Influences of pH and Iron Concentration on the Salivary Microbiome in Individual Humans with and without Caries

Abstract

This study aimed to identify the differences in the oral microbial communities in saliva from patients with and without caries by performing sequencing with the Illumina MiSeq platform, as well as to further assess their relationships with environmental factors (salivary pH and iron concentration). Forty-three volunteers were selected, including 21 subjects with and 22 without caries, from one village in Gansu, China. Based on 966,255 trimmed sequences and clustering at the 97% similarity level, 1,303 species-level operational taxonomic units were generated. The sequencing data for the two groups revealed that (i) particular distribution patterns (synergistic effects or competition) existed in the subjects with and without caries at both the genus and species levels and (ii) both the salivary pH and iron concentration had significant influences on the microbial community structure.

Importance: The significant influences of the oral environment observed in this study increase the current understanding of the salivary microbiome in caries. These results will be useful for expanding research directions and for improving disease diagnosis, prognosis, and therapy.

Keywords: caries; iron; microbial communities; pH; saliva.

FIG 1

Comparison of bacterial taxonomy (>1% relative abundance) of samples from individuals with (C) and without (H) caries at the phylum (a), genus (b), and species (c) levels. *, P < 0.05.

FIG 2

Non-caries- and caries-associated genera (a) and species (b), as determined based on the LEfSe method. The nonparametric factorial Kruskal-Wallis rank sum test was used to detect the taxa with significant differential relative abundance between the healthy group and the caries group at a significance level of 0.05. The relative abundance (the histograms) and prevalence (the dotted lines) of each species are also shown in panel b. Green and red indicate data from healthy and caries groups, respectively.

FIG 3

Heatmaps displaying genus distribution patterns for the healthy (a) and caries (b) groups. Increasing values are translated into colors from red to green. Trees were clustered based on the similarity of the Schoener's values. (c) The cooccurrence probabilities based on the Schoener's index were divided into 13 intervals, including <0, 0, 0 to 0.1, 0.1 to 0.2, 0.2 to 0.3, 0.3 to 0.4, 0.4 to 0.5, 0.5 to 0.6, 0.6 to 0.7, 0.7 to 0.8, 0.8 to 0.9, 0.9 to 1, and 1. The frequency of the Schoener's index falling into each interval was calculated in both the caries group and the healthy group. The number of involved genera at the different intervals and a global comparison of the Schoener's index between the two groups (using a t test) are also shown within the plot. (d) All pairs with Schoener's index greater than 0.8.

FIG 4

Salivary pH and iron concentration. Significant differences in the salivary pH (a) and iron concentration (b) between samples from individuals with (C) and without (H) caries. ***, P < 0.001; **, P < 0.01. (c) Significant correlations between the salivary pH and iron concentration. (d) Significant correlations between the salivary pH and DMFT index.

FIG 5

Redundancy analysis (RDA) of the significant influences of the salivary pH, iron concentration, and DMFT index on the bacterial community structure at the genus level. Colors indicate the non-caries-associated (underlined) and caries-associated genera. These distinct genera with high correlations with one or more variables are explained by the Venn diagram inserted in the plot: yellow, blue, and red indicate genera with high correlations (>0.6) with iron concentration, pH, and DMFT, respectively.