The human mouth is home to more than 300 species of bacteria living in a mucky film of proteins, carbohydrates and germs known as a biofilm. Swirling around in our saliva, this biofilm contains one particularly nasty bug, named Streptococcus mutans, that secretes an enzyme that attacks tooth enamel and causes cavities. Now, however, scientists have designed a genetically engineered bacterium that does toothpaste one better: it actually kills the Streptococcus in your mouth.

British and Swedish researchers took the harmless Lactobacillus zeae bacterium commonly found in yogurt and added a gene to it that enables it to produce antibodies. Thus armed, the Lactobacillus then goes hunting for Streptococcus mutans and poisons it by sticking the antibodies to its surface. In theory, human saliva could be stripped of its meanest bug.