In this study, extracted deciduous teeth with large carious cavity from school-age children were collected and topically intervened in vitro with 38% SDF solution. Dental plaque from the carious cavity pre- and post-intervention were collected for microbial sequencing and the results were compared. The amount of most of the bacteria, the microbial diversity and the microbial community composition in the dental plaque after SDF intervention were obviously decreased or changed, and the relative contents of Pseudomonas, Fusobacterium and Pseudoramibacter were higher than before, although no statistically significant difference was found (P > 0.05). After SDF intervention, microbial association in the dental plaque became more complex with positive connections overwhelm the negative ones, and carbohydrate transportation and metabolic functions in the dental plaque were significantly reduced (P < 0.05).
The results supported that 38% SDF had broad spectrum of bactericidal effect, which suppressed the dental plaque as a whole. As a result, microbial diversity and the microbial community composition was reduced and shifted. The statistical indifference in this study may be due to the small sample size. A recent study on 38% SDF, which compared pre- and one-month-post-intervention dental plaque sample from the mouth of adult patients [29], found no significant difference in the plaque microorganisms composition either. However, difference in dentition, sampling condition and the follow-up interval should also be noticed.
Most of the bacteria presented before the intervention (Olsenella, Streptococcus, Bifidobacterium, Prevotella_7, Lactobacillus, Actinomyces, Leptotrichia, Selenomonas_3, Veillonella) were sensitive to SDF, and the amount decreased obviously. Streptococcus, Lactobacillus and Actinomyces has long been recognized as classical cariogenic bacteria. Prevotella_7, Selenomonas_3, and Bifidobacterium were often found in deep dentin caries [30]. Olsenella, which could produce lactic acid, was seldom mentioned in the past, have been found being associated with dentin caries and root caries recently [31, 32]. Leptotrichia has a strong glycolysis effect [33], and its detection rate increased in the process of caries [34, 35]. Veillonella has a unique intracellular pH control mechanism [36], which might promote the cariogenicity of Streptococcus mutans [35, 37]. By inhibition of these bacteria, cariogenicity of the dental plaque was suppressed, microbial new balance was needed and the chance became possible.
It was also found that Pseudomonas, Fusobacterium and Pseudoramibacter could withstand the effect of topically applied SDF. The possible relation of Pseudomonas in dental caries was indicated recently. NavNeet Kaur et al. [38] detected a high level of Pseudomonas in the dentin caries. In Liang's study [39], Pseudomonas was mentioned as a potential pathogen of caries, but considered the relationship be further studied. Pseudomonas was obligate aerobic and negative gram staining, with capsular bacillus and no bud, belonged to the non-fermentation bacteria. It had no special nutritional requirements, was common in soil, fresh water, seawater. A total of twenty-nine species have been identified, of which at least three are pathogenic to animals or humans, causing infections. Pseudomonas aeruginosa was in the oral common flora and was a conditioned pathogen [40], it was one of the main pathogens of nosocomial infection, usually secondary infection. Small doses of the bacteria could produce local abscesses, while large doses of it could lead to death from systemic infection, which was also the main cause of pulmonary cystic fibrosis [41]. Fluorescent pseudomonas could cause spoilage of frozen meat, eggs, milk and dairy products. Pseudomonas birensis was also known as bacillus birensis, could be infected through mouth, respiratory tract or wound. Pseudoramibacter was a nonmotile, nonsporeforming, strictly anaerobic, Gram-positive rod, which was saccharolytic. The end products of its fermentation were formate, acetate, butyrate, caproate, and hydrogen [42]. It was the pathogen that could cause periapical infection [43–45], often found in deep dentin caries [30, 46] and infected root canals [42]. Similarly, Fusobacterium was often detected in deep dentin caries [30, 46] and infected root canals [44, 45, 47], It was a strict obligate anaerobe, with negative gram staining, normally living in oral cavity, upper digestive tract, intestinal tract, urogenital tract of human or animal and soil. It was most commonly seen in oral dental plaque. Most of the strains did not ferment any sugars, only a few strains showed weak fermentation reaction to glucose and fructose.
Restricted by ethic issues, extracted deciduous teeth was used in this study. As a result, intervention and sampling after it were all carried out in vitro. In vitro study was different from real clinical situation. The effect of common oral flora, saliva, food and environmental microorganisms on the formation of new microbial balance could hardly be mimic and evaluated. New micro-ecological balance formation after SDF intervention in this study could only be based on the before-intervention status as well as the effect of SDF. The bacteria which was sensitive to SDF were controlled, while those with relatively high tolerance to SDF got better growth and development after the intervention. After SDF intervention, microbial association in the dental plaque became more complex with positive connections overwhelm the negative ones, indicating that the connections and collaborations among the remaining bacterial communities were enhanced.
The functional characteristics of plaque in caries were also compared and analyzed based on the COG database for homologous classification of gene products, carbohydrate transportation and metabolism function in plaque were significantly reduced 24 hours and 1 week after intervention (shown in yellow in Fig. 5 bar chart, G), and the replication, recombination and repair function in plaque (shown in pink in bar chart in Fig. 5, L) were also significantly reduced, revealed from another aspect the overall inhibitory effect of SDF on the bacterial community in dental plaque, which affecting the properties of the whole plaque, especially the cariogenicity within the plaque micro-ecology. The function of signal transduction mechanism was significantly increased, which could be correlated with the enhancement of synergistic effect between residual bacteria in plaque after intervention.
School-age children had mixed dentition, deciduous teeth with extensive carious cavity or obvious dental pulp symptoms may also affect the health of their permanent teeth. The results of the dental plaque from the extensive carious cavity could also be guiding information for the prevention and control of deciduous caries associated with pulp infection or periapical disease.
Most parameters in this study showed that the greatest microbial difference was found at one week after SDF intervention, indicating a possible time effect of SDF. However, the observation period in this study was relatively short which may has its limitations, and there were differences between this in vitro study and the real mouth situation, time effect of SDF application on dental plaque may not be well explained, and should be studied further.