Streptococcus mutans adherence to conventional and self-ligating brackets: an in vitro study

ABSTRACT Introduction: Although self-ligating brackets presumably provide better hygiene conditions, no consensus has been reached so far. Objective: Therefore, the objective of this study was to evaluate, in an in vitro experimental design, the adherence of Streptococcus mutans (SM) in self-ligating and conventional brackets of different manufacturers and ligature types. Methods: Four commercial brands of maxillary premolar metal brackets were tested (Abzil®; Morelli®; 3M Unitek®; and GAC®). Each one was subdivided into three groups, which varied according to the type of ligature and bracket model (metallic, elastic, and self-ligating), totalizing twelve groups, composed of six brackets each. Previously sterilized brackets were initially immersed in saliva for one hour, and subsequently washed and added in a bacterial suspension, maintained in aerobiosis for 72 hours. The adhered bacteria were then separated and quantified by colony forming units (CFU/mL) counting after 48 hours of growth. The groups were compared by Kruskal-Wallis and Dunn post-hoc tests (p< 0.05). Results: Regardless of the commercial brand, self-ligating brackets had significantly less CFU/mL. However, according to comparisons performed within each commercial brand, only Abzil® self-ligating brackets had significantly lower biofilm adhesion. Among all of the self-ligating models, GAC® brackets presented the highest bacterial adhesion rate. Conclusions: Self-ligating brackets are likely to present lower rates of biofilm adhesion. Particularly, Abzil® and GAC® self-ligating brackets are less likely to accumulate biofilm. Although such results are derived from an in vitro study, practitioners might acknowledge findings concerning bacterial adhesion as one of the relevant features to be considered during bracket selection.

One of the most recent advances in Orthodontics refers to the development of the self-ligating brackets, originally designed to facilitate wire insertion and removal. 27,28 These brackets feature an active or passive opening and closing device that ensures a safe and effective engagement of the wire into the bracket slot, 29 with no need for metallic or elastic ligatures.
Although some authors have claimed that self-ligating brackets provide better hygiene conditions, 29,30 no consensus has been found indicating that self-ligating brackets are actually more advantageous in this aspect. 31 Therefore, the objective of this study was to evaluate, in an in vitro experimental design, For each of these commercial brands, two bracket models were selected (conventional and self-ligating brackets); and two types of ligatures - metallic (0.025 mm; Morelli ® , Sorocaba/SP, Brazil) or elastic (gray color; Morelli ® , Sorocaba/SP, Brazil)were attached to the conventional brackets (Table 1). Thus, a total of twelve groups composed of six brackets each were formed (Fig 1). Each set of brackets was sterilized (Cristófoli ® , Curitiba/PR, Brazil) at 122ºC for 15 minutes, and then reserved until the experiment.  The donors were 30 to 36 years old, had good oral health and, at the time of the collection, had fasted for eight hours without having brushed their teeth. In addition, they had not undergone professional cleaning or antibiotic therapy in the three months prior to collection, nor had they had caries or periodontal disease at the time. After collection, the saliva was kept on ice until its use.

STREPTOCOCCUS MUTANS BIOFILM FORMATION
A Streptococcus mutans UA159 strain was initially reactivated from stock cultures in liquid BHI (Brain-heart infusion) medium for 18 to 24 hours at 37ºC, 5% CO 2 , and then cultured in BHI

DESCRIPTION OF THE EXPERIMENT
Into a 96-well plate, the previously sterilized brackets were carefully immersed in saliva for one hour, so that each bracket would occupy a well. After this period, the saliva was removed, the brackets were washed with phosphate buffer solution (PBS), and added to another plate along with 200 μL (in each well) of the bacterial suspension prepared as described in Figure 2.
After inoculation, the plate was kept at 37°C, 5% CO 2 for 72 hours.
Posteriorly, the brackets were removed from the wells and carefully transferred to Eppendorf-type tubes (Eppendorf ® , Hamburg, Germany) containing 1mL of PBS, which were sonicated for 10 minutes, to separate bacteria adhered to biofilm from the brackets. To quantify bacterial adherence, serial dilution and plating on BHI agar plates added with sheep's blood were performed.
Colony Forming Units (CFU) counts were performed after 48 hours of bacterial growth on the plates at 37ºC and 5% CO 2 .
Thus, the higher the number of CFUs, the greater the number of viable bacteria that adhered to the bracket surface throughout the experiment.
However, no statistically significant differences were found between the bracket / ligature types in comparisons performed within each commercial brand individually, except for the Abzil ® bracket models (Table 2). In the paired comparison between Abz-SL and Abz-Ela, a significantly higher amount of CFU/mL was observed for the latter. In addition, when the Abz-SL and Abz-Met groups were compared, significantly higher amounts of CFU/mL were observed for Abz-Met; but without significant differences when these and the conventional brackets with elastic ligatures (Abz-Ela) were compared with each other (

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The authors report no commercial, proprietary or financial interest in the products or companies described in this article.