Maturation of the oral microbiota during primary teeth eruption: a longitudinal, preliminary study

Abstract

Introduction: Oral microbiota that established in the early years of life may influence the child's oral health in the long term. Until now, no consensus is reached about whether the development of the oral microbiota is more related with age increase or more with teeth eruption.

Objective: To analyze the microbiota development of both saliva and supragingival plaque during the gradual eruption of primary teeth in caries-free infants and toddlers.

Methods: Saliva and plaque samples were collected at five and four dentition states, respectively, and were identified by bacterial 16S rRNA gene sequencing.

Results: During the longitudinal observation, the saliva ecosystem seemed more complex and dynamic than the plaque, with larger bacteria quantity and more significantly varied species over time. About 70% of the initial colonized OTUs in plaque persisted until the completion of the primary dentition. Transient bacteria were mostly detected in the early saliva and plaque microbiota, which came from the environment and other sites of the human body. Microbial diversity in both saliva and plaque varied greatly from pre-dentition to full eruption of eight anterior teeth, but not during the eruption of primary molars.

Conclusion: Oral bacterial development follows an ordered sequence during the primary teeth eruption. 'Fully eruption of all primary anterior teeth' is a critical stage in this process.

Keywords: Saliva; cohort studies; growth/development; microbiota; plaque/plaque biofilms.

Figure 1.

Flowchart of sampling in this study. S1: Infants are around 4–5 months old and with no teeth eruption. Saliva samples are obtained. S2: Infants are around 6 months old, with only two lower primary central incisors erupted. S3: Infants are around 9 months old, with all the upper and lower primary incisors (8 teeth) erupted. S4: Toddlers are around 14–19 months old, with all the first primary molars (16 teeth) erupted. S5: Toddlers are around 24–36 months old, with all the second primary molars (20 teeth) erupted. Both saliva and supragingival plaque samples are obtained at each dentition state from S2 to S5.

Figure 2.

The community structure and relative abundance variation at the genus level. Taxonomic distribution of the relative abundance of major genera in saliva (a) and supragingival plaque (b) samples. Each column represents the relative abundance of microbial components in a single sample. (c) The relative abundances of major bacterial genera in saliva (blue) and supragingival plaque (red) samples over time. Significant differences are indicated by asterisks: *P < 0.05, **P < 0.01. Differences over time within the respective sample type is analyzed using the Kruskal–Wallis test, with P-value correction by the Bonferroni method. At each time point, between-group comparison is done by the Wilcoxon method.

Figure 3.

Relative abundance of bacterial species with significant variation over time in saliva and plaque. Species with significant variation over time (P < 0.05) that could be detected from all the sampling dentition states are shown. Species with (a) increasing and (b) decreasing trend in saliva over time. Species with (c) increasing and (d) varying trend in plaque over time. Intra-group differences in both saliva and plaque groups are analyzed using the Kruskal–Wallis test, with P-value correction by the Bonferroni method.

Figure 4.

Comparison of α and β diversity of the oral microbiota over time. The α diversity of microbial profiles in saliva (blue boxes) and plaque (red boxes) of children over time. (a) The α diversity variation within each group in bacterial richness index of Chao1, bacterial phylogenetic diversity index of PD whole tree, and bacterial community diversity indexes of Shannon and Simpson. (b) Principal coordinate analysis (PCoA) plots for saliva (left) and supragingival (right) samples (principal coordinates 1 and 2) based on the unweighted UniFrac distance. Each point represents the microbiota composition of one sample. Saliva microbiota of infants in S1 (left) and plaque microbiota of infants in S2 (right) differed significantly from the older groups.

Figure A1.

The community structure and relative abundance variation at the phylum level. Taxonomic distribution of the relative abundance of reads (%) at the phylum level in saliva (a) and supragingival plaque (b) samples. (c) The relative abundances variation of major bacterial phyla in saliva (blue) and supragingival plaque (red) samples over time. Significant differences are indicated by asterisks: *P < 0.05, **P < 0.01. Differences over time within the respective sample type are analyzed using the Kruskal–Wallis test, with P-value correction by the Bonferroni method. At each time point, between-group comparison is done by the Wilcoxon method.

Figure A2.

Venn diagrams depict the shared and unique OTUs within saliva (a) and supragingival plaque (b) groups over time. All OTUs are included.

Figure A3.

Venn diagrams depict the shared and unique OTUs within saliva and plaque samples at each state. (a) Venn diagrams at State 2, when only two lower primary incisors erupt. (b) Venn diagrams at State 3, when all eight primary incisors erupt. (c) Venn diagrams at State 4, when four primary first molars erupt. (d) Venn diagrams at State 5, when four primary second molars erupt. All OTUs are included.

Figure A4.

Comparison of OTU relative abundances at the species level between saliva and supragingival plaque. Differences in relative abundance are analyzed using the Wilcoxon rank-sum test, P < 0.05 is considered to reflect a significant difference. Not significant difference is not shown. (a) Comparison of relative abundances at State 3, when all eight primary incisors erupt. (b) Comparison of relative abundances at State 4, when four primary first molars erupt. (c) Comparison of relative abundances at State 5, when four primary second molars erupt.