In Silico Analysis of Shiga Toxin-ProducingO157:H7 Strains from Presumptive Super- and Low-Shedder Cattle.

Abstract

Cattle are the primary reservoir for STEC O157, with some shedding >10CFU/g in feces, a phenomenon known as super-shedding (SS). The mechanism(s) responsible for SS are not understood but have been attributed to the environment, host, and pathogen. This study aimed to compare genetic characteristics of STEC O157 strains from cattle in the same commercial feedlot pens with SS or low-shedding (LS) status. Strains from SS (n = 35) and LS (n = 28) collected from 11 pens in three feedlots were analyzed for virulence genes, Shiga toxin-carrying bacteriophage insertion sites, and phylogenetic relationships. In silico analysis showed limited variation regarding virulence gene profiles.-encoding prophage insertion sitesandforand, respectively, were all occupied, but two isolates had fragments of the-carrying phage inandloci withoutand. All strains screened for lineage-specific polymorphism assay (LSPA-6) were 111111, lineage I. Of the isolates, 61 and 2 were clades 1 and 8, respectively. Phylogenetic analysis revealed that pens with more than one SS had multiple distantly related clusters of SS and LS isolates. Although virulence genes and lineage were largely similar within and across feedlots, multiple genetic origins of strains within a single feedlot pen illustrate challenges for on-farm control of STEC.