Soon a spoonful of yogurt will be able to protect your teeth against decay. Biologists at the Karolinska Institute in Stockholm, Sweden, have engineered Lactobacillus zeae-a bacterium commonly found in dairy products-to produce antibodies against Streptococcus mutans, the mouth bacterium that causes most cavities. When the researchers gave the modified Lactobacillus to rats, the antibodies on its surface bound to the Streptococcus on the rats’ teeth, forming microscopic clumps that slid down the animals’ throats. Immunologist Lennart Hammarstrm hopes to test the treatment next year in people with underactive salivary glands, such as patients undergoing radiation treatment for head and neck cancer, who have a greater risk of developing cavities. The Swedish team is also engineering Lactobacillus to produce antibodies against Rotavirus, which causes severe diarrhea in children, and Helicobacter pylori, a bacterium that causes ulcers and stomach cancer. Patients will take pills or eat yogurt containing modified Lactobacillus, which would colonize their stomachs. The treatment could prove cheaper and more effective than antibiotics, says Hammarstrm.