Arginine-induced metabolomic perturbation in Streptococcus mutans

Arginine and sodium fluoride affect the microbial composition and reduce biofilm metabolism and enamel mineral loss in an oral microcosm model

OBJECTIVE

To assess the effects of arginine, with or without sodium fluoride (NaF; 1,450 ppm), on saliva-derived microcosm biofilms and enamel demineralization.

METHODS

Saliva-derived biofilms were grown on bovine enamel blocks in 0.2 % sucrose-containing modified McBain medium, according to six experimental groups: control (McBain 0.2 %); 2.5 % arginine; 8 % arginine; NaF; 2.5 % arginine with NaF; and 8 % arginine with NaF. After 5 days of growth, biofilm viability was assessed by colony-forming units counting, laser scanning confocal microscopy was used to determine biofilm vitality and extracellular polysaccharide (EPS) production, while biofilm metabolism was evaluated using the resazurin assay and lactic acid quantification. Demineralization was evaluated by measuring pH in the culture medium and calcium release. Data were analyzed by Kruskal-Wallis' and Dunn's tests (p < 0.05).

RESULTS

8 % arginine with NaF showed the strongest reduction in total streptococci and total microorganism counts, with no significant difference compared to arginine without NaF. Neither 2.5 % arginine alone nor NaF alone significantly reduced microbial counts compared to the control, although in combination, a reduction in all microbial groups was observed. Similar trends were found for biofilm vitality and EPS, and calcium released to the growth medium.

CONCLUSIONS

8 % Arginine, with or without NaF, exhibited the strongest antimicrobial activity and reduced enamel calcium loss. Also, NaF enhanced the effects of 2.5 % arginine, yielding similar results to 8 % arginine for most parameters analyzed.

CLINICAL SIGNIFICANCE

The results provided further evidence on how arginine, with or without NaF, affects oral microcosm biofilms and enamel mineral loss.

Metabolomics reveals high fructose-1,6-bisphosphate from fluoride-resistant Streptococcus mutans

BACKGROUND

Fluoride-resistant Streptococcus mutans (S. mutans) strains have developed due to the wide use of fluoride in dental caries prevention. However, the metabolomics of fluoride-resistant S. mutans remains unclear.

OBJECTIVE

This study aimed to identify metabolites that discriminate fluoride-resistant from wild-type S. mutans.

MATERIALS AND METHODS

Cell supernatants from fluoride-resistant and wild-type S. mutans were collected and analyzed by liquid chromatography-mass spectrometry. Principal components analysis and partial least-squares discriminant analysis were performed for the statistical analysis by variable influence on projection (VIP > 2.0) and p value (Mann-Whitney test, p < 0.05). Metabolites were assessed qualitatively using the Human Metabolome Database version 2.0 ( http://www.hmdb.ca ), or Kyoto Encyclopedia of Genes and Genomes ( http://www.kegg.jp ), and Metaboanalyst 6.0 ( https://www.metaboanalyst.ca ).

RESULTS

Fourteen metabolites differed significantly between fluoride-resistant and wild-type strains in the early log phase. Among these metabolites, 5 were identified. There were 32 differential metabolites between the two strains in the stationary phase, 13 of which were identified. The pyrimidine metabolism for S. mutans FR was matched with the metabolic pathway.

CONCLUSIONS

The fructose-1,6-bisphosphate concentration increased in fluoride-resistant strains under acidic conditions, suggesting enhanced acidogenicity and acid tolerance. This metabolite may be a promising target for elucidating the cariogenic and fluoride resistant mechanisms of S. mutans.

Addition of cariogenic pathogens to complex oral microflora drives significant changes in biofilm compositions and functionalities

BACKGROUND

Dental caries is a microbe and sugar-mediated biofilm-dependent oral disease. Of particular significance, a virulent type of dental caries, known as severe early childhood caries (S-ECC), is characterized by the synergistic polymicrobial interaction between the cariogenic bacterium, Streptococcus mutans, and an opportunistic fungal pathogen, Candida albicans. Although cross-sectional studies reveal their important roles in caries development, these exhibit limitations in determining the significance of these microbial interactions in the pathogenesis of the disease. Thus, it remains unclear the mechanism(s) through which the cross-kingdom interaction modulates the composition of the plaque microbiome. Here, we employed a novel ex vivo saliva-derived microcosm biofilm model to assess how exogenous pathogens could impact the structural and functional characteristics of the indigenous native oral microbiota.

RESULTS

Through shotgun whole metagenome sequencing, we observed that saliva-derived biofilm has decreased richness and diversity but increased sugar-related metabolism relative to the planktonic phase. Addition of S. mutans and/or C. albicans to the native microbiome drove significant changes in its bacterial composition. In addition, the effect of the exogenous pathogens on microbiome diversity and taxonomic abundances varied depending on the sugar type. While the addition of S. mutans induced a broader effect on Kyoto Encyclopedia of Genes and Genomes (KEGG) ortholog abundances with glucose/fructose, S. mutans-C. albicans combination under sucrose conditions triggered unique and specific changes in microbiota composition/diversity as well as specific effects on KEGG pathways. Finally, we observed the presence of human epithelial cells within the biofilms via confocal microscopy imaging.

CONCLUSIONS

Our data revealed that the presence of S. mutans and C. albicans, alone or in combination, as well as the addition of different sugars, induced unique alterations in both the composition and functional attributes of the biofilms. In particular, the combination of S. mutans and C. albicans seemed to drive the development (and perhaps the severity) of a dysbiotic/cariogenic oral microbiome. Our work provides a unique and pragmatic biofilm model for investigating the functional microbiome in health and disease as well as developing strategies to modulate the microbiome. Video Abstract.

Correlation between caries activity and salivary microbiota in preschool children

Early childhood caries (ECC) is the most common chronic infectious oral disease in preschool children worldwide. It is closely related to the caries activity (CA) of children. However, the distribution characteristics of oral saliva microbiomes in children with different CA are largely underexplored. The aim of this study was to investigate the microbial community in saliva of preschool children with different CA and caries status, and to analyze the difference of microbial community in saliva of children with different CA and its correlation with ECC. Subjects were divided into 3 groups based on the Cariostat caries activity test: Group H, high CA (n=30); Group M, medium CA (n = 30); Group L, low CA (n=30). Questionnaire survey was used to explore the related influencing factors of CA. According to the caries status (on the basis of decayed mising filled teeth), these subjects were divided into caries-free group (dmft=0, n=19), caries-low group (0 < dmft ≤ 4, n=27) and caries-high group (dmft > 4, n=44). Microbial profiles of oral saliva were analyzed using 16S rRNA gene sequencing. There were significant differences in the microbial structure (P < 0.05). Scardovia and Selenomonas were the biomarkers of both H group and high caries group. The genus Abiotrophia and Lautropia were the biomarkers of both the L group and the low caries group, while the Lactobacillus and Arthrospira spp. were significantly enriched in the M group. The area under the ROC curve of the combined application of dmft score, age, frequency of sugary beverage intake, and the genus Scardovia, Selenomonas, and Campylobacter in screening children with high CA was 0.842. Moreover, function prediction using the MetaCyc database showed that there were significant differences in 11 metabolic pathways of salivary microbiota among different CA groups. Certain bacteria genera in saliva such as Scardovia and Selenomonas may be helpful in screening children with high CA.

Genomic and phenotypic characterization of Streptococcus mutans isolates suggests key gene clusters in regulating its interaction with Streptococcus gordonii

Streptococcus mutans (S. mutans) is one of the primary pathogens responsible for dental caries. Streptococcus gordonii (S. gordonii) is one of the early colonizers of dental plaque and can compete with S. mutans for growth. In the present analysis, we explored key target genes against S. gordonii in S. mutans using 80 S. mutans clinical isolates with varying capabilities against S. gordonii. A principal coordinate analysis revealed significant genetic diversity differences between antagonistic and non-antagonistic groups. Genomic comparisons revealed 33 and 61 genes that were, respectively, positively and negatively correlated with S. mutans against S. gordonii, with RNA-sequencing (RNA-seq) highlighting 11 and 43 genes that were, respectively, upregulated and downregulated in the antagonistic group. Through a combination of these results and antiSMASH analysis, we selected 16 genes for qRT-PCR validation in which the expression levels of SMU_137 (malate dehydrogenase, mleS), SMU_138 (malate permease, mleP), SMU_139 (oxalate decarboxylase, oxdC), and SMU_140 (glutathione reductase) were consistent with RNA-seq results. SMU_1315c-1317c (SMU_1315c transport-related gene) and SMU_1908c-1909c were, respectively, downregulated and upregulated in the antagonistic group. The expression patterns of adjacent genes were closely related, with correlation coefficient values greater than 0.9. These data reveal new targets (SMU_137–140, SMU_1315c-1317c, and SMU_1908c-1909c) for investigating the critical gene clusters against S. gordonii in S. mutans clinical isolates.

AhrC Negatively Regulates Streptococcus mutans Arginine Biosynthesis

Streptococcus mutans is a primary cariogenic pathogen in humans. Arginine metabolism is required for bacterial growth. In S. mutans, however, the involvement of transcription factors in regulating arginine metabolism is unclear. The purpose of this study was to investigate the function and mechanism of ArgR family transcription factors in S. mutans. Here, we identified an ArgR (arginine repressor) family transcription factor named AhrC, which negatively regulates arginine biosynthesis and biofilm formation in S. mutans. The ahrC in-frame deletion strain exhibited slow growth and significantly increased intracellular arginine content. The strain overexpressing ahrC showed reduced intracellular arginine content, decreased biofilm biomass, reduced production of water-insoluble exopolysaccharides (EPS), and different biofilm structures. Furthermore, global gene expression profiles revealed differential expression levels of 233 genes in the ahrC-deficient strain, among which genes related to arginine biosynthesis (argJ, argB, argC, argD, argF, argG, argH) were significantly upregulated. In the ahrC overexpression strain, there are 89 differentially expressed genes, mostly related to arginine biosynthesis. The conserved DNA patterns bound by AhrC were identified by electrophoretic mobility shift assay (EMSA) and DNase I footprinting. In addition, the analysis of β-galactosidase activity showed that AhrC acted as a negative regulator. Taken together, our findings suggest that AhrC is an important transcription factor that regulates arginine biosynthesis gene expression and biofilm formation in S. mutans. These findings add new aspects to the complexity of regulating the expression of genes involved in arginine biosynthesis and biofilm formation in S. mutans.

IMPORTANCE Arginine metabolism is essential for bacterial growth. The regulation of intracellular arginine metabolism in Streptococcus mutans, one of the major pathogens of dental caries, is unclear. In this study, we found that the transcription factor AhrC can directly and negatively regulate the expression of N-acetyl-gamma-glutamyl-phosphate reductase (argC), thus regulating arginine biosynthesis in S. mutans. In addition, the ahrC overexpression strain exhibited a significant decrease in biofilm and water-insoluble extracellular polysaccharides (EPS). This study adds new support to our understanding of the regulation of intracellular arginine metabolism in S. mutans.