Final answer:
Pathogenicity islands in E. coli O157:H7 contain genes for virulence factors such as the Shiga toxin, which are responsible for its deadly impact compared to non-virulent strains that do not possess such islands with these specific virulence genes.
Step-by-step explanation:
The deadly nature of E. coli O157:H7 compared to non-virulent strains such as HS can be explained by the presence of pathogenicity islands which contain genes coding for virulence factors. In the case of O157:H7, these pathogenicity islands contain genes for Shiga toxin production, acquired through horizontal gene transfer from Shigella dysenteriae. This particular virulence factor inhibits protein synthesis, leading to severe and potentially fatal symptoms.
Therefore, the correct answer to the question "How does pathogenicity islands explain how O157:H7 is deadly and HS is not?" would be B) Pathogenicity islands in O157:H7 contain genes for virulence factors, while those in HS do not. This highlights the significant impact that pathogenicity islands have on the virulence of bacterial strains, ultimately influencing their potential to cause serious disease.