Now meet a different bacterium, less well known than E. coli, but equally pervasive. In fact, infection with Streptococcus mutans is the most prevalent human disorder next to the common cold. It's the busy, ubiquitous bug that drills holes in the teeth of young and old — a disease more properly known as dental caries.

In curbing caries, prevention is everything — brushing teeth with fluoride toothpaste, removing plaque, watching that sweet tooth. Therapy, so far, is limited to filling or extracting cavity-riddled teeth.

Immunologists at the United Medical and Dental Schools of Guy's Hospital, in London, are developing a vaccine aimed at eradicating S. mutans where it lives — in, on and around the teeth. They derived their experimental vaccine — oral in more ways than one — not from potatoes but from transgenic tobacco plants.

Their article, also in the May 1998 issue of Nature Medicine, bears the title: "Characterization of a recombinant plant monoclonal secretory antibody and preventive immunotherapy in humans."

Their experiment began with six human volunteers, who had their teeth and mouths thoroughly sterilized in a nine-day treatment to get rid of permanently colonized S. mutans. Then a pipette delivered a solution of vaccine to each tooth surface. This was held in place for five minutes by an individually cast dentition tray.

Two of the four participants received the vaccine twice weekly for three weeks. Two controls got a placebo instead.

The tobacco plants, transfected with genes encoding IgA/G dimeric antibodies, provided the mucosal-tissue-targeting vaccine, combined with the more all-purpose IgG. This IgA/G antibody was detectable for 72 hours or more on teeth and for 48 hours in saliva.

By 58 days, S. mutans had begun recolonizing in the saliva of controls, whereas the subjects who received vaccine remained bacteria-free at least until the experiment ended at 120 days. — David Leff