SAT Biology Subject Test Practice Questions

Answer: C, Mutualistic. Humans and E. coli both benefit from the relationship, making it mutualistic.

Key Takeaway: In this relationship, E. coli receive warmth, shelter, and nutrients in exchange for producing vitamin K and sodium for humans. E. coli also protect humans against pathogenic microorganisms. For this relationship to be parasitic, one organism would have to benefit at the expense of the other. For this relationship to be commensal, one organism would have to benefit without any benefit or detriment to the other.

Photo: E. Coli, Eric Erbe and Christopher Pooley