Abstract
Streptococcus mutans (S. mutans) is considered a leading cause of dental caries. The capability of S. mutans to tolerate low pH is essential for its cariogenicity. Aciduricity of S. mutans is linked to its adaptation to environmental stress in oral cavity. This study aimed to investigate the effect of biofilm age and starvation condition on acid tolerance of biofilm formed by S. mutans clinical isolates. S. mutans clinical strains isolated from caries-active (SM593) and caries-free (SM18) adults and a reference strain (ATCC25175) were used for biofilm formation. (1) Both young and mature biofilms were formed and then exposed to pH 3.0 for 30 min with (acid-adapted group) or without (non-adapted group) pre-exposure to pH 5.5 for three hours. (2) The mature biofilms were cultured with phosphate-buffered saline (PBS) (starved group) or TPY (polypeptone-yeast extract) medium (non-starved group) at pH 7.0 for 24 h and then immersed in medium of pH 3.0 for 30 min. Biofilms were analyzed through viability staining and confocal laser scanning microscopy. In all three strains, mature, acid-adapted and starved biofilms showed significantly less destructive structure and more viable bacteria after acid shock than young, non-adapted and non-starved biofilms, respectively (all p < 0.05). Furthermore, in each condition, SM593 biofilm was denser, with a significantly larger number of viable bacteria than that of SM18 and ATCC25175 (all p < 0.05). Findings demonstrated that mature, acid-adapted and starvation might protect biofilms of all three S. mutans strains against acid shock. Additionally, SM593 exhibited greater aciduricity compared to SM18 and ATCC25175, which indicated that the colonization of high cariogenicity of clinical strains may lead to high caries risk in individuals.
Highlights
Dental caries is an infectious and transmissible bacterial disease [1,2]
We examined the biofilms of S. mutans clinical strains SM593 and SM18 and the standard strain ATCC 25175 in terms of aciduricity in different conditions including biofilm formation in different phases, acid adaptation and starvation using confocal laser scanning microscopy (CLSM) with molecular probes LIVE/DEAD BacLight viability stain
S. mutans clinical strains isolated from caries-active (SM593) and caries-free (SM18) adults and the standard strain ATCC 25175 have been used to analyze the association between the aciduricity of strains and caries activity under different conditions to identify potential factors influencing the aciduricity of S. mutans
Summary
Dental caries is an infectious and transmissible bacterial disease [1,2]. Streptococcus mutans (S. mutans) has been described as the principal etiologic agent of human dental caries and an important constituent of dental plaque [3]. It is widely accepted that strong capability of S. mutans to tolerate low pH, namely aciduricity, is essential for its cariogenicity [5]. The constitutive acid tolerance properties of S. mutans, as well as its adaptive acid tolerance response (ATR) provide a holistic benefit to withstand continual changes in acid shock [6]. It has been reported that ATR plays a crucial role in protecting S. mutans from more severe acid stress [7]. Acid adaption triggered by prior exposure to low but nonlethal pH enables S. mutans to survive in severe acidification
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