Salivary density of Streptococcus mutans and Streptococcus sobrinus and dental caries in children and adolescents with Down syndrome

Abstract Streptococcus mutans and Streptococcus sobrinus are strongly associated with dental caries. However, the relationship between oral streptococci and dental caries in children with Down syndrome is not well characterized. Objective To assess and compare dental caries experience and salivary S. mutans, S. sobrinus, and streptococci counts between groups of Down syndrome and non-Down syndrome children and adolescents. Material and Methods This study included a sample of 30 Down syndrome children and adolescents (G-DS) and 30 age- and sex-matched non-Down syndrome subjects (G-ND). Dental caries experience was estimated by the number of decayed, missing, and filled teeth in the primary dentition and the permanent dentition. Unstimulated whole saliva samples were collected from all participants. The fluorescence in situ hybridization technique was used to identify the presence and counts of the bacteria. The statistical analysis included chi-square, Student’s t-test and Spearman’s correlation. Results The G-DS exhibited a significantly higher caries-free rate (p<0.001) and a lower S. mutans salivary density (p<0.001). No significant differences were found in the salivary densities of S. sobrinus or streptococci between the groups (p=0.09 and p=0.21, respectively). The salivary S. mutans or S. sobrinus densities were not associated with dental caries experience in neither group. Conclusion The reduced dental caries experience observed in this group of Down syndrome children and adolescents cannot be attributed to lower salivary S. mutans densities, as determined with the fluorescence in situ hybridization technique.


Introduction
Down syndrome (DS) is a genetic disorder described in 1866 by John Longden Hayden Down.
DS is the most common chromosomal anomaly of the human species with an incidence of 1:800 to 1:1,000 births. The main clinical characteristics of DS include mental retardation and cardiovascular, haematopoietic, musculoskeletal, nervous system, and immunological system anomalies. These effects, particularly those in the immune system, result in an increased susceptibility to infection 8,12,25 .
Numerous oral abnormalities have been described in DS individuals including malformations of the small palate and maxilla, mouth breathing resulting in eruption, dental agenesis, low incidence of dental caries, high incidence of periodontal diseases, high incidence of mucosal ulcers, candidiasis, and acute necrotizing ulcerative gingivitis, compared with healthy individuals. Patients with DS also demonstrate macrologlossia, imbalanced occlusal and soft tissues forces, open bite, impaired chewing and consequent 5, 12,25 .
One of the most prominent oral manifestations in DS subjects is a low prevalence of dental caries 4,8,9,12,13 , despite exposure to risk factors, such as a cariogenic diet, decreased salivary flow, mouth breathing, unbalanced occlusal forces, and poor access to oral hygiene 25,28 . Some studies have addressed the etiology of this low prevalence of dental caries, but the exact mechanism remains unclear. Some of the hypotheses suggested to explain the low prevalence of dental caries include the following: delayed tooth eruption in combination with an altered chronology of eruption; the high frequency of hypodontia; differences in the composition, pH, and buffering capacity of the saliva and the salivary flow 8,9,25 , and differences in the cariogenic microbiota 7,9,28 .
Microbial diversity comprises the number of species present (species richness) and the number of individuals of each species (uniformity). The knowledge about microbial diversity is important, given that a microbial community may change in terms of the number of individuals per species in response to changing conditions that favor their growth 27 .
Streptococcus mutans and Streptococcus sobrinus are strongly associated with dental caries. However, the relationship between oral streptococci and dental caries in children with DS is not well characterized.
While some studies have shown that the occurrence of dental caries is associated with S. mutans counts in children and adolescents with DS 2,9,20,28

Dental caries experience
Prior to the beginning of the study, a single dental examiner (FARS) was trained by a specialist in pediatric dentistry (i.e., the gold standard -RAR) to ensure consistency in the dental caries diagnoses.
Six non-Down syndrome children (10% of the total sample) were previously examined twice, with were =0.94 for the intra-examiner agreement and =0.92 for the inter-examiner agreement. The dental the participants seated in a dental chair.

Statistical analysis
The data were organized into a database using the Spearman's correlation test was used to assess the associations between the salivary counts of S. mutans and S. sobrinus and the dental caries experience. The

Results
The total sample included 60 children and adolescents between 3 and 12 years of age who resided in the city of Juiz de Fora.   .
The results of dental caries experience revealed a and adolescents in the G-DS, which is consistent with results reported in previous investigations 4,6,[9][10][11]23,25,28 .
in the frequencies of caries-free children and   and is a rapid and objective method 14,18,22 . Moreover, the FISH technique has been proven effective in the detection of S. mutans and S. sobrinus 14,18,24,29 .
To detect and quantify cariogenic oral bacteria, unstimulated saliva samples were collected 12,20 .
Saliva represents an easy and non-invasive means for obtaining bacterial samples from all of the oral sites 16 . The samples were collected preferentially in the morning, to minimize the effects of circadian rhythms 5 .

Conclusions
In conclusion, we found that DS children and adolescents present a lower dental caries experience and a lower salivary density of S. mutans than non-Down controls. However, the reduced dental caries experience observed in this group of Down syndrome subjects could not be attributed to the lower salivary S. mutans in situ hybridization technique.