Mucus is part of the 0.5 percent of saliva that is not water. It contains salivary mucins, which work to protect teeth from cavities. Previously it was believed that the salivary mucins' only function was to give the mucus a slippery, elastic consistency and to contribute to its gel-like properties. It has since been proven that the mucins actively defend against pathogens and keep the human microbiome healthy.
Recent findings suggest that the body's natural defenses may be more effective for preventing tooth decay than external defenses such as fluoride treatments and sealants. Over a 24 hour period, the salivary mucins don't alter the S. mutans or get rid of the bacteria. Instead they maintain a liquid medium in which the bacteria are suspended, which decreases their capacity to create biofilms on the teeth.
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S. mutans brings about tooth decay by attaching itself to a tooth to create a biofilm. A biofilm is a closely packed microbe community that latches onto surfaces and then secretes sticky polymers that it uses to surround itself. As the bacterium grows under the biofilm layer, organic acid byproducts attack the tooth enamel, resulting in cavities. The new findings are most significant for the S. mutans because it only causes cavities when it attaches to a tooth or when it gets into the biofilm on a tooth's surface.
When naturally present species are not wiped out, the oral microbiome, or friendly bacteria, is better preserved. Ideally you want to simply weaken the ability of the bacteria to cause disease. There are a number of common diseases including asthma, cystic fibrosis, and ulcerative colitis that are linked to issues with mucin production. This correlation offers further evidence that mucins have an active role in guarding the host from pathogens and keeping a healthy microbial environment.