Alert reader (I've always wanted to say that) Frank writes in to say that he recalled seeing news articles about bacteria being a cause of the rash of deformed frogs. I did a little more Googling, and found that bacteria has in fact been suspected as a cause of the Minnesota frogs' deformity:
[A] parasite called a trematode may be involved in some frog deformities. Trematodes burrow into the limb buds of tadpoles and can, in fact, cause at least one of the deformities seen in Minnesota frogs.
Posted by Charles Kuffner on June 26, 2002 to Technology, science, and math
UPDATE: Paul Orwin informs me that trematodes, though classified as microorganisms, are not bacteria. Read the comments for a fuller explanation. Thanks, Paul!