Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2011
Publication Date: 1/15/2012
Citation: Parker, C., Kyle, J.L., Huynh, S., Carter, M.Q., Brandl, M., Mandrell, R.E. 2012. Distinct transcriptional profiles and phenotypes exhibited by Escherichia coli O157:H7 isolates related to the 2006 spinach-associated outbreak. Applied and Environmental Microbiology. 78:455-463.
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: In 2006, an outbreak of Escherichia coli O157:H7 was linked to the consumption of ready-to-eat bagged spinach. The likely sources of pre-harvest spinach contamination were soil and water that became contaminated via cattle or feral pigs in the proximity of the spinach fields. In this study, we compared the transcriptional profiles of 12 E. coli O157:H7 strains that possess the same two enzyme PFGE profile and are related temporally or geographically to the 2006 E. coli O157:H7 outbreak attributed to bagged spinach in the United States. These E. coli O157:H7 strains included three clinical isolates, five isolates from separate bags of spinach, and single isolates from pasture soil, river water, cow feces, and a feral pig. The three clinical strains and two spinach bag strains grown in cultures to stationary phase showed decreased expression of many's -regulated genes including gadA, osmE, osmY, and katE, as compared to the soil, water, cow, feral pig, and the other three spinach bag strains. The decreased expression of these 'S -regulated genes was correlated with the decreased resistance to acid stress (pH 2.5), osmotic stress (2.5 M NaCl), and oxidative stress (12.5 mM H2O2); but an increase in scavenging ability. We also observed intrastrain variability among the strains that was much more pronounced among the clinical and spinach strains. Together, the transcriptional and phenotypic differences of the spinach outbreak strains of E. coli O157:H7 support the hypothesis that some variants within the spinach bag retained characteristics of the pre-harvest isolates, whereas other variants with altered gene expression and phenotypes infected the human host.