Genetic Mutations That Confer Fluoride Resistance Modify Gene Expression and Virulence Traits of Streptococcus mutans

Abstract

Fluoride is an inorganic monatomic anion that is widely used as an anti-cariogenic agent for the control of caries development. The aims of this study were to identify the mutated genes that give rise to fluoride-resistant (FR) strains of the cariogenic pathogen Streptococcus mutans and explore how genetic alterations in the genome of an S. mutans FR strain optimize the metabolism(s) implicated in the expression of virulence-associated traits. Here, we derived an S. mutans FR strain from a wild-type UA159 strain by continuous shifts to a medium supplemented with increasing concentrations of fluoride. The FR strain exhibited a slow growth rate and low yield under aerobic and oxidative stress conditions and was highly sensitive to acid stress. Notably, microscopy observation displayed morphological changes in which the FR strain had a slightly shorter cell length. Next, using the sequencing analyses, we found six mutations in the FR genome, which decreased the gene expression of the phosphoenolpyruvate-dependent phosphotransferase system (PTS). Indeed, the ability to intake carbohydrates was relatively reduced in the FR strain. Collectively, our results provide evidence that the genetic mutations in the genome of the FR strain modulate the expression of gene(s) for carbon metabolism(s) and cellular processes, leading to diminished fitness with respect to virulence and persistence.

Influence of Fluoride-Resistant Streptococcus mutans Within Antagonistic Dual-Species Biofilms Under Fluoride In Vitro

“…The lower ratio of the fluoride-resistant strain within the dual-species biofilm without NaF compared to its wild strain might be partly attributed to the slow growth rate of the fluoride-resistant strain in our study (data not shown). The slow growth rate of the fluoride-resistant strain was consistent with previous reports, which might have resulted from bacterial-deficient carbohydrate uptake ( Liao et al., 2015 ; Lee et al., 2021 ). The ratio of either S. mutans or its fluoride-resistant dual-species was raised without NaF with the development of biofilm when compared between 24 and 48 h, which was confirmed by FISH and qRT-PCR.…”

“…At present, there is no unified conclusion about the variation in acid production ability of wild-type S. mutans compared with fluoride-resistant S. mutans . Some studies found that fluoride-resistant S. mutans had a weaker acid production ability, while others found it to be stronger when compared to its related wild strain, and further studies found that there was no significant difference between these two strains ( Eisenberg et al., 1985 ; Van Loveren et al., 1991 ; Hoelscher and Hudson, 1996 ; Cai et al., 2017 ; Lee et al., 2021 ). This diversity might be derived from different culture conditions and bacterial strains and induced by fluoride-resistant strains.…”

“…Some studies have reported the phenotypic characteristics of fluoride-resistant strains, such as the stability of fluoride resistance, fitness, growth, acidogenicity, and cariogenicity. These results showed that there are many significant differences in these phenotypic characteristics between the fluoride-resistant strains and their wild strains, such as higher fluoride resistance, higher acid tolerance, lower growth, and some controversial cariogenitic characteristics ( Van Loveren et al., 1991 ; Zhu et al., 2012 ; Liao et al., 2015 ; Cai et al., 2017 ; Liao et al., 2017 ; Liao et al., 2018 ; Lee et al., 2021 ). These genetic stability differences could be caused by genetic mutations, as revealed by gene sequencing ( Liao et al., 2015 ; Liao et al., 2018 , Lee et al., 2021 ).…”

“…These results showed that there are many significant differences in these phenotypic characteristics between the fluoride-resistant strains and their wild strains, such as higher fluoride resistance, higher acid tolerance, lower growth, and some controversial cariogenitic characteristics (Van Loveren et al, 1991;Zhu et al, 2012;Liao et al, 2015;Cai et al, 2017;Liao et al, 2017;Liao et al, 2018;Lee et al, 2021). These genetic stability differences could be caused by genetic mutations, as revealed by gene sequencing (Liao et al, 2015;Liao et al, 2018, Lee et al, 2021. Although the complete mechanism of S. mutans fluoride resistance needs to be further studied, some genes or genetic loci have been found to be responsible for the fluoride resistance of S. mutans (Liao et al, 2016;Men et al, 2016;Murata and Hanada, 2016;Tang et al, 2019;Lu et al, 2020;Yu et al, 2020).…”

Identification of the antibacterial action mechanism of diterpenoids through transcriptome profiling

“…Additionally, murB, htrA, SMU.753, and spxA regulated by LiaSR and SMU.862 regulated by BceRS were also upregulated. Lee et al suggested that genetic mutations in S. mutans fluoride-resistant strains resulted in the reduced activity of PTSs, such as ManXYZ and ManLMN, which strongly affect carbon inflow and energy production via the central carbon metabolism, whereas the upregulation of MsmEFGK and MalXFGK can overcome the restricted carbon flow (Lee et al, 2021). Transcriptomic analyses demonstrated that diterpenoids downregulated genes associated with most PTSs.…”