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Background  Various H-serotypes of the Shiga toxin-producing  Escherichia coli  (STEC) O104, including H4, H7, H21, and H ¯ , have been associated with sporadic cases of illness and have caused food-borne outbreaks globally. In the U.S., STEC O104:H21 caused an outbreak associated with milk in 1994. However, there is little known on the evolutionary origins of STEC O104 strains, and how genotypic diversity contributes to pathogenic potential of various O104 H-antigen serotypes isolated from different ecological niches and/or geographical regions.    Results  Two STEC O104:H21 (milk outbreak strain) and O104:H7 (cattle isolate) strains were shot-gun sequenced, and the genomes were closed. The intimin ( eae ) gene, involved in the attaching-effacing phenotype of diarrheagenic  E. coli , was not found in either strain. Examining various O104 genome sequences, we found that two “complete” left and right end portions of the locus of enterocyte effacement (LEE) pathogenicity island were present in 13 O104 strains; however, the central portion of LEE was missing, where the  eae  gene is located. In O104:H4 strains, the missing central portion of the LEE locus was replaced by a pathogenicity island carrying the  aidA  (adhesin involved in diffuse adherence) gene and antibiotic resistance genes commonly carried on plasmids. Enteroaggregative  E. coli -specific virulence genes and European outbreak O104:H4-specific  stx2 -encoding  Escherichia  P13374 or  Escherichia  TL-2011c bacteriophages were missing in some of the O104:H4 genome sequences available from public databases. Most of the genomic variations in the strains examined were due to the presence of different mobile genetic elements, including prophages and genomic island regions. The presence of plasmids carrying virulence-associated genes may play a role in the pathogenic potential of O104 strains.    Conclusions  The two strains sequenced in this study (O104:H21 and O104:H7) are genetically more similar to each other than to the O104:H4 strains that caused an outbreak in Germany in 2011 and strains found in Central Africa. A hypothesis on strain evolution and pathogenic potential of various H-serotypes of  E. coli  O104 strains is proposed.    Electronic supplementary material  The online version of this article (doi:10.1186/s12866-015-0413-9) contains supplementary material, which is available to authorized users.

bmc microbiology

Genome sequencing and comparative genomics provides insights on the evolutionary dynamics and pathogenic potential of different H-serotypes of Shiga toxin-producing Escherichia coli O104