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United States Department of Agriculture

Agricultural Research Service

Research Project: Protection of Food and Water Supplies from Pathogen Contamination Title: Effects of Fumigants on Microbial Diversity and Persistence of E. coli O15:H7 in Contrasting Soil Microcosms

Authors
item Ibekwe, Abasiofiok
item Ma, Jincai -

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 15, 2011
Publication Date: July 14, 2011
Citation: Ibekwe, A.M., Ma, J. 2011. Effects of Fumigants on Microbial Diversity and Persistence of E. coli O15:H7 in Contrasting Soil Microcosms. Science of the Total Environment. Available: http://www.sciencedirect.com/science/article/pii/S0048969711006589.

Interpretive Summary: Fumigation of soil is a common practice used to kill many soil borne pathogens. However, little research has been done on the long term effects of fumigants on total microbial composition and the persistence behavior of pathogenic E. coli O157:H7 after fumigation with methyl bromide and methyl iodide. Our study showed that E. coli O157:H7 survived for about 35 days after fumigation using plate count method. However, low concentrations of the pathogen by real time PCR were observed for more than 86 days. This suggests the presence of dormant but viable cells in the microcosms. Enrichment and immunomagnetic separation revealed the presence of viable cells indicating that very low levels of the pathogen were still present in the soil. Therefore, the presence of dormant E. coli O157:H7 cells in the soil may be a major source of recontamination of the environment by this pathogen. This research will directly benefit farmers, preharvest food safety microbiologists, and the public.

Technical Abstract: Persistence of E. coli O157 in the environment is a serious public health concern. However, little is known about the persistence of this pathogen after exposure to chemical compounds like fumigants in the environment. In this study, the persistence behavior of pathogenic E. coli O157:H7 was investigated after fumigation with methyl bromide (MeBr; CH3Br) and methyl iodide (MeI, iodomethane; CH3I) in soil microcosms under laboratory conditions. Our goal was to assess changes in soil microbial community structure and persistence of E. coli O157:H7 in microcosm soils after fumigation. PCR was used to amplify 16S rRNA genes from total bacterial community composition, and the products were subjected to denaturing gradient gel electrophoresis (DGGE). Microbial diversity as determined by DGGE was significantly higher in clay soil than sandy soil. Real-time PCR and plate counts were used to quantify the survival of E. coli O157:H7 in the two soils after fumigation with MeBr and MeI. The survival of the pathogen was higher in the non fumigated controls than the fumigated treatments when determined using plate counts. These results were confirmed by real time PCR analysis targeting the stx1, stx2, and the eae genes. E. coli O157:H7 survived for about 35 days when determined using the plate count method but continued to be detected at about the detection limit of 102 by real time PCR for more than 86 days. Our results showed that there was a fast inactivation of the pathogen during the first 35 days. After this period, a small proportion of the pathogen continued to survive in the soil microcosms. Subsequent enrichment of soil samples and immunomagnetic separation revealed the continuous presence of viable cells after 86 days of incubation. The data presented contribute to a better understanding of the behavior of E. coli O157:H7 in soil, and showed the need for more investigation of the role of dormant cells in soil that may be a source for recontamination of the environment.

Last Modified: 11/28/2014