Abstract
Enteroinvasive Escherichia coli (EIEC) cause intestinal illness through the same pathogenic mechanism used by Shigella spp. The latter species can be typed through genomic and phenotypic methods used for E. coli and have been proposed for reclassification within E. coli species. Recently the first appearance of a highly pathogenic EIEC O96:H19 was described in Europe as the causative agent of two large outbreaks that occurred in Italy and in the United Kingdom. In contrast to Shigella spp and to the majority of EIEC strains, EIEC O96:H19 fermented lactose, lacked pathoadaptive mutations, and showed good fitness in extracellular environment, similarly to non-pathogenic E. coli, suggesting they have emerged following acquisition of the invasion plasmid by a non-pathogenic E. coli. Here we describe the whole genome comparison of two EIEC O96:H19 strains isolated from severe cases of diarrhea in Uruguay in 2014 with the sequences of EIEC O96:H19 available in the public domain. The phylogenetic comparison grouped all the O96:H19 strains in a single cluster, while reference EIEC strains branched into different clades with Shigella strains occupying apical positions. The comparison of the virulence plasmids showed the presence of a complete conjugation region in at least one O96:H19 EIEC. Reverse Transcriptase Real Time PCR experiments confirmed in this strain the expression of the pilin-encoding gene and conjugation experiments suggested its ability to mobilize an accessory plasmid in a recipient strain. Noteworthy, the tra region was comprised between two reversely oriented IS600 elements, which were also found as remnants in another EIEC O96:H19 plasmid lacking the tra locus. We hypothesize that an IS-mediated recombination mechanism may have caused the loss of the conjugation region commonly observed in EIEC and Shigella virulence plasmids. The results of this study support the hypothesis of EIEC originating from non-pathogenic E. coli through the acquisition of the virulence plasmid via conjugation. Remarkably, this study showed the ability of a circulating EIEC strain to mobilize plasmids through conjugation, suggesting a mechanism for the emergence of novel EIEC clones.
Highlights
Enteroinvasive Escherichia coli (EIEC) cause disease in humans, characterized by abdominal cramps, bloody and mucous diarrhea (van den Beld and Reubsaet, 2012)
The virulence gene content and the presence of pathoadaptive mutations was investigated in the strains from Uruguay and compared to the same information obtained from the whole genome sequences of all the EIEC O96:H19 strains included in the study
The key genetic event characterizing the evolution of EIEC and Shigella consisted in the acquisition of the pINV virulence plasmid, which harbors the majority of genes involved in the invasion mechanism, in combination with the accumulation of pathoadaptive mutations in anti-virulence loci, which has been extensively demonstrated either in EIEC or Shigella strains (Pasqua et al, 2017)
Summary
Enteroinvasive Escherichia coli (EIEC) cause disease in humans, characterized by abdominal cramps, bloody and mucous diarrhea (van den Beld and Reubsaet, 2012). The main virulence genes of EIEC and Shigella are harbored on a large virulence plasmid termed pINV. These include the mxi and spa genes encoding a type three secretion system (T3SS) as well as the ipaB, ipaC, ipaD, and icsA genes encoding effectors necessary to invade and disseminate into the host cells (Pasqua et al, 2017). During the last decade an increase in the number of cases of EIEC infections has been observed in Europe, with two large outbreaks, suspected to be linked to the consumption of contaminated food, occurred in Italy and in the United Kingdom between 2012 (Escher et al, 2014) and 2014 (Newitt et al, 2016). A third isolate sharing the same characteristics was identified as the cause of a sporadic case of EIEC infection occurred in Spain in 2013, confirming the circulation of such clone in Europe (Michelacci et al, 2016)
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