No evidence for a bovine mastitis Escherichia coli pathotype

Andreas Leimbach,Anja Poehlein,John Vollmers,Dennis Görlich,Rolf Daniel,Ulrich Dobrindt

doi:10.1186/s12864-017-3739-x

Abstract

BackgroundEscherichia coli bovine mastitis is a disease of significant economic importance in the dairy industry. Molecular characterization of mastitis-associated E. coli (MAEC) did not result in the identification of common traits. Nevertheless, a mammary pathogenic E. coli (MPEC) pathotype has been proposed suggesting virulence traits that differentiate MAEC from commensal E. coli. The present study was designed to investigate the MPEC pathotype hypothesis by comparing the genomes of MAEC and commensal bovine E. coli.ResultsWe sequenced the genomes of eight E. coli isolated from bovine mastitis cases and six fecal commensal isolates from udder-healthy cows. We analyzed the phylogenetic history of bovine E. coli genomes by supplementing this strain panel with eleven bovine-associated E. coli from public databases. The majority of the isolates originate from phylogroups A and B1, but neither MAEC nor commensal strains could be unambiguously distinguished by phylogenetic lineage. The gene content of both MAEC and commensal strains is highly diverse and dominated by their phylogenetic background. Although individual strains carry some typical E. coli virulence-associated genes, no traits important for pathogenicity could be specifically attributed to MAEC. Instead, both commensal strains and MAEC have very few gene families enriched in either pathotype. Only the aerobactin siderophore gene cluster was enriched in commensal E. coli within our strain panel.ConclusionsThis is the first characterization of a phylogenetically diverse strain panel including several MAEC and commensal isolates. With our comparative genomics approach we could not confirm previous studies that argue for a positive selection of specific traits enabling MAEC to elicit bovine mastitis. Instead, MAEC are facultative and opportunistic pathogens recruited from the highly diverse bovine gastrointestinal microbiota. Virulence-associated genes implicated in mastitis are a by-product of commensalism with the primary function to enhance fitness in the bovine gastrointestinal tract. Therefore, we put the definition of the MPEC pathotype into question and suggest to designate corresponding isolates as MAEC.

Highlights

Escherichia coli bovine mastitis is a disease of significant economic importance in the dairy industry
Bovine-associated E. coli are phylogenetically highly diverse and dominated by phylogroups A and B1 We compiled a strain panel of eight mastitis-associated E. coli (MAEC) and six fecal commensal strains and supplemented it with the genomes from eleven reference strains from public databases (Table 1)
In order to obtain a more detailed view of the phylogenetic relationship of the strains, we calculated a core genome phylogeny based on a multiple whole genome nucleotide alignment (WGA) with 39 reference E. coli strains, four Shigella spp., and one Escherichia fergusonii strain as an outgroup

Summary

Introduction

Escherichia coli bovine mastitis is a disease of significant economic importance in the dairy industry. Molecular characterization of mastitis-associated E. coli (MAEC) did not result in the identification of common traits. A mammary pathogenic E. coli (MPEC) pathotype has been proposed suggesting virulence traits that differentiate MAEC from commensal E. coli. The present study was designed to investigate the MPEC pathotype hypothesis by comparing the genomes of MAEC and commensal bovine E. coli. Bovine mastitis is a common disease in dairy cows with a global economic impact [1]. Escherichia coli is a major causative agent involved in acute bovine mastitis with a usually fast recovery rate. The bovine gastrointestinal tract is a natural reservoir for commensal and pathogenic E. coli of high phylogenetic and genotypic diversity with the putative ability to cause mastitis [9]. It was proposed that various genotypes of E. coli with specific phenotypes are better suited to elicit mastitis than others [3, 10, 11]

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