Actinomyces spp. gene expression in root caries lesions

Background: studies of the distribution of Actinomyces spp. on carious and non-carious root surfaces have not been able to confirm the association of these bacteria with root caries, although they were extensively implicated as a prime suspect in root caries. Objective: the aim of this study was to observe the gene expression of Actinomyces spp. in the microbiota of root surfaces with and without caries. Design: the oral biofilms from exposed sound root surface (SRS; n=10) and active root caries (RC; n=30) samples were collected. The total bacterial RNA was extracted and the mRNA was isolated. Samples with low RNA concentration were pooled, yielding a final sample size of SRS=10 and RC=9. cDNA libraries were prepared and sequenced on the Illumina Hi-Seq2500. Sequence reads were mapped to eight Actinomyces genomes. Count data were normalized using DESeq2 to analyse differential gene expression applying the Benjamini-Hochberg correction (FDR<0.001). Results: Actinomyces spp. had similar numbers of reads (U-Mann Whitney test; p>0.05), except for Actinomyces OT178 (p=0.001) and A. gerencseriae (p=0.004), which had higher read count in the SRS. Genes that code for stress proteins (clp, dnaK and groEL), enzymes of glycolysis pathways (including, enolase and phosphoenolpyruvate carboxykinase), adhesion (Type-2 fimbrial and collagen-binding protein) and cell growth (EF-Tu) were highly, but not differentially (p>0.001) expressed in both groups. Genes with the most significant up-regulation in RC were those coding for hypothetical proteins and uracil DNA glycosylase (p=2.61E-17). The gene with the most significant up-regulation in SRS was a peptide ABC transporter substrate-binding protein (log2FC= -6.00, FDR= 2.37E-05). Conclusion: there were similar levels of Actinomyces gene expression in both sound and carious root biofilms. These bacteria can be commensal in root surface sites, but may be cariogenic due to survival mechanisms allowing them to exist in acid environment and metabolize sugars saving energy.