Genome sequencing and comparative genomics of enterohemorrhagic Escherichia coli O145:H25 and O145:H28 reveal distinct evolutionary paths and marked variations in traits associated with virulence & colonization

Sandra C Lorenz,Michael L Kotewicz,Markus Fischer,Julie A Kase,Narjol Gonzalez-Escalona

doi:10.1186/s12866-017-1094-3

Abstract

BackgroundEnterohemorrhagic Escherichia coli (EHEC) O145 are among the top non-O157 serogroups associated with severe human disease worldwide. Two serotypes, O145:H25 and O145:H28 have been isolated from human patients but little information is available regarding the virulence repertoire, origin and evolutionary relatedness of O145:H25. Hence, we sequenced the complete genome of two O145:H25 strains associated with hemolytic uremic syndrome (HUS) and compared the genomes with those of previously sequenced O145:H28 and other EHEC strains.ResultsThe genomes of the two O145:H25 strains were 5.3 Mbp in size; slightly smaller than those of O145:H28 and other EHEC strains. Both strains contained three nearly identical plasmids and several prophages and integrative elements, many of which differed significantly in size, gene content and organization as compared to those present in O145:H28 and other EHECs. Furthermore, notable variations were observed in several fimbrial gene cluster and intimin types possessed by O145:H25 and O145:H28 indicating potential adaptation to distinct areas of host colonization. Comparative genomics further revealed that O145:H25 are genetically more similar to other non-O157 EHEC strains than to O145:H28.ConclusionPhylogenetic analysis accompanied by comparative genomics revealed that O145:H25 and O145:H28 evolved from two separate clonal lineages and that horizontal gene transfer and gene loss played a major role in the divergence of these EHEC serotypes. The data provide further evidence that ruminants might be a possible reservoir for O145:H25 but that they might be impaired in their ability to establish a persistent colonization as compared to other EHEC strains.

Highlights

Enterohemorrhagic Escherichia coli (EHEC) O145 are among the top non-O157 serogroups associated with severe human disease worldwide
Phylogenetic analysis accompanied by comparative genomics revealed that O145:H25 and O145:H28 evolved from two separate clonal lineages and that horizontal gene transfer and gene loss played a major role in the divergence of these EHEC serotypes
The data provide further evidence that ruminants might be a possible reservoir for O145:H25 but that they might be impaired in their ability to establish a persistent colonization as compared to other EHEC strains

Summary

Introduction

Enterohemorrhagic Escherichia coli (EHEC) O145 are among the top non-O157 serogroups associated with severe human disease worldwide. Certain STEC strains are capable of causing more severe human diseases such as hemorrhagic colitis (HC) and the life-threatening hemolytic uremic syndrome (HUS); these STEC are commonly referred to as enterohemorrhagic E. coli (EHEC). While EHEC O157:H7 is considered to be the most frequent cause of severe disease, nonO157 EHEC and STEC (LEE-negative STEC e.g. O104 and O113) strains are increasingly recognized as the cause of similar illnesses worldwide [1, 2, 6,7,8,9]. It is estimated that at least 50% of all STEC infections in the US are caused by non-O157 STEC/EHEC strains, many belonging to serogroups O26, O45, O103, O111, O121 and O145 known as the “big six” [1, 9]. In addition to O157 all non-intact beef products in the US are required to be tested for the presence of these six serogroups [10]

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