Timing of colonization of caries-producing bacteria: an approach based on studying monozygotic twin pairs

How does the early life environment influence the oral microbiome and determine oral health outcomes in childhood?

The first 1000 days of life, from conception to 2 years, are a critical window for the influence of environmental exposures on the assembly of the oral microbiome, which is the precursor to dental caries (decay), one of the most prevalent microbially induced disorders worldwide. While it is known that the human microbiome is susceptible to environmental exposures, there is limited understanding of the impact of prenatal and early childhood exposures on the oral microbiome trajectory and oral health. A barrier has been the lack of technology to directly measure the foetal "exposome", which includes nutritional and toxic exposures crossing the placenta. Another barrier has been the lack of statistical methods to account for the high dimensional data generated by-omic assays. Through identifying which early life exposures influence the oral microbiome and modify oral health, these findings can be translated into interventions to reduce dental decay prevalence.

Acquisition and establishment of the oral microbiota

Acquisition and establishment of the oral microbiota occur in a dynamic process over various stages and involve close and continuous interactions with the host and its environment. In the present review, we discuss the stages of this process in chronological order. We start with the prenatal period and address the following questions: ‘Is the fetus exposed to maternal microbiota during pregnancy?’ and ‘If so, what is the potential role of this exposure?’ We comment on recent reports of finding bacterial DNA in placenta during pregnancies, and provide current views on the potential functions of prenatal microbial encounters. Next, we discuss the physiological adaptations that take place in the newborn during the birth process and the effect of this phase of life on the acquisition of the oral microbiota. Is it really just exposure to maternal vaginal microbes that results in the difference between vaginally and Cesarian section‐born infants? Then, we review the postnatal phase, in which we focus on transmission of microbes, the intraoral niche specificity, the effects of the host behavior and environment, as well as the role of genetic background of the host on shaping the oral microbial ecosystem. We discuss the changes in oral microbiota during the transition from deciduous to permanent dentition and during puberty. We also address the finite knowledge on colonization of the oral cavity by microbes other than the bacterial component. Finally, we identify the main outstanding questions that limit our understanding of the acquisition and establishment of a healthy microbiome at an individual level.

Heritability of Oral Microbiota and Immune Responses to Oral Bacteria

Maintaining a symbiotic oral microbiota is essential for oral and dental health, and host genetic factors may affect the composition or function of the oral microbiota through a range of possible mechanisms, including immune pathways. The study included 836 Swedish twins divided into separate groups of adolescents (n = 418) and unrelated adults (n = 418). Oral microbiota composition and functions of non-enzymatically lysed oral bacteria samples were evaluated using 16S rRNA gene sequencing and functional bioinformatics tools in the adolescents. Adaptive immune responses were assessed by testing for serum IgG antibodies against a panel of common oral bacteria in adults. In the adolescents, host genetic factors were associated with both the detection and abundance of microbial species, but with considerable variation between species. Host genetic factors were associated with predicted microbiota functions, including several functions related to bacterial sucrose, fructose, and carbohydrate metabolism. In adults, genetic factors were associated with serum antibodies against oral bacteria. In conclusion, host genetic factors affect the composition of the oral microbiota at a species level, and host-governed adaptive immune responses, and also affect the concerted functions of the oral microbiota as a whole. This may help explain why some people are genetically predisposed to the major dental diseases of caries and periodontitis.

Emergence stages of permanent teeth in twins: A comparative study

BACKGROUND

The relative effects of genetic factors on the timing of tooth eruption can be demonstrated through twin studies; however, twin studies on the eruption time of permanent teeth were limited.

AIM

To compare the emergence stages of permanent teeth in twins and non-twin children.

DESIGN

A total of 480 patients aged from 5.0 to 13.11 years who attended to the Department of Pediatric Dentistry for any dental reason were included in the study. Twins were compared with the control group in terms of the emergence stages for all permanent teeth.

RESULTS

At the age of 5 years, girls in the control group showed statistically significant differences related to the central incisors, the first molars in the mandibular dentition and the first molars in the maxillary dentition, showing more advanced emergence stages. In the 6- to 8- and 12- to 13-year-old age groups, no gender-related differences in emergence stages were observed. Gestation age was found to have a statistically significant effect on the emergence stages of permanent teeth. There were no statistically significant differences between birthweights and tooth emergence stages in twins.

CONCLUSION

Longitudinal studies are needed to determine whether there is a strong genetic effect on emergence stages of permanent teeth in twins.

Longitudinal Study of Oral Microbiome Variation in Twins

Humans are host to a multitude of microorganisms that rapidly populate the body at birth, subject to a complex interplay that is dependent on host genetics, lifestyle, and environment. The host-associated microbiome, including the oral microbiome, presents itself in a complex ecosystem important to health and disease. As the most common chronic disease globally, dental caries is induced by host-microbial dysbiosis in children and adults. Multiple biological and environmental factors are likely to impact disease predisposition, onset, progression, and severity, yet longitudinal studies able to capture these influences are missing. To investigate how host genetics and environment influenced the oral microbial communities over time, we profiled supragingival plaque microbiomes of dizygotic and monozygotic twins during 3 visits over 12-months. Dental plaque DNA samples were amplified by targeting the 16S rRNA gene V4 region, and microbial findings were correlated with clinical, diet and genetic metadata. We observed that the oral microbiome variances were shaped primarily by the environment when compared to host genetics. Among the environmental factors shaping microbial changes of our subjects, significant metadata included age of the subject, and the age by which subjects initiated brushing habits, and the types of actions post-brushing. Relevant heritability of the microbiome included Actinomyces and Capnocytophaga in monozygotic twins and Kingella in dizygotic twins. Corynebacterium and Veillonella abundances were associated with age, whereas Aggregatibacter was associated with younger subjects. Streptococcus abundance showed an inverse association over time, and Selenomonas abundances increased with brushing frequency per day. Unraveling the exact biological mechanisms in caries has the potential to reveal novel host-microbial biomarkers, pathways, and targets important to effective preventive measures, and early disease control in children.

Research on oral microbiota of monozygotic twins with discordant caries experience - in vitro and in vivo study

Oral microbiome is potentially correlated with many diseases, such as dental caries, periodontitis, oral cancer and some systemic diseases. Twin model, as an effective method for studying human microbiota, is widely used in research of relationship between oral microbiota and dental caries. However, there were few researches focusing on caries discordant twins. In this study, in vitro assays were conducted combined with 16S rRNA sequencing analysis on oral microbiota sampled from twins who presented discordant caries experience and mice model was developed as well. Results showed that oral microbiota from caries-active twin possessed higher metabolic activity and produced more lactic production. 16S rRNA sequencing analysis showed that more than 80% of family taxa could be transferred into gnotobiotic-mice. Key caries-associated genera were significantly different between twins and the same difference in genus level could be found in mice as well (p < 0.05). This study suggested that oral microbiota of twins could be distinguished from each other despite the similarities in genetic make-up, living environment, and lifestyle. The difference in microbiota was applied to develop a mice model which may facilitate the investigation of core microbiota of dental caries.

Bacterial colonization, enamel defects and dental caries in 4-6-year-old mono- and dizygotic twins

BACKGROUND

Knowledge of the genetic and environmental influences in caries aetiology has relevance for preventive dentistry.

AIMS

This classical twin study compared concordance of mutans streptococci (MS) and lactobacilli (LB) colonization, enamel defects, and caries in a cohort of 4-6-year-old mono- (MZ) and dizygotic (DZ) twin pairs.

DESIGN

The twins were examined for prevalence and concordance of enamel opacities and hypoplasia, oral counts of MS and LB, and dental caries. Bacterial counts were assessed using a commercial microbiological kit.

RESULTS

Thirty-four MZ and 50 DZ twins (mean gestational age 35.0 ± 2.4 weeks, and birthweight 2.4 ± 0.6 kg) were examined. There were no statistically significant differences between MZ and DZ twins in the prevalence of MS, LB, and enamel hypoplasia. Concordance rates for MS and LB presence and prevalence of enamel defects within MZ and DZ twin pairs were not significantly different. There were more children with caries in DZ compared with MZ twins (18% vs 3%, P = 0.0029), most likely due to increased daily frequency of sugar consumption and less toothbrushing.

CONCLUSIONS

Concordance data from MZ and DZ twins did not demonstrate any statistically significant difference in susceptibility for enamel defects and colonization of MS and LB.

Australian Twin Registry: 30 years of progress

The Australian Twin Registry (ATR) is a national volunteer resource of twin pairs and higher-order multiples willing to consider participating in health, medical, and scientific research. The vision of the ATR is 'to realize the full potential of research involving twins to improve the health and well-being of all Australians'. The ATR has been funded continuously by the National Health and Medical Council for more than 30 years. Its core functions entail the recruitment and retention of twin members, the maintenance of an up-to-date database containing members' contact details and baseline information, and the promotion and provision of open access to researchers from all institutes in Australia, and their collaborators, in a fair and equitable manner. The ATR is administered by The University of Melbourne, which acts as custodian. Since the late 1970s the ATR has enrolled more than 40,000 twin pairs of all zygosities and facilitated more than 500 studies that have produced at least 700 peer-reviewed publications from classical twin studies, co-twin control studies, within-pair comparisons, twin family studies, longitudinal twin studies, randomized controlled trials, and epigenetics studies, as well as studies of issues specific to twins. New initiatives include: a Health and Life Style Questionnaire; data collection, management, and archiving using a secure online software program (The Ark); and the International Network of Twin Registries. The ATR's expertise and 30 years of experience in providing services to national and international twin studies has made it an important resource for research across a broad range of disciplines.

Association between early childhood caries, streptococcus mutans level and genetic sensitivity levels to the bitter taste of, 6-N propylthiouracil among the children below 71 months of age

BACKGROUND

Oral health is an integral component of pre-school health and well-being. Unfortunately, many children are afflicted with dental caries at an early age, even those as young as 12 months of age. The purpose of this study is to determine the association between Early Childhood Caries (ECC), Streptococcus mutans and genetic sensitivity levels to the bitter taste of, PROP among the children below 71 months of age.

MATERIALS AND METHODS

Total of 119 children belonging to the age group of 36 to 71 months of both sexes, were recruited from A. J. Institute of Dental Sciences, Mangalore (Karnataka). PROP sensitivity test was carried out to determine the inherent genetic ability to taste a bitter or sweet substance. One who tasted bitter as taster and one who was not able to differentiate/tasted like paper as non-tasters. Facial expression was observed during the tasting to support the verbal response. Estimation of S. mutans level and caries experience was recorded. The results were statistically analyzed using Mann'Whiteney-U Test and Kruskal value test.

RESULTS

In the total of 119 children, the mean DMFS was definitely higher in non-taster children compared to tasters and also had a high S. mutans level. Tasters had low ECC experience, low S. mutans level. The tasters had a mean DMFS value of 9.5120 (S.D. 7.0543) and non-tasters had a value of 7.7250 (S.D. 8.33147), which was statistically significant.

CONCLUSION

Children who had higher level S. mutans had ECC and were non tasters. The PROP sensitivity test (filter paper test) proved to be a useful diagnostic tool in determining the genetic sensitivity levels of bitter taste. Age and low socio-economic status of pre-school children suggest a complex multifactorial relationship between S. mutans colonization, ECC and taste perception.

Genetic, Epigenetic, and Environmental Influences on Dentofacial Structures and Oral Health: Ongoing Studies of Australian Twins and Their Families

The Craniofacial Biology Research Group in the School of Dentistry at The University of Adelaide is entering an exciting new phase of its studies of dental development and oral health in twins and their families. Studies of the teeth and faces of Australian twins have been continuing for nearly 30 years, with three major cohorts of twins recruited over that time, and currently we are working with twins aged 2 years old to adults. Cross-sectional data and records relating to teeth and faces of twins are available for around 300 pairs of teenage twins, as well as longitudinal data for 300 pairs of twins examined at three different stages of development, once with primary teeth, once at the mixed dentition stage, and then again when the permanent teeth had emerged. The third cohort of twins comprises over 600 pairs of twins recruited at around birth, together with other family members. The emphasis in this third group of twins has been to record the timing of emergence of the primary teeth and also to sample saliva and dental plaque to establish the timing of colonization of decay-forming bacteria in the mouth. Analyses have confirmed that genetic factors strongly influence variation in timing of primary tooth emergence. The research team is now beginning to carry out clinical examinations of the twins to see whether those who become colonized earlier with decay-forming bacteria develop dental decay at an earlier age. By making comparisons within and between monozygotic (MZ) and dizygotic (DZ) twin pairs and applying modern molecular approaches, we are now teasing out how genetic, epigenetic, and environmental factors interact to influence dental development and also oral health.