Which pathogen is usually associated with contaminating leafy green produce such as romaine lettuce?

E. coli, specifically strains like O157:H7, is commonly associated with leafy green produce, including romaine lettuce. This bacterium can contaminate crops through several routes, such as contaminated water used for irrigation, improper handling during processing, or contact with contaminated soil or feces. E. coli can thrive in the environment and leads to serious foodborne illness, often resulting in severe gastrointestinal symptoms and complications.

Other pathogens listed do not typically target leafy greens in the same way. Trichinella is primarily associated with undercooked pork, while Hepatitis B Virus is transmitted through blood or bodily fluids and is not related to food contamination. Staphylococcus aureus is associated more with improperly handled ready-to-eat foods and can cause illness through improper food handling, rather than being specifically linked to leafy greens. Therefore, E. coli is the most relevant pathogen in this context.