title: Fitness of Outbreak and Environmental Strains of Escherichia coli O157:H7 in Aerosolizable Soil and Association of Clonal Variation in Stress Gene Regulation.
abstract: Airborne dust from feedlots is a potential mechanism of contamination of nearby vegetable crops with Escherichia coli O157:H7 (EcO157). We compared the fitness of clinical and environmental strains of EcO157 in <45 µm soil from a spinach farm. Differences in survival were observed among the 35 strains with D-values (days for 90% decreases) ranging from 1-12 days. Strains that survived longer, generally, were from environmental sources and lacked expression of curli, a protein associated with attachment and virulence. Furthermore, the proportion of curli-positive (C+) variants of EcO157 strains decreased with repeated soil exposure and the strains that were curli-negative (C-) remained C- post-soil exposure. Soil exposure altered expression of stress-response genes linked to fitness of EcO157, but significant clonal variation in expression was measured. Mutations were detected in the stress-related sigma factor, rpoS, with a greater percentage occurring in parental strains of clinical origin prior to soil exposure. We speculate that these mutations in rpoS may confer a differential expression of genes, associated with mechanisms of survival and/or virulence, and thus may influence the fitness of EcO157.
authors.affiliation: Produce Safety and Microbiology Research Unit, United States Department of Agriculture, Agriculture Research Service, Western Regional Research Center, Albany, CA 94710, USA. subbarao.ravva@ars.usda.gov.
authors.forename: Subbarao V
authors.initials: SV
authors.lastname: Ravva
authors.affiliation: Produce Safety and Microbiology Research Unit, United States Department of Agriculture, Agriculture Research Service, Western Regional Research Center, Albany, CA 94710, USA. michael.cooley@ars.usda.gov.
authors.forename: Michael B
authors.initials: MB
authors.lastname: Cooley
authors.affiliation: Produce Safety and Microbiology Research Unit, United States Department of Agriculture, Agriculture Research Service, Western Regional Research Center, Albany, CA 94710, USA. chester.sarreal@ars.usda.gov.
authors.forename: Chester Z
authors.initials: CZ
authors.lastname: Sarreal
authors.affiliation: Produce Safety and Microbiology Research Unit, United States Department of Agriculture, Agriculture Research Service, Western Regional Research Center, Albany, CA 94710, USA. remandrell@gmail.com.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Mandrell