Fitness of outbreak and environmental strains of Escherichia coli O157:H7 in aerosolizable soil and association of clonal variation in stress gene regulation

Authors: Ravva, Subbarao; Cooley, Michael; Sarreal, Chester; MANDRELL, ROBERT - Retired Non ARS Employee

Submitted to: Pathogens
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2014
Publication Date: 6/30/2014
Citation: Ravva, S.V., Cooley, M.B., Sarreal, C.Z., Mandrell, R.E. 2014. Fitness of outbreak and environmental strains of Escherichia coli O157:H7 in aerosolizable soil and association of clonal variation in stress gene regulation. Pathogens. 3:528-548. DOI: 10.3390/pathogens3030528.

Interpretive Summary: Major outbreaks associated with produce indicate that pre-harvest contamination has occurred in the field, so it is critical to identify sources of pathogens in the environment and interventions for minimizing them. Animal manure is a potential pathogen reservoir, and transport of these pathogens through aerosols to crops grown in close proximity to dairy operations to croplands in California cannot be ignored. Since contamination of 'ready to eat' produce cannot be washed off, on-site prevention of contamination is vital. Any such attempts require an understanding of the biological and environmental factors that regulate the proliferation of pathogens during their transport from animal reservoirs to produce grown in proximity to livestock operations. We have worked on developing improved detection methods for pathogens, and on understanding how pathogens survive, transport and re-grow in agricultural environments. Our long-term goals are to develop on-site prevention and control of pathogens at the source prior to contaminating produce we consume. This study evaluated the survival of Escherichia coli O157:H7 strains, isolated from human clinical samples and various environmental sources near the produce fields linked to 2006 spinach outbreak, in fine soil dust that is likely to be airborne from feedlots and dairies and contaminate nearby produce fields. We observed differences in soil survival between different pathogenic strains and also between colonies isolated for each strain after soil exposure. We also observed that repeated exposure to soil selects variants of pathogenic strains that withstand and survive the stress factors in the enviroment. Mutations detected in the stress response gene rpoS may be linked with the environmental survival of these deadly pathogens.

Technical 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.