Identification and characterization of a peculiar vtx2-converting phage frequently present in verocytotoxin-producing Escherichia coli O157 isolated from human infections

Certain verocytotoxin-producing Escherichia coli (VTEC) O157 phage types (PTs), such as PT8 and PT2, are associated with severe human infections, while others, such as PT21, seem to be restricted to cattle. In an attempt to delve into the mechanisms underlying such a differential distribution of PTs, we performed microarray comparison of human PT8 and animal PT21 VTEC O157 isolates. The main differences observed were in the vtx2-converting phages, with the PT21 strains bearing a phage identical to that present in the reference strain EDL933, BP933W, and all the PT8 isolates displaying lack of hybridization in some regions of the phage genome. We focused on the region spanning the gam and cII genes and developed a PCR tool to investigate the presence of PT8-like phages in a panel of VTEC O157 strains belonging to different PTs and determined that a vtx2 phage reacting with the primers deployed, which we named Φ8, was more frequent in VTEC O157 strains from human disease than in bovine strains. No differences were observed in the production of the VT2 mRNA when Φ8-positive strains were compared with VTEC O157 possessing BP933W. Nevertheless, we show that the gam-cII region of phage Φ8 might carry genetic determinants downregulating the transcription of the genes encoding the components of the type III secretion system borne on the locus of enterocyte effacement pathogenicity island.