Genetic mutations caused the Black Death

The article seems to say contradictory things about the role of biofilms.

As I understand it though, the ancestor of Yersinia Pestis had a set of genes which produced a biofilm that allowed it to survive in the lower intestine of fleas. A mutation then occurred, stripping the bacteria of its ability to make this particular biofilm.

Instead, it produced a different kind of biofilm, which permitted it to survive in the upper GI tract – increasing the chances that the bacteria would be transmitted via flea bites.

H/t: Instapundit

Which reminds me of History Teachers‘ Black Death (based on “Hollaback Girl” by Gwen Stefani):