Microbes Determine Mood?

I've always known that how we eat significantly affects our sense of well being, but a segment I heard on NPR this morning took it to a whole new level.  Could it be that microbes in the gut influence a wide range of brain function?  You can listen to the broadcast or view its transcript here.  So perhaps I owe my generally happy disposition to the eight active probiotics in my favorite Cascade Fresh yogurts?