You have isolated a bacterium that you believe to be the causative agent of a new disease in frogs. How would you test the third of Koch's postulates?

A) Determine the shape of the bacterial cells.
B) Inject the bacteria into a healthy frog.
C) Isolate the bacterium from a sick frog.
D) Show that the bacterium is not present in healthy frogs.
E) Grow a pure culture of the bacterium outside the frog.