Final answer:
Pathogenicity islands contain genes for virulence factors like the Shiga-like toxin in O157:H7, making it deadly, while harmless strains like HS lack these islands, rendering them non-pathogenic.
Step-by-step explanation:
The difference in pathogenicity between O157:H7 and harmless strains of E. coli can be explained by pathogenicity islands. O157:H7 is considered a highly virulent strain due to the presence of virulence factors such as a Shiga-like toxin, the genes for which were acquired through horizontal gene transfer from Shigella dysenteriae. This toxin, which can be life-threatening, inhibits protein synthesis within the host cells, leading to severe symptoms such as bloody diarrhea and potential kidney failure. In contrast, harmless strains like HS do not contain these pathogenicity islands and therefore are less likely to cause disease.