Dongjin Park

　　Department of Biological Sciences, Keimyung University, S. Korea

　　(Department of Food Science and technology, University of Nebraska-Lincoln, USA)

　　Escherichia coli O157:H7 is a human pathogen that resides asymptomatically in its bovine host. The phylogeny of E. coli O157:H7 is comprised of three lineages (I, II and I/II), which are highly heterogeneous in genome structures and virulence signatures, such as Shiga toxin (Stx) subtypes. Lineage I and I/II strains are commonly associated with human infection and produce high levels of Stx2. Stx genotypes and the level of Stx production are highly variable among E. coli O157:H7 isolates, the basis of which has not been clearly understood.

　　To study the genetic and population dynamics of E. coli  O157:H7 in its natural host, chronological collections (8 years) of bovine isolates from Wisconsin dairy farms, R and X, were analyzed for evolutionary lineages, stx2 subtypes, and Stx toxin production. Each farm had a distinct phylogenetic history and shigatoxigenic profile. Both farms initially had dominant Stx-negative lineage II strains until new Stx2-positive lineage I strains emerged in 1999 on farm R and 1994 on farm X, respectively. The Stx2-positive strain on farm R disappeared and on farm X was modified (stx2::IS genotype) during the following years. Later years, Stx-negative lineage II strains repopulated farm R while Stx-negative linage I with the new stx2::IS insertional mutation continued to dominate farm X. This data shows the presence of highly dynamic populations of Stx-positive and Stx-negative E. coli O157:H7 on the farms over time. Interestingly, Stx-negative strains often outcompeted Stx-positive strains, implying the Stx production is not essential for bacterial persistence in cattle hosts. The lack of selective pressure for Stx in cattle may result in stx inactivation during long-term persistence in cattle. Genetic analysis indicates that Stx-negative strains resulted from inactivation of stx gene itself or stx-encoding prophage genes. It is worth noting that the Stx-negative strain from farm X carries stx2::IS and has the potential to revert to the original Stx-positive strain when IS is lost in the stx locus. Together, the data suggest the bovine and farm environments can be niches where E. coli O157:H7 with variable Stx toxigenicity emerge and persist, frequently finding a route for their transmission to food production systems and ultimately humans. Furthermore, the virulence outcome of E. coli O157:H7 is shown to the result of a complex interplay of evolutionary lineages, stx genotypes, the integrity of stx-prophage structure, and host source.