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Escherichia coli O157:H7 strains fall into three major genetic lineages that differ in their distribution among humans and cattle. Several recent studies have reported differences in the expression of virulence factors between E. coli O157:H7 strains from these two host species. In this study, we wished to determine if important virulence-associated "mobile genetic elements" such as Shiga toxin 2 (Stx2)-encoding prophage are lineage restricted or are host source related and acquired independently of the pathogen genotype. DNA sequencing of the stx 2 flanking region from a lineage II (LII) strain, EC970520, revealed that the transcriptional activator gene Q in LI strain EDL933 (upstream of stx 2 ) is replaced by a pphA (serine/threonine phosphatase) homologue and an altered Q gene in this and all other LII strains tested. In addition, nearly all LI strains carried stx 2 , whereas all LII strains carried variant stx 2c and 4 of 14 LI/II strains had copies of both stx 2 and variant stx 2c . Real-time PCR (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) demonstrated that LI and LI/IIstrains produce significantly more stx 2 mRNA and Stx2 than LII strains. However, among LI strains significantly more Stx2 is also produced by strains from humans than from cattle. Therefore, lineage-associated differences among E. coli O157:H7 strains such as prophage content, toxin type, and toxin expression may contribute to host isolation bias. However, the level of Stx2 production alone may also play an important role in the within-lineage association of E. coli O157:H7 strains with human clinical disease.

Aims The objective of this study was to determine antimicrobial activities of essential oils (EOs) against bovine respiratory disease (BRD) pathogens and nasopharyngeal commensal bacteria, as well as cytotoxicity in bovine turbinate (BT) cells in vitro. Methods and Results The chemical composition of 16 EOs was determined using gas chromatography–mass spectrometry. All EOs were first evaluated for growth inhibition of a single BRD pathogen Mannheimia haemolytica serotype 1 strain (L024A). The most inhibitory EOs (n = 6) were then tested for antimicrobial activity against multidrug‐resistant strains of M. haemolytica (serotypes 1, 2 and 6); the BRD pathogens Pasteurella multocida and Histophilus somni, as well as commensal bacteria that were isolated from the nasopharynx of feedlot cattle. The cytotoxicity of 10 EOs was also evaluated using a BT cell line. The EOs ajowan, thyme and fennel most effectively inhibited all BRD pathogens tested including multidrug‐resistant strains with minimum inhibitory concentrations (MIC) of ≤0·025% (volume/volume, v/v). For these EOs, the MIC was 2–32 fold greater against commensal bacteria, compared to BRD‐associated pathogens. No cytotoxic effects of EOs against BT cells were observed within the tested range of concentrations (0·0125–0·4%, v/v). Conclusions The EOs ajowan, thyme and fennel inhibited M. haemolytica, P. multocida and H. somni at a concentration of 0·025% and had minimal antimicrobial activity against nasopharyngeal commensal bacteria and cytotoxicity against BT cells. Significance and Impact of the Study This study demonstrated that EOs may have potential for intra‐nasal administration to mitigate bovine respiratory pathogens in feedlot cattle.

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The objectives of this study were to identify endemic bacteriophages (phages) in the feedlot environment and determine relationships of these phages to Escherichia coli O157:H7 from cattle shedding high and low numbers of naturally occurring E. coli O157:H7. Angus crossbred steers were purchased from a southern Alberta (Canada) feedlot where cattle excreting >10 4 CFU · g ؊1 of E. coli O157:H7 in feces at a single time point were identified as supershedders (SS; n ‫؍‬ 6), and cattle excreting <10 4 CFU · g ؊1 of feces were identified as low shedders (LS; n ‫؍‬ 5). Fecal pats or fecal grabs were collected daily from individual cattle for 5 weeks. E. coli O157:H7 in feces was detected by immunomagnetic separation and enumerated by direct plating, and phages were isolated using short-and overnight-enrichment methods. The total prevalence of E. coli O157:H7 isolated from feces was 14.4% and did not differ between LS and SS (P ‫؍‬ 0.972). The total prevalence of phages was higher in the LS group (20.9%) than in the SS group (8.3%; P ‫؍‬ 0.01). Based on genome size estimated by pulsed-field gel electrophoresis and morphology determined by transmission electron microscopy, T4-and O1-like phages of Myoviridae and T1-like phage of Siphoviridae were isolated. Compared to T1-and O1-like phages, T4-like phages exhibited a broad host range and strong lytic capability when targeting E. coli O157:H7. Moreover, the T4-like phages were more frequently isolated from feces of LS than SS, suggesting that endemic phages may impact the shedding dynamics of E. coli O157:H7 in cattle.