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United States Department of Agriculture

Agricultural Research Service

Research Project: Biology and Control of Human Pathogens on Fresh Produce

Location: Produce Safety and Microbiology Research

Title: Intra and inter-strain differences in fitness of Escherichia coli O157:H7 to protozoan predation and survival in soil

Authors
item Ravva, Subbarao
item Sarreal, Chester
item Mandrell, Robert -

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: March 18, 2013
Publication Date: N/A

Interpretive Summary: Contamination of produce is known to be associated with some human illness outbreaks. In order to understand how this contamination occurs, it is important to know how pathogens survive in the environment, croplands and feedlots. It is known that pathogens persist under harsh environmental conditions. In this work, we compared the fate of E. coli pathogens from 2006 baby spinach outbreak for survival in dry field soil and predation by protozoa isolated from dairy wastewater. We found pathogens previously exposed to harsh environments survive longer and resist predation and may be the ones that may contaminate and proliferate on produce.

Technical Abstract: Introduction: Escherichia coli O157:H7 (EcO157) associated with 2006 spinach outbreak appears to have persisted as the organism was later isolated from environmental samples in the produce production areas of the central coast of California. Survival in harsh environments may be linked to the inherent fitness characteristics of EcO157. Purpose: This study evaluated the comparative fitness of outbreak-related clinical and environmental strains to resist protozoan predation and survive in soil from a spinach field in the general vicinity of isolation of strains genetically indistinguishable from the 2006 outbreak strains. Methods: The decreases in the populations of EcO157 strains during predation by Vorticella microstoma and Colpoda aspera, isolated previously from dairy wastewater, in decreased strength Sonneborn medium were measured. Strains with different proportions of curli-positive (C+) and curli-negative (C-) phenotypes were evaluated to determine whether protozoa are selective in grazing on them. We compared if specific phenotype subpopulations that evade predation are increased in fitness for survival in soil. Results: Environmental strains from soil and feral pig feces survived longer (11 to 35 days for 90% decreases, D-value) with both protozoa; these D-values correlated (P<0.05) negatively with protozoan growth. Similarly, strains from cow and pig feces survived significantly longer in soil compared to genetically indistinguishable clinical isolates. The C+ phenotype, a fitness trait linked with attachment in ruminant and human gut, decreased after exposure to protozoa and in soils only C- cells remained after 7 days. The C+ phenotype correlated negatively with D-values of EcO157 exposed to soil (rs=-0.683; P=0.036), Vorticella (rs=-0.465; P=0.05) or Colpoda (rs=-0.750; P=0.0001). In contrast, protozoan growth correlated positively with C+ phenotype (Vorticella, rs=0.730, P=0.0004; Colpoda, rs=0.625, P=0.006) suggesting a preference for C+ cells. Significance: We speculate that the C- phenotype is a selective trait for survival and possibly transport of the pathogen in soil and water environments.

Last Modified: 8/30/2014
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