Abstract
Cattle are the primary reservoir of the foodborne pathogen Escherichia coli O157:H7, with the concentration and frequency of E. coli O157:H7 shedding varying substantially among individual hosts. The term ‘‘super-shedder” has been applied to cattle that shed ≥104 cfu E. coli O157:H7/g of feces. Super-shedders have been reported to be responsible for the majority of E. coli O157:H7 shed into the environment. The objective of this study was to determine if there are phenotypic and/or genotypic differences between E. coli O157:H7 isolates obtained from super-shedder compared to low-shedder cattle. From a total of 784 isolates, four were selected from low-shedder steers and six isolates from super-shedder steers (4.01–8.45 log cfu/g feces) for whole genome sequencing. Isolates were phage and clade typed, screened for substrate utilization, pH sensitivity, virulence gene profiles and Stx bacteriophage insertion (SBI) sites. A range of 89–2473 total single nucleotide polymorphisms (SNPs) were identified when sequenced strains were compared to E. coli O157:H7 strain Sakai. More non-synonymous SNP mutations were observed in low-shedder isolates. Pan-genomic and SNPs comparisons did not identify genetic segregation between super-shedder or low-shedder isolates. All super-shedder isolates and 3 of 4 of low-shedder isolates were typed as phage type 14a, SBI cluster 3 and SNP clade 2. Super-shedder isolates displayed increased utilization of galactitol, thymidine and 3-O-β-D-galactopyranosyl-D-arabinose when compared to low-shedder isolates, but no differences in SNPs were observed in genes encoding for proteins involved in the metabolism of these substrates. While genetic traits specific to super-shedder isolates were not identified in this study, differences in the level of gene expression or genes of unknown function may still contribute to some strains of E. coli O157:H7 reaching high densities within bovine feces.
Highlights
Escherichia coli O157:H7 is a major Shiga toxin–producing foodborne pathogen that is a serious public health concern and economic problem worldwide
Matthews et al [6] and Omisakin et al [7] estimated that super-shedders accounted for 80 and 96%, respectively, of the total E. coli O157:H7 shed into the environment by cattle
The objective of this study was first to determine if there are genetic differences between E. coli O157:H7 isolates obtained from super-shedder and low-shedder cattle and compare them to other closed E. coli O157:H7 genomes reported in the literature
Summary
Escherichia coli O157:H7 is a major Shiga toxin–producing foodborne pathogen that is a serious public health concern and economic problem worldwide. Healthy cattle are a primary reservoir of E. coli O157:H7, hosting the pathogen within their gastrointestinal tract (GIT) and shedding the organism into the environment via their feces. Cattle that shed the organism at levels 104 cfu/g of feces have been termed “super-shedders” and are reported to have a substantial impact on on-farm prevalence, transmission of and the contamination of food products with E. coli O157:H7. Matthews et al [6] and Omisakin et al [7] estimated that super-shedders accounted for 80 and 96%, respectively, of the total E. coli O157:H7 shed into the environment by cattle. Arthur et al [8] showed that 95% of feedlot pens housing at least one super-shedder had a hide prevalence of E. coli O157:H7 of >80%, whereas only 29% of pens lacking a supershedder exceeded this level
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