A child’s genes do not assist bacteria that cause tooth decay

THAT children should be taught to brush their teeth regularly is a sine qua non of bringing them up. But the suspicion remains among some people, dentists included, that even so, certain children are doomed to develop dental cavities. The hypothesis behind this fear is that some combinations of genes may give succour to the sorts of oral bacteria which are responsible for cavities. If true, that would be sad for the youngsters concerned. But a study just published in Cell Host and Microbe, by Andres Gomez and Karen Nelson of the J. Craig Venter Institute, in San Diego, suggests it isn’t true.

The mouth is home to many species of microbes. Most are benign. Some, though, are well known to secrete acidic waste products when fed sugar. This acidity weakens teeth, causing them to decay. To try to find out whether a child’s genes play any role in encouraging such acid-secreting bugs, Dr Gomez and Dr Nelson set up an experiment with twins.

Their “volunteers” (actually volunteered by their parents, of course) were 280 pairs of fraternal twins and 205 pairs of identical twins, all aged between five and 11, who had not taken antibiotics during the previous six months. The children were asked to refrain from brushing their teeth the evening and the morning before the crucial moment of data collection. This was when the researchers swabbed the children’s gingival sulci (the clefts between teeth and gums, in which bacteria collect) to find out what was there. The children also had their teeth scored by dentists as belonging to one of three categories: having no signs of current or previous dental cavities; having signs of current or previous cavities affecting the enamel (a tooth’s hard, outer layer); or having signs of cavities that penetrated the enamel and affected the underlying dentine as well.

Dr Gomez and Dr Nelson found that, though identical twins shared many groups of bacteria which were not shared by fraternal twins, none of these was a type responsible for cavities. Moreover, similarities in bacterial flora were greatest among five- to seven-year-olds, weaker among seven- to nine-year-olds and weakest among nine- to 11-year-olds. This suggests that any role genes do play in regulating the mouth’s ecology fades with time.

Far from supporting the idea that some children are fated to suffer from cavities no matter how well they brush their teeth, these results make it clear that the power to control the growth of the relevant bacteria is very much within reach of children and their parents. Brushing, however, may not be the only approach. Avoiding sugary foods is obviously de rigueur. It seems likely, though, that which other foods a child eats may help shape his oral ecosystem, too. This is an area of ongoing research. But, as in the intestines, so in the mouth, scientific medicine is at last coming to grips with the fact that the mixture of microbes present is both important and capable of manipulation, to the benefit of the host.