Pathogenomes of Atypical Non-shigatoxigenic NSF/SF O157:H7/NM: Comprehensive Phylogenomic Analysis Using Closed Genomes.

The toxigenic conversion of strains by Shiga toxin-converting (Stx) bacteriophages were prominent and recurring events in the stepwise evolution of enterohemorrhagic (EHEC) O157:H7 from an enteropathogenic (EPEC) O55:H7 ancestor. Atypical, attenuated isolates have been described for both non-sorbitol fermenting (NSF) O157:H7 and SF O157:NM serotypes, which are distinguished by the absence of Stx, the characteristic virulence hallmark of Stx-producing (STEC). Such atypical isolates either never acquired Stx-phages or may have secondarily lost during the course of infection, isolation, or routine subculture; the latter are commonly referred to as LST (Lost Shiga Toxin)-isolates.