“We’ve known for a long time that if you want to find Strep salivarius, you have to look on the tongue,” says Dr. Mark Welch. With genetic sequencing, methods became more precise, and researchers found that specific bacteria can be found at different sites such as on the tongue, teeth, and gums. “There’s some that only live on the roof of the mouth, for example,” she says. The Hedgehog The standard model for plaque formation, as explained by Drs. Paul Kolenbrander and Robert Palmer, postulates that when people brush their teeth, enamel is clean and barren of microbial life. Within seconds, enamel is coated with salivary pellicle, a glycoprotein, as the mucins in the saliva bind to the enamel. The salivary pellicle has proteins and carbohydrates to which certain bacteria can then bind. “You can think of it as what happens to an ecosystem after a forest fire, where a few plants start to re-colonize the land, then other plants can come in,” says Dr. Mark Welch. “The bacteria that can bind directly to salivary pellicle are the early colonizers and most of those are streptococcus and actinomyces. Then other bacteria bind to the early colonizers and build up in succession.” The standard model suggests that there are early colonizing bacteria, a bridging bacterium called Fusobacterium nucleatum, and then late colonizers that can cause periodontitis. “But what we saw was a different bacterium providing much of the structure,” says Dr. Mark Welch. The hedgehog structure that her images capture has a key bacterium, Corynebacterium, that makes up the core of the enormous bush of filaments. “Corynebacterium seemed to start at the base of the biofilm and grow all the way out to the tips and other bacteria were stuck to its tips,” she says. Fusobacterium nucleatum created an intermediate layer but it didn’t have structural continuity. Other bacteria live within the Corynebacterium structure and streptococci live around the outside. The hedgehog structure seems to be typical. “We saw these hedgehog structures in every person that we sampled more than once,” says Dr. Mark Welch. As well, the bacteria in the hedgehog structure are all intertwined. In plaque samples from healthy people, there are never big clusters of only one kind of bacteria. “They need each other,” she says. “In a healthy mouth, the bacteria live in balance, like in any ecosystem.” The Oral Microbiome in Dental Practice During a dental exam, Dr. Mark Welch was given a ridged plastic tongue scraper and told she had a furry tongue. “Then, of course, I felt compelled to look at what was on the tongue with our microscopy method,” she said. “And we found these amazing structures that are completely different from dental plaque.” On the filiform papillae of the tongue, there is a core of human epithelial cells, then bacteria grow out from them in columns, triangles, and pyramids. There are hypotheses that tongue bacteria may have an influence on the transformation of nitrate. Dr. Gena Tribble has published research that suggests that the microbiome of people who clean their tongues, at least occasionally, are different from those who never do.5 “Tongue cleaning does seem to shift the structure of the microbiome, probably in a positive direction,” says Dr. Mark Welch. There are more than 700 different bacteria in the oral microbiome database, but a sample of bacteria from any one person’s mouth will likely include only about 150 different types. “The general pattern is that we all have pretty much the same species of bacteria,” says Dr. Mark Welch. “And among them, there are some really major players that are almost always there, and each of them makes up like 2% or 5% of the community.” Though people have the same types of bacteria in their mouths, each person has their own strains. “You can tell people apart by their oral microbiomes,” says Dr. Mark Welch. “If I tested everyone at a party, and then tested them a year later, I’d be able to identify who was who by the strains, their abundance, and the proportion of different bacteria. It’s quite stable over time.” Dr. Mark Welch did a study where she sampled the oral bacteria of people who had been married for 10 years or longer. “We didn’t tell the person analyzing our data who Therearemore than700different bacteria in the oral microbiome database, but a sample of bacteria from any one person’s mouth will likely include only about 150 different types. 28 | 2024 | Issue 1 Issues and People
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