I have heard that rats digestive systems can't deal with a living yeast culture.
Rather than being killed in the digestive system of rats, it continues to grow and produce carbon dioxide until the rats stomach and intestines rupture.
Have not tried it, but if you have a rat problem, you might want to try treating some bait with a yeast culture and see how it goes.
Edit: did a web search and found nothing that verifies the use of yeast, so it might be just a legend.