Free Shiga toxin bacteriophages isolated from sewage showed diversity although the stx genes appeared conserved

Phages carrying the stx₂ gene were detected in a range of sewage samples using a plaque hybridization-based method. After detection, phages were isolated and propagated with a laboratory strain of Escherichia coli as host for characterization purposes. Although it was not possible to conduct propagation or transduction experiments on most of the phages, 11 reached a sufficiently high titre for studies of host infectivity, electron microscopy and sequencing of the stx₂ flanking regions to be performed. These phages showed a wide range of host infectivity and morphology. The genetic structure of the 5′ stx flanking region appeared conserved whereas the 3′ region differed from that of previously described phages. This is the first description of infectious stx-phages isolated as free particles in the environment, and as such constitutes a new contribution to the study of the ecology of these phages.