Effect of milk and yogurt on streptococcus sobrinus counts and caries score in rats.

BACKGROUND: An anti-cariogenic diet containing probiotics can be effective in caries prevention. This animal study compared the effects of milk and yogurt on Streptococcus sobrinus counts and caries score.
MATERIALS AND METHODS: A total of 36 male rats were infected with S. sobrinus (27,607) and divided into three groups. Group A and B received 200 mL of milk and 100 g of yogurt per day, respectively, and a control group received 2.5 mL of NCP number 2 diet twice daily for 21 days. After killing the animals, their lower left jaws were removed and sonicated to quantify the colonies of S. sobrinus. Dental caries was scored using Keyes technique. Data were analyzed using ANOVA and Kruskal-Wallis, Mann-Whitney and Wilcoxon-Signed Rank tests. Statistical significance was set at P < 0.05.
RESULTS: The mean (±standard error of the mean) of S. sobrinus colonies in the milk, yogurt and control groups were determined at 119666.67 (±20733), 46416.666 (±12846) and 163,250 (±33493), respectively. Microbial counts decreased in the yogurt group compared with the milk and control groups (P = 0.004 and P = 0.000; respectively). There were significant differences between caries scores of smooth surfaces in the milk and yogurt groups compared with the control group (P = 0.000 and P = 0.000, respectively). Both milk and yogurt significantly reduced caries score of fissured surfaces compared with controls (P = 0.004 and P = 0.000, respectively).
CONCLUSION: Considering the limitations of this study, yogurt administration reduces S. sobrinus counts. In addition, yogurt and milk regimens reduce the caries scores of smooth and fissured surfaces.

INTRODUCTION

Tooth decay is the most common chronic disease of early childhood. Although it can be preventable, dental caries is considered a multi-factorial infectious and transmissible disease.[1] The most acceptable theory of caries development is the chemicoparasitic theory in which the presence of cariogenic bacteria, susceptible host and fermentable carbohydrate as a substrate for microbial action are critical; therefore, chemical and mechanical microbial plaque removal, as well as sugar discipline, are commonly advised.[23]

Diet plays a key role in caries development. In other words, an anticariogenic diet can be effective in caries prevention.[4] The low cariogenic potential of food is attributed to high protein and mineral content as well as moderate fat content and high buffering capacity facilitating the saliva action to protect the teeth.[5] Dairy products such as milk and yogurt are excellent foods containing protein, providing essential amino acids and organic nitrogen. They also contain calcium, phosphate, casein, and lipids, which are considered factors with anticariogenic effects. These ingredients modulate the acidity of saliva and plaque, causing the remineralization of early carious lesions, in addition to having some degrees of antibacterial properties.[5]

In addition, yogurt contains probiotics, living beneficial microorganisms with an inhibitory effect on pathogenic bacteria.[6] Yogurt and other fermented milk-based products have been demonstrated to be beneficial for general health, especially because of their probiotic content. They were proposed as an alternative to manage many disorders such as infectious diseases, cancers and gastrointestinal problems in particular.[7] Because of so many interactions known between different species of microorganisms, it has been shown that the growth of pathogenic strains can be inhibited by interfering with colonization and their interaction with nonpathogenic neighbors.[8] In this respect, probiotic therapy refers to selected removal of pathogenic bacteria by nonpathogenic competitors.[9] The most common probiotic bacteria used for caries control are some species of Bifidobacteria and Lactobacilli, which are tested against Streptococcus mutans.[10]

Mutans streptococci are the major pathogenic bacteria in the caries process. This group mainly includes S. mutans and Streptococcus sobrinus, which are responsible for caries development in both animals and humans.[5] S. sobrinus is a primary bacterial pathogen on smooth surface dental caries. It presents in 43-60% of plaque cultures of children with early childhood caries.[11] It is supposed that the inhibition of these microorganisms leads to caries prevention.

This comparative study was carried out to evaluate the effects of milk and yogurt on S. sobrinus counts and caries score in rats.

MATERIALS AND METHODS

The present study was approved by the Ethic Committee of Babol University of Medical Sciences, and all experiments were conducted in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (NIH Publications No. 80-23 revised in 1996).

Thirty-six male Wistar rats aged 19 days and weighing 25 ± 5 g were used in this study. The S. sobrinus-free rats were screened by culturing the saliva samples on streptococcus selective agar medium. Then, they were infected with S. sobrinus PTCC 27607 (Persian Type Culture Collection, Tehran, Iran) under the cariogenic diet 2000 sterilized by deionized distilled water with 10% sucrose ad libitum to establish the infection for 6 days.[12] Plating of oral swabs showed the rats were successfully infected with S. sobrinus.

All the animals were then anesthetized with intraperitoneal injection of a mixture of ketamine (8 mg/kg) and xylazine (2.5 mg/kg)[13] and their submandibular and parotid salivary glands were surgically removed when aged 25 days. After surgery, they were divided into three experimental groups (12 in each group). Group A and B were respectively fed ad libitum with 200 mL of milk and 100 g of yogurt daily and the control group received gavage of 2.5 mL of liquid diet NCP number 2 twice daily.[14] Milk and yogurt were chosen in the same brand, with similar fat, protein and carbohydrate contents. The animals were housed one per cage (42 cm × 26 cm) and maintained on a 12-h light/12-h dark cycle (lights on at 6.00 a.m.), at a temperature of 21°C ± 1°C and relative humidity of 50-70%. They had free access to water and diet related to each group.

After 21 days, the animals were sacrificed, and their lower left jaws were removed and suspended in 10 mL of 0.9% normal saline.[15] The samples were sonicated (BAN Delin Sonoplus, Germany) for 30 s in duty cycle 1 × 10. One loop was inoculated on streptococcus selective agar medium (Merck, Germany) and incubated at 37°C for 2 days for detecting and counting the colonies of S. sobrinus.

Dental caries was detected under a stereomicroscope at magnification of ×40 (Micro-Optic Industrial Group Company, China) and then scored using the Keyes technique.[16] Microbiological data and the total number of lesions on smooth and fissured surfaces were assessed separately.

Data were analyzed using SPSS 18 (SPSS Inc., Chicago, IL, USA). Kruskal-Wallis analysis of variance was used to compare the levels of S. sobrinus. Ordinal caries scores were analyzed using Kruskal-Wallis, Mann-Whitney and Wilcoxon-Signed Rank methods. Statistical significance was defined at P < 0.05.

RESULTS

The mean (±standard error of the mean) of S. sobrinus colonies in milk, yogurt and control groups were estimated at 119666.67 (±20733), 46416.666 (±12846) and 163,250 (±33493), respectively. A significant reduction in S. sobrinus counts was found in the yogurt group compared with the milk and control groups (P = 0.004 and P = 0.000; respectively), but milk consumption did not significantly reduce S. sobrinus counts. S. sobrinus counts are displayed in Figure 1.

Figure 1

Streptococcus sobrinus counts in experimental groups.

There were significant differences between all the groups in caries scores of smooth surfaces (P = 0.000).

Consumption of milk and yogurt resulted in a significant decrease in dental caries scores on smooth surfaces compared with the controls (P = 0.000 and P = 0.000, respectively) but there was no significant difference between yogurt and milk groups. The caries scores of smooth surfaces are displayed in Table 1.

Table 1

Caries scores on smooth surfaces

In relation to caries scores of fissured surfaces, a significant difference was shown between all the groups (P = 0.000). Both milk and yogurt significantly decreased the caries scores of fissured surfaces compared to the controls (P = 0.004 and P = 0.000, respectively) while there were no significant differences between yogurt and milk groups. The caries scores of fissured surfaces are presented in Table 2.

Table 2

Caries scores on fissured surfaces

Figure 2 illustrates the rats’ second molars, which were sectioned mesio-distally at the central fissure to find carious lesion based on the Keyes technique [Figure 2].

Figure 2

The rat second molar sectioned mesio-distally at the central fissure.

Wilcoxon-Signed rank did not reveal any significant differences between caries scores on smooth and fissured surfaces in all the groups.

DISCUSSION

This study evaluated S. sobrinus counts and caries scores by administration of milk and yogurt in rats. The results indicated that consumption of yogurt had an inhibitory effect on S. sobrinus, consistent with observations made by Caglar et al. and Petti et al.[1718]

It suggested that the yogurt affects the oral micro-flora because of its probiotic microorganisms.[19] These beneficial bacteria are suggested as the nonpathogenic competitors against cariogenic bacteria.[9] It is supposed that interactions between microbial species prevent colonization of pathogenic bacteria by interfering with cellular adhesion and biofilm formation.[920] Furthermore, some probiotic strains produce and secrete organic acids, hydrogen peroxide and bacteriocins, making the ecosystem unsafe for pathogenic neighbors.[20]

Probiotic microorganisms naturally exist in some dietary products such as yogurt. In addition, they may be added to dairy products, chewing gums, food supplements, pills, etc.[21] However, probiotics-containing dairy products appear to be the most natural and the best approach for probiotic therapy.[22] Not only the probiotics content but also the other components such as high mineral and protein and moderate lipid contents in these products play a major role in caries prevention.[5]

Milk consumption did not significantly decrease S. sobrinus counts compared to the control group. The results of two studies conducted by Engström et al. and Lexner et al.[2324] are consistent with the present study. This outcome can be probably explained by considering the lack of probiotics in milk, since the efficacy of probiotics-containing milk in reduction of caries-associated bacteria has been previously demonstrated.[2526]

The anticariogenic effect of probiotic milk containing Lactobacillus rhamnosus GG was evaluated on children aged 1-6 years during 7 months. It was found that the incidence of tooth decay may decrease by a long-term daily consumption of this product.[25]

Additionally, it was observed that consumption of milk and yogurt significantly decreased caries scores of smooth and fissured surfaces compared with the control group. In this regard, Tanaka et al.[27] found that the frequent use of yogurt (more than or up to 4 times a week) might be associated with a lower incidence of dental caries in children. In addition, Levy et al.[28] showed that regular administration of milk had a protective effect on primary teeth.

It is suggested that milk ingredients such as fat, casein, calcium and phosphate have a protective effect against tooth decay.[29] Aimutis[30] reported that the casein phosphopeptides (CPPs) and glycomacropeptide in dairy products prevented demineralization as well as stimulated remineralization of tooth enamel. The above-mentioned effect was also demonstrated by Ferrazzano et al.[31] about CPPs of yogurt.

The CPPs stabilize high concentrations of calcium and phosphate together with fluoride ions on the enamel surface. These ions are freely bioavailable to diffuse into early carious lesions, thereby effectively enhancing remineralization.[32] They also have a great capacity to neutralize the acid produced by cariogenic bacteria to prevent demineralization.[33]

This study was designed as an animal study on rats mainly due to ethical considerations existing in human studies. In spite of natural differences between rats and humans, based on previous studies,[17182728] it seems that if this project had been performed on humans, similar findings would have been achieved. However, further studies on humans, comparing the anticaries properties of dairy products on S. mutans and S. sobrinus are recommended.

CONCLUSION

This study showed that yogurt administration significantly reduced S. sobrinus counts. Although it was not statistically significant, S. sobrinus counts dropped by consumption of milk. Additionally, taking yogurt and milk reduced the caries scores of smooth and fissured surfaces significantly. These results confirmed the anticariogenic capacity of yogurt and milk.

Financial support and sponsorship

This project was financially supported by Babol University of Medical Sciences.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.