB proved most inhibitory to competitive soil microflora of the media studied with recovery of 1.5x106 for APEC O8 and O157. Leafy green O157 outbreak strain counts varied with soil-amendment, 1.0x107 in poultry litter and 1.0x108 in anaerobically-digested biosolids. Gram stains and microscopy confirmed presumptive E. coli counts. Additionally, APEC O157 (1.3x106 CFU/g) and O8 (3.0x106 CFU/g) strains could be recovered from soil inoculated with 108 CFU/g using E. coli O157 pathatrix beads and plated onto TSA-NA without the need for further selective media.
Significance: This work provides novel techniques to evaluate the survival and transport of pathogenic E. coli within different manure and biosolids applied during cultivation of spinach.