My dental bridge smells. What could be the cause?

Though cleaning under / around the bridge is part of everyday routine what you describe sounds more like a sign of that bridge failing. There is most likely a leaking margin possible food trap or both. Have your dentist thoroughly examining it or get a second opinion. Good luck, Dr. Daniyar 

A healthy dental bridge should not smell.  What are you doing to take care of it?  Are you cleaning under the bridge.  There are a variety of tools that can help  you clean under the bridge.  There is something called Superfloss.  It's floss that looks like a shoe string.  The ends are tight but the middle is thicker. You thread the floss under the bridge and then use the thicker part to clean under the bridge.  You can also use a water pick.  A water pick dispenses water under pressure.  You can use the tip like a pencil and trace your gum line, interproximal areas, and under the bridge to flush everything out.  Any plaque or remaining food particles that remains under the bridge can contribute to a bad smell. Hope this helps!  Good Luck!