ARS | Publication request: Transcriptome and phenotypic difference of Escherichia coli O157:H7 isolates related to the 2006 spinach-associated outbreak reveals variants in the bag

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: March 2, 2010
Publication Date: N/A

Interpretive Summary: Food-borne outbreaks linked to the consumption of ready-to-eat leafy vegetables, such as lettuce and spinach contaminated with Escherichia coli O157:H7 are a mounting concern. Likely sources of pre-harvest contamination are soil and water that become contaminated via cattle and feral pigs that are located proximately to the vegetable fields. In this study, we compared the gene expression of twelve isolates that were genetically indistinguishable and related by time or geography to the 2006 E. coli O157:H7 outbreak attributed to fresh spinach in the United States. These E. coli O157:H7 isolates included three isolates from patients, five isolates from the bagged spinach, and an isolate from soil, an isolate from water, a bovine-derived isolate and a porcine-derived isolate located near the spinach fields. From stationary phase cultures, we observed the decreased expression of many RpoS-regulated genes including gadA, osmE, osmY, and katE, in all three clinical isolates and two spinach isolates as compared to the soil, water, bovine, porcine, and the other three spinach isolates. Indeed, the decreased expression of several RpoS-regulated genes was most pronounced in the clinical isolates and correlated with decreased resistance to acid stress, oxidative stress (H2O2), and osmotic stress in these isolates. The gene expression and phenotypic differences of the clinical isolates suggest that certain variants in E. coli O157:H7 populations within the spinach bag promote rapid adaptation to the human host.

Technical Abstract: Food-borne outbreaks of Escherichia coli O157:H7 illness linked to the consumption of ready-to-eat leafy vegetables, such as lettuce and spinach, are a mounting concern. Likely sources of pre-harvest contamination are soil and water that become contaminated via cattle and feral pigs in the proximity of the vegetable fields. In this study, we compared the transcriptional profiles of twelve isolates that possess the same PFGE pattern and are related temporally or geographically to the 2006 E. coli O157:H7 outbreak attributed to fresh spinach in the United States. These E.coli O157:H7 isolates included three clinical isolates, five isolates from separate bags of spinach, and single isolates from soil, water, a cow and a feral pig. From stationary phase cultures, we observed the decreased expression of many RpoS-regulated genes including gadA, osmE, osmY, and katE, in all three clinical isolates and two spinach isolates as compared to the soil, water, cow, feral pig, and the other three spinach bag isolates. The decreased expression of several RpoS-regulated genes was most pronounced in the clinical isolates and correlated with decreased resistance to acid stress (pH 2.5), oxidative stress (15 mM H2O2), and osmotic stress (2.5 M NaCl) in these isolates. The transcriptional and phenotypic differences of the spinach bag isolates of E .coli O157:H7 suggest that certain variants within the bag retained characteristics of the pre-harvest isolates while other variants with altered expression and phenotype infected the human host.