Colonizing the mouth with benign bacteria.

Colonizing the mouth with benign bacteria

Mutant bacteria especially adept at establishing themselves in the human mouth have been developed by scientists at the Forsyth Dental Center in Boston. They expect these bacteria to serve as the missing link in attempts to replace cavity-causing bacteria with mutant strains that do not generate the large amounts of acid that decay teeth.

About nine years ago Jeffrey D. Hillman and his colleagues at Forsyth, working with colonies of the bacterium Streptococus mutans, isolated a mutant that produces only low levels of acid. Later, when they introduced the mutant strain into mouths of germ-free rats, the rats remained cavity-free throughout their lives despite a diet high in sugar (SN: 12/8/79, p. 394). But this promising bacterium never succeeded as a dental therapy, because it was unable to take over a human mouth already occupied by the natural, cavity-causing strain. "The human mouth is a more complex ecosystem than that of the rat,' Hillman says.

So Hillman and his colleagues went back to natural bacterial populations from human mouths to find a strain that is a better colonizer. They now report they isolated such a bacterium (called JH1001) and created a mutant of it that is an even more successful colonizer (JH1005). These bacteria produce a chemical, called a bacteriocin, that stops the growth of closely related strains.

Three Forsyth staff members had their teeth cleaned, then brushed and flossed with JH1005 cells for five minutes. Now, almost a year later, that strain is the dominant S. mutans in their mouths. An unexpected finding is that in these volunteers the total number of all S. mutans bacteria in their mouths is significantly depressed. However, levels of a similar bacterium, S. sanguis, that occupies a similar niche was not depressed by the S. mutans mutant.

Now the scientists plan to combine in a single strain the characteristics of low acid production and successful colonization. "Hopefully we'll have such a strain in a couple years,' Hillman says. "Colonization has been the major stumbling block.' According to the American Association for Dental Research, "The findings to date lend strong support to the practical application of replacement therapy for the prevention of tooth decay.'