Have you ever had a cavity before? The CDC estimates that around 21% of adults between ages 20 and 64 have at least 1 untreated cavity (see more “fun” dental facts here, then go brush your teeth).
The human body serves as a wonderful host to a collection of dynamic microbe food-webs that directly affect our health in positive, negative, and neutral ways. Lactobacillus acidophilus helps us break down the food we eat, Staphylococcus epidermidis is mostly neutral on your skin, and depending on the strain Escherichia coli can either help the human digestive system…or give you food poisoning. There is a whole world of bacteria literally at our fingertips, and it is so diverse! Like a scene straight out of Hayao Miyazaki’s Nausicaä of the Valley of the Wind.
Image from Nausicaä of the Valley of the Wind (1984) directed by Hayao Miyazaki
Caries (not to be confused with cavities) are a wearing down of the enamel and dentin on your teeth, causing the tooth to decay and erode into cavities. The culprit? Cariogenic bacteria! Your mouth is the second most diverse microbiome (beaten out by the gut microbiome), with a collection of bacteria, fungi, viruses, and protozoa that is completely unique to your oral biosphere (learn more about the oral biosphere here).
If the thought of being the host to a microbe utopia disturbs you, and you are about to go out and buy Listerine to try and kill all of the microbes in your mouth, let me tell you about a research article that I read recently about Streptococcus mutans, just one of the (many) oral microbes that is naturally found in the mouth.
S. mutans is a gram positive, round, cariogenic bacteria that makes its mark in the overall oral biosphere by contributing to an oral biofilm (spoiler alert: biofilms are not movies that bacteria make, but you can learn more about them here).
The oral biofilm refers to the communities of bacteria that hang around the surface of our teeth and the gaps in between the gums, and they produce a layer of polymers that better let them adhere to the stones in our mouths. For S. mutans, it is well-studied for its glucan-dependent biofilm formation that progresses dysbiosis, a state of unbalance where there are too many certain microbes and not enough of others. When S. mutans is in a diverse biofilm community, it is balanced out by surrounding bacteria, and isn’t able to form the same strong biofilm that is associated with virulence. S. mutans becomes more of an issue for human health when it has access to an abundance of sucrose and little competition. Essentially, our oral biofilm, like any other environment, benefits from diversity and balance.
Glucan is a type of polysaccharide made by a class of enzymes known as glycosyltransferases (GTFs) that turn sugars in their environment into long, sticky chains. These chains of glucan build up in areas with more sucrose and they form an adhesive matrix that is incredibly resistant to breakages, a biofilm. Each glucan chain is like a piece of duct tape, not very strong on its own, but if there’s enough you could adhere someone to a wall (an elementary school showed this was possible, see the story here), or a cell to the surface of your tooth.
But S. mutans don't want to be anchored anywhere in your mouth, they want to be in the best possible area, where it has the most food and least competition. This pathway for glucan production and biofilm formation is one that scientists are very keen on trying to understand. The health of our oral biosphere actually plays a big role in fighting off infections elsewhere in the body, so understanding why the biosphere might become unbalanced is important. When S. mutans adheres to our teeth, the biofilm also makes a nice landing spot for other microbes. S. mutans is an early colonizing bacteria (learn more here), a microbe trendsetter, and one microbe that follows S. mutans is C. albicans.
C. albicans is a fungi that doesn’t have as much success latching onto teeth without S. mutans present. The environment of the mouth is actually a really brutal one: saliva contains antimicrobial enzymes, the tongue moves around and the space is crowded with tons of other microbes that all want a spot. That’s why C. albicans and S. mutans do the hard work of resisting the harsh environment conditions and laying down a strong matrix, together. S. mutans form a dense biofilm and C. albicans forms hyphae that strengthen the biofilm.
While scanning through a transposon library of mutants, Treerat et. al (2024) came across a fascinating phenotype when they removed a gene encoding for a homolog of the protein Prp (Phage-related Ribosomal Protease). Prps within bacteria are responsible for cleaving the N-terminal of the ribosomal protein L27, helping to assemble the functional ribosome. Interestingly, they found that S. mutans that lacked the SMU_848 gene, had droplets of glucan forming on the surface of the S. mutans biofilm!
The discovery of this phenotype was unintentional, but the group recognized it had potential for helping to uncover an important pathway that leads to the formation of S. mutans biofilms. To investigate this gene further, the group purposefully removed the SMU_848 gene (ΔSMU_848), stained the glucan to be fluorescent, then observed and quantified the glucan production between S. mutans with or without the SMU_848 gene (Figure 3A). Doing further analysis on the contents of the droplets, the group confirmed that this mutant S. mutans, when grown in the presence of sucrose, was producing more glucan (Figure 3B); however, the glucan produced was a mix of insoluble and soluble glucan (Figure 2 in the paper). Soluble glucan is unable to be formed into a biofilm.
Figure 3 from Treerat et. al 2024 paper. A) fluorescent imaging of wild type S. mutans and S. mutans lacking SMU_848 gene (ΔSMU_848). Cells were grown in the presence of sucrose and glucan was stained blue to fluoresce. B) Intensity of fluorescence was used to quantify the difference in the amount of glucan produced.
But that begged the question, would S. mutans lacking the SMU_848 gene still be able to form interkingdom interactions with C. albicans? If S. mutans lacking SMU_848 were producing more glucan, would that mean the bacteria and fungi could interact better and therefore become more virulent?
To test this, the group grew C. albicans that contained Green Fluorescent Protein (GFP) in the presence of S. mutans strains with or without the SMU_848 gene (Figure 7). S. mutans glucan within biofilms was then stained blue to appear visible.
Figure 7. Fluorescent imaging of C. albicans (fluorescent strain PHL2, denoted C. a PHL2, shines green due to GFP) co-inoculated with S. mutans with or without SMU_848 gene. A) wild type S. mutans with C. a PHL2 forms a biofilm with C. a PHL2 forming hyphae. B) S. mutans without SMU_848 have hyper glucan production and C. a PHL2 has noticeably less hyphae formation.
The results are stunning, even if you can’t appreciate the interkingdom friendships this bacterium and fungus make, you have to marvel at the beauty of fluorescent imaging!
Overally, the group saw a drastic reduction in the ability of C. albicans to form hyphae when grown with S. mutans lacking SMU_848, despite the increase in glucan production. With this, the group concluded that the SMU_848 gene seems to play a direct role in the pathway that leads to glucan production and an indirect role in interkingdom interactions for S. mutans.
With the characterization of this gene and the pathways it affects, I’m curious to see if there’s any potential in the targeting of SMU_848 to disrupt biofilm formation. When microbes like S. mutans and C. albicans form thick biofilms, they increase their chances of becoming antibiotic/antimicrobial resistant. Preventing the formation of a dysbiotic biofilm becomes key to slowing the prevalence of resistant microbes in our very delicate environment.
You might be asking yourself, why not just buy a super strong antimicrobial mouthwash and wipe those suckers out for good? Well, for starters, using antimicrobial products too frequently can increase the number of antimicrobial resistant microbes. Every time you use an antimicrobial, the vast majority of your microbes are wiped out, both the good and the bad. The biofilms microbes produce make it harder for antimicrobials to reach the actual microbe. Additionally, antimicrobials force microbes to either adapt and become resistant, or die. And if only antimicrobial resistant microbes are left to repopulate the oral biosphere, then the following generations are also going to be resistant.
Our best defense against the oral biosphere from becoming dysbiotic is by taking care of our oral environment. Bacteria and other microbes are not always something to be feared, they are contributors to our ecosystem, and we can help them and ourselves by taking care of our oral health. Brushing your teeth, gums, and tongue, flossing, and drinking water can help to balance the oral biosphere. So tonight, when you go to brush your teeth, just know you are a participant in a large microscopic world.
About the Author:
Thank you for reading my post! My name is Jen and I am a soon-to-be Mount Holyoke alumnae, class of 2025, who loves bacteria, human-microbe interactions, and art. I believe that microbes, much like humans, are wonderfully diverse and sometimes overlooked, so I am always looking for articles to deepen my understanding of the big, small world around me. I have been studying protein interactions that lead to spore formation in Bacillus subtilis for 2.5 years at MHC’s Camp Lab!
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