Evolution of the Stx2-Encoding Prophage in Persistent Bovine Escherichia coli O157:H7 Strains

Abstract

e Escherichia coli O157:H7 is a human pathogen that resides asymptomatically in its bovine host. The level of Shiga toxin (Stx) produced is variable in bovine-derived strains in contrast to human isolates that mostly produce high levels of Stx. To understand the genetic basis for varied Stx production, chronological collections of bovine isolates from Wisconsin dairy farms, R and X, were analyzed for multilocus prophage polymorphisms, stx 2 subtypes, and the levels of stx 2 transcript and toxin. The E. coli O157:H7 that persisted on both farms were phylogenetically distinct and yet produced little to no Stx2 due to gene deletions in Stx2c-encoding prophage (farm R) or insertional inactivation of stx 2a by IS1203v (farm X). Loss of key regulatory and lysis genes in Stx2c-encoding prophage abolished stx 2c transcription and induction of the prophage and stx 2a ::IS1203v in Stx2a-encoding prophage generated a truncated stx 2a mRNA without affecting phage production. Stx2-producing strains were transiently present (farm R) and became Stx2 negative on farm X (i.e., stx 2a ::IS1203v). To our knowledge, this is the first study that details the evolution of E. coli O157:H7 and its Stx2-encoding prophage in a chronological collection of natural isolates. The data suggest the bovine and farm environments can be niches where Stx2-negative E. coli O157:H7 emerge and persist, which explains the Stx variability in bovine isolates and may be part of an evolutionary step toward becoming bovine specialists.

Shiga toxins and stx phages: highly diverse entities

“…Genomic differences amongst stx phages have been made evident by several approaches, including RFLP patterns (Osawa et al, 2000), polymorphic prophage patterns (Park et al, 2013) and a multilocus characterization scheme (Smith et al, 2007). Developments in sequencing technologies over recent years have allowed for complete nucleotide sequencing of several stx phages.…”

Phylogenetic characterization of Escherichia coli O157 : H7 based on IS629 distribution and Shiga toxin genotype

“…The prevalence of IS629 elements in E. coli O157 : H7, together with the ability to cause genetic changes and a possible bias towards integration within or nearby to MGE, creates the potential for IS629 to disrupt and deactivate prophages and various virulence factors, as shown in the insertional inactivation of both stx1 and stx2 (Suzuki et al, 2004;Park et al, 2013;Kusumoto et al, 1999).…”

Horizontally Transferred Genetic Elements and Their Role in Pathogenesis of Bacterial Disease

“…82 In contrast to human isolates of EHEC O157:H7, which usually produce high amounts of Stx, bovine isolates produce variable amounts of Stx. 101,126 Differences in inducibility of the Stx phage and in regulation of expression of the stx gene may contribute to this situation. In some cases, low or no production of Stx in bovine strains has been attributed to inactivation of the stx gene by an IS element.…”

“…In some cases, low or no production of Stx in bovine strains has been attributed to inactivation of the stx gene by an IS element. 101 Stx may also play a role in adherence. 111 The outer membrane protein intimin, encoded by the LEE, binds not only to Tir (also encoded by the LEE) but also to surface-expressed nucleolin.…”