Abstract
Escherichia coli AW1.7 is a heat resistant food isolate and the occurrence of pathogenic strains with comparable heat resistance may pose a risk to food safety. To identify the genetic determinants of heat resistance, 29 strains of E. coli that differed in their of heat resistance were analyzed by comparative genomics. Strains were classified as highly heat resistant strains, exhibiting a D60-value of more than 6 min; moderately heat resistant strains, exhibiting a D60-value of more than 1 min; or as heat sensitive. A ~14 kb genomic island containing 16 predicted open reading frames encoding putative heat shock proteins and proteases was identified only in highly heat resistant strains. The genomic island was termed the locus of heat resistance (LHR). This putative operon is flanked by mobile elements and possesses >99% sequence identity to genomic islands contributing to heat resistance in Cronobacter sakazakii and Klebsiella pneumoniae. An additional 41 LHR sequences with >87% sequence identity were identified in 11 different species of β- and γ-proteobacteria. Cloning of the full length LHR conferred high heat resistance to the heat sensitive E. coli AW1.7ΔpHR1 and DH5α. The presence of the LHR correlates perfectly to heat resistance in several species of Enterobacteriaceae and occurs at a frequency of 2% of all E. coli genomes, including pathogenic strains. This study suggests the LHR has been laterally exchanged among the β- and γ-proteobacteria and is a reliable indicator of high heat resistance in E. coli.
Highlights
Escherichia coli are commensals in the human and animal gut but the species comprises intestinal and extraintestinal pathogens
The conservation of the open reading frames (ORFs) among E. coli, C. sakazakii, and K. pneumonia is remarkable; most ORFs share more than 99% nucleotide identity to the corresponding genes in E. coli AW1.7 (Figure 4B)
Core-genome phylogenetic analysis and phylogenetic analysis of the locus of heat resistance (LHR) support the conclusion that the LHR is transmitted via lateral gene transfer
Summary
Escherichia coli are commensals in the human and animal gut but the species comprises intestinal and extraintestinal pathogens. The ecological versatility of E. coli is reflected in its genome plasticity. The average E. coli genome is approximately 5.16 Mb, encoding an average of 5190 genes. Lateral gene transfer promotes the evolution and diversity of E. coli, and allows acquisition of virulence factors (Dobrindt et al, 2004; Croxen et al, 2013; Gordienko et al, 2013). Genes responsible for colonization, toxin production and antibiotic resistance are encoded on mobile genetic elements and are transmitted between strains of E. coli (Croxen et al, 2013). The horizontal transfer of large gene clusters, called genomic islands, provides
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