The Fitness Cost of Fluoride Resistance for Different Streptococcus mutans Strains in Biofilms

Abstract

The cariogenic bacterium Streptococcus mutans can develop stable resistance to fluoride through chromosomal mutations in vitro. Fluoride-resistant S. mutans has seldom been isolated in clinical settings, despite the wide application of fluoride in oral-care products. One explanation is that the fluoride-resistant S. mutans strains have decreased fitness. However, so far, there has been no conclusive evidence to support this idea. The aim of this study was to investigate the fitness cost of 48-h biofilms of two fluoride-resistant S. mutans strains, UF35 and UA159-FR (UAFR), using the wild-type fluoride-sensitive strain UA159 as a reference. The engineered UF35 strain contains one point mutation, whereas UAFR, selected from NaF-containing agar plates, has multiple chromosomal mutations. All biofilms were formed for 48 h under a constantly neutral pH or a pH-cycling (8 h of neutral pH and 16 h of pH 5.5) condition in the absence of fluoride. The biomass of the biofilms was quantified with a crystal violet assay. The biofilms were also treated with chlorhexidine or solutions at pH 3.0, after which their lactic acid production was quantified. Compared to the UF35 and UA159 biofilms, the biomass of UAFR biofilms was two–four fold higher, and the UAFR biofilms were more resistant to chlorhexidine and low pH in terms of lactic acid production. No difference in biomass and lactic acid production was detected between UF35 and UA159 biofilms. The fluoride resistance of UAFR and UF35 strains in biofilms was further confirmed by treating the biofilms with NaF solutions. The level of NaF resistance of the three biofilms is generally ranked as follows: UAFR > UF35 > UA159. In conclusion, there is indeed a fitness consequence in UAFR, but surprisingly, this fluoride-resistant strain performs better than UF35 and UA159 under the described conditions. In addition, UF35 did not display a reduced fitness; it performed as well as the wild-type fluoride-sensitive strain.

Resistance Toward Chlorhexidine in Oral Bacteria – Is There Cause for Concern?

“…For this purpose, resistant strains must either be isolated from patient samples or induced in vitro by multiple passaging of bacteria under CHX-treatment (see Figure 4), whereby the latter has the advantage that the mutant strain exhibiting phenotypic adaptation to CHX can be compared to the wildtype strain. Finally, it is important to consider the respective fitness cost of a given mutation related to CHX resistance as the magnitude of this cost is the main biological parameter determining the rate of development of resistance (Andersson and Hughes, 2010; Cai et al, 2017).…”

Drug resistance of oral bacteria to new antibacterial dental monomer dimethylaminohexadecyl methacrylate

“…Furthermore, Enterococcus faecalis ( E. faecalis ) was also shown to develop drug resistance to CHX with ten repeated exposures 29 . In addition, a study reported that cariogenic bacterium S. mutans developed resistance to fluoride in vitro 44 . Therefore, bacterial resistance against widely used agents such as CHX and fluoride in the oral environment does appear to be a common problem.…”

The evolutionary history of the human oral microbiota and its implications for modern health

“…For example, a novel preventative dentistry approach in rural communities, dubbed the “Big Bang”, includes numerous preventative measures administered simultaneously once a year 117 ; the approach has already been shown to improve oral health in rural communities, as well as significantly impact the diversity and composition of oral microbiota 105 . Large‐scale social and political decisions may also have individual impacts on oral microbiota; for example, the presence, or not, of fluoridation in urban water distribution services in some communities and countries may have long‐term impacts on oral microbiota and future responses to other antimicrobial compounds 118 . Lastly, oral microbiota changes that happened in past European populations are still actively occurring in many parts of the world.…”