Virulence Genes in Escherichia coli Populations Are Lost During Manure Storage on a Commercial Swine Farm

Agriculture and Agri-Food Canada, London ON Canada N5V 4T3

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	Abstract

Confined livestock production farms typically store their wastes prior to land application. Here, we employed three complementary approaches to evaluate changes in the population structure and stability of virulence genes in E. coli during manure storage on a commercial farm that housed healthy swine. Isolates were genotyped by rep-PCR using the BOXA1R primer, and evaluated for the presence of selected virulence genes by PCR. Isolates obtained from the manure holding tank (n= 392) carried estB, fedA, Stx2e, astA, paa, aidA-I and sepA at a lower frequency than isolates obtained from fresh feces (n= 412). Fresh fecal material from the barn was added into diffusion chambers, and immersed in the manure holding tank for seven weeks. The fecal E. coli population was initially dominated by a single genotype, all isolates of which carried fedA and aidA-1. Following seven weeks, a genotype that did not carry any virulence genes dominated the surviving population. In a second experiment, 48 fecal isolates of E. coli that varied in their genotype and virulence gene complement were incubated in diffusion chambers in the manure holding tank for three weeks. Over 95% of the inoculum population carried at least one virulence gene, whereas after three weeks 90% of the recovered isolates carried no virulence genes. Taken together, these results indicate that during commercial manure storage, there was a significant reduction in the carriage of these virulence genes by E. coli. We propose that loss of virulence genes from enteric pathogens in the farm and in natural environments may, if generalized, contribute to the attenuation of public health risk from contamination with agricultural wastes.