Slimy water bottles? A review of oral biofilms

Have you ever wondered why sometimes, when you run your tongue along your teeth it feels slimy? Similar to sliding your finger on the inside of a water bottle you haven’t washed in a while? What could your teeth and the inside of a slimy water bottle have? The answer is biofilms: local environments where a collection of bacterial species exhibit mutualistic behaviors to create an optimal habitat for protection and growth (Klein et al. 2015).

The primary bacteria present in oral biofilms is Streptococcus mutans which produces the enzyme glucosyltransferase which turns glucose into extracellular polymeric substance (EPS) which is a sticky-slimey substance that provides the structure of the biofilms that the bacteria uses for protection (Bowen & Koo 2011). This oral biofilm is more predominantly known as plaque which brushing attempts to disrupt before tartar formation where plaque hardens into a complex with dissolved calcium and phosphate ions known as dental calculus (Samaranayake & Matsubara 2017).

Where do the calcium and phosphate ions that form a complex with EPS come from? They are the remnants of the formation of carious lesions, which we call cavities, small holes in the hydroxyapatite crystal lattice structure that makes up our tooth enamel(Klein et al. 2015). When there is a large sugar influx, the bacteria hiding in the biofilm metabolises the glucose using the lactic acid pathway, turning sugars into lactic acid, which then diffuses into the hydroxyapatite crystal structure and protonates phosphate and calcium ions dissolving them out of the crystal lattice (Figure 1). The result is small holes in the hydroxyapatite that weaken it and lead to dental caries, also known as cavities, and helps EPS turn into its tartarated form (Fox et al. 1978).

The physical process of brushing disrupts the formation of dental calculus, so where does toothpaste come in? Toothpaste’s active ingredient is fluoride, also present in drinking water, which amplifies the remineralisation process. Fluoride diffuses into hydroxyapatite and forms a new complex where calcium and phosphate ions are much more closely bound to the lattice structure and allows for the rebinding of calcium and phosphate due to higher affinity with the new fluoride-hydroxyapatite structure resulting in remineralisation (Shi et al. 2023).

An important consideration when discussing hydroxyapatite dissolution is critical pH, the pH at which the acid is able to dissolve the calcium and phosphate ions out of solution. In typical hydroxyapatite the critical pH is 5.5, however fluoride-hydroxyapatite has a much lower critical pH of 4.5, which is very difficult for bacterially produced lactic acid to reach. Additionally, although discussion often focuses on critical pH, time below critical pH is an equally important consideration, as the longer the environment remains acidic, the more demineralization occurs (Bowen 2016).

Finally, one of the most interesting physical laws that heavily impacts all that has been discussed above is diffusion. Although touched on briefly, the importance of diffusion and kinetics of acids and ions in hydroxyapatite’s crystal lattice can not be exaggerated, it is the governing force behind almost all of oral health.

References

Klein MI, Hwang G, Santos PHS, Campanella OH, Koo H. Streptococcus mutans-derived extracellular matrix in cariogenic oral biofilms. Front Cell Infect Microbiol. 2015;5. doi:10.3389/fcimb.2015.00010

Bowen WH, Koo H. Biology of streptococcus mutans-derived glucosyltransferases: role in extracellular matrix formation of cariogenic biofilms. Caries Res. 2011;45(1):69-86. doi:10.1159/000324598

Samaranayake L, Matsubara VH. Normal oral flora and the oral ecosystem. Dental Clinics of North America. 2017;61(2):199-215. doi:10.1016/j.cden.2016.11.002

Fox JL, Higuchi WI, Fawzi MB, Wu MS. A new two-site model for hydroxyapatite dissolution in acidic media. Journal of Colloid and Interface Science. 1978;67(2):312-330. doi:10.1016/0021-9797(78)90016-4

Shi W, Liu Z, Yamada I, et al. Effective thermal diffusion of eu(Iii) and f ions into hydroxyapatite nanoparticles by citric acid coordinative mediation. ACS Appl Nano Mater. 2023;6(4):2346-2355. doi:10.1021/acsanm.2c04358

Bowen WH. Dental caries – not just holes in teeth! A perspective. Molecular Oral Microbiology. 2016;31(3):228-233. doi:10.1111/omi.12132

İnan S. Inorganic ion exchangers for strontium removal from radioactive waste : a review. J Radioanal Nucl Chem. 2022;331(3):1137-1154. doi:10.1007/s10967-022-08206-3