In a Dutch oven or a large deep skillet with a tight-fitting lid heat broth over high heat until boiling.
Add mussels, cover and cook 3 to 5 minutes or until mussels open fully. (Discard any mussels that do not open.)
Using a slotted spoon, transfer mussels to a platter. Reserve cooking liquid. Let mussels stand until cool enough to handle. Pry off top shells of mussels; discard top shells. Set shells filled with mussels aside.
Preheat broiler to high. In a small bowl combine Parmesan, breadcrumbs, bacon and parsley.
Using a teaspoon, spoon breadcrumb mixture evenly over each mussel. Spoon 1/2 teaspoon reserved cooking liquid over each mussel. Place on a baking sheet. Broil 5 1/2 inches from heat source 2 to 3 minutes or until cheese melts. Serve immediately.