Shifts in Composition and Activity of Oral Biofilms After Fluoride Exposure


Oral diseases are biofilm-mediated diseases caused by imbalances in the ecology of resident microflora. Among them, dental caries (tooth decay) is considered the most common disease worldwide, and toothbrushing, which physically eliminates the oral biofilm, is the most widespread prevention strategy. Although it is well established that fluoride increases enamel resistance to acidic pH and promotes tooth remineralization, its effect on the biofilm bacterial communities’ composition and metabolism is not fully understood. We have grown in vitro oral biofilms and used 16S rRNA Illumina sequencing to study the effect of fluoride on DNA- and RNA-based bacterial populations. In addition, a metatranscriptomic approach has also been performed, in which total RNA has been sequenced to study gene expression profiles in the presence/absence of 500 ppm sodium fluoride. Our data show a lower pH drop and a clear shift in total and metabolically active bacterial composition after fluoride exposure. Streptococcus oralis was the species most affected, with a 10-fold reduction in both DNA and RNA samples, whereas Rothia mucilaginosa underwent an 8-fold increase in the DNA and S. salivarius a 4- and 5-fold increase in the RNA and DNA samples, respectively. The metatranscriptomes indicated that fluoride exposure induced a dramatic shutdown of sugar metabolism, including significant under-expression of different sugar transporters, fucosidases, and a pyruvate oxidase, among others. The reduction in saccharolytic organisms and the inhibition of sugar fermentation pathways by fluoride may therefore be considered instrumental for the beneficial effect of fluoride-containing oral hygiene products.

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We especially appreciate the involvement of Núria Jiménez and Javier Pons in the massive sequencing and bioinformatic analyses, respectively.


The research leading to these results has received funding from the Spanish Government under grant agreement RTI 2018-102032-B-100.

Author information




AM and ALL conceived the work. ALL worked in the designing and performance of the experiments. AM and ALL made the analyses and interpretation of data and wrote the manuscript.

Corresponding author

Correspondence to A. Mira.

Ethics declarations

All saliva donors signed an informed consent form and the protocol was approved by the Ethics Committee of the Valencian Public Health Authority (FISABIO-DGSP).

Conflict of Interest

The authors declare that they have no conflict of interest.

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López-López, A., Mira, A. Shifts in Composition and Activity of Oral Biofilms After Fluoride Exposure. Microb Ecol (2020).

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  • Dental caries
  • Fluoride
  • High-throughput sequencing
  • Biofilm composition
  • Differential gene expression