Shiga toxin-producing Escherichia coli strains from cattle as a source of the Stx2a bacteriophages present in enteroaggregative Escherichia coli 0104:H4 strains

Enteroaggregative, Shiga toxin-producing E. colt (EAEC-STEC) 0104:H4 strains are emerging pathogens causing life threatening diseases in humans. EAEC-STEC 0104:H4 strains isolated between 2001 and 2011 were found to harbor a distinct type of Shiga toxin 2a- (Stx2a) encoding prophage. This phage type shows only <65% genetic similarity to so far described viable Stx phages due to differences in the modules for DNA replication, metabolism, regulation and host specificity. Stx production in EAEC is rarely observed and the source of the Stx2a phage in the EAEC-STEC 0104:H4 strains is not known. We identified two DNA segments derived from orf15 and the cl gene of the 0104:H4 Stx2a phage P13374 that are characteristic for Stx2a prophages present in EAEC-STEC 0104:H4 strains. By PCR, these sequences were detected in 14 (5.8%) of 241 Stx2-positive STEC from animals and food. Infectious Stx2a phages could be isolated from four bovine STEC strains. These were found highly similar to P13374 for orf15, cl and stx2a sequences, the chromosomal integration site (wrbA), for phage DNA restriction profiles, virion morphology and superinfection immunity. Stx2a phages of the four bovine STEC strains formed lysogens on the E. colt K-12 strain C600. Phage P13374 from an EAEC-STEC 0104:H4 outbreak strain and one of the bovine STEC phages (P13803) lysogenized the Stx-negative EAEC 0104:H4 strain CB14647 by integrating in the wrbA gene of CB14647 and converted it into a Stx2a producer. Our findings provide experimental evidence that EAEC-STEC 0104:H4 strains have evolved by uptake of Stx2a phages from the bovine reservoir. (C) 2013 Elsevier GmbH. All rights reserved.