DENTAL CARIES IS A TRANSMISSIBLE DISEASE

A Post-Graduate MDS Essay in Pediatric and Preventive Dentistry

INTRODUCTION

HISTORICAL FOUNDATIONS: THE KEYES-FITZGERALD EXPERIMENTS

1. Caries-free animals raised in isolation or in the company of caries-free animals remained caries-free even when fed a cariogenic diet.
2. When caries-free animals were caged with caries-active animals, they developed dental caries, demonstrating horizontal transmission of the causative agent.
3. Surgical removal or sterilization of the oral flora in caries-active animals reduced caries development, confirming the microbial etiology.
4. The transmission was reproduced by direct fecal or oral inoculation from caries-active donors into germ-free recipients.

THE CONCEPT REVISITED: CAUFIELD'S POSITION

• Dental caries satisfies the criteria of an infectious disease because it is caused by bacteria (specifically MS) that colonize tooth surfaces from an external source.
• Unlike most classic infectious diseases, caries results from an imbalance of indigenous oral biota rather than invasion by a purely exogenous pathogen. The introduction of refined dietary sugars into modern diets shifts the ecological balance of the oral biofilm toward acidogenic/aciduric species, tipping the scale from health toward disease.
• The host-parasite model - applicable to other infectious diseases - can be directly applied to understand the pathogenicity of the caries process.
• The primary cariogenic species, Streptococcus mutans and Streptococcus sobrinus, exhibit classical virulence properties including adhesion to tooth surfaces via glucosyltransferases, acid production (acidogenicity), and acid tolerance (aciduricity).

VERTICAL TRANSMISSION: FROM MOTHER TO CHILD

The Window of Infectivity

• MS was initially acquired by 38 of 46 children at a median age of 26 months, coinciding with the eruption of primary molars - a period of increased tooth surface area and microbial habitat.
• This acquisition occurred during a discrete, defined window - the "window of infectivity" - roughly between 19 and 31 months of age.
• Eight children (17%) remained MS-free throughout the study period.
• Children cared for exclusively by their mothers during ages 1-2 years had significantly higher colonization rates compared with children attended by other caregivers.
• The mothers' MS levels were the primary determinant of infant colonization.

Molecular Evidence of Vertical Transmission

HORIZONTAL TRANSMISSION: AMONG CHILDREN AND PEERS

• MS can colonize the mouths of predentate infants, preceding tooth eruption.
• Horizontal transmission among children in daycare settings, schools, and among siblings does occur.
• These newer findings expanded the understanding of transmission dynamics and highlighted new opportunities for prevention beyond the mother-infant dyad alone.

Systematic Review and Meta-Analysis: Horizontal Transmission

• Children sharing only one S. mutans genotype with peers had a 40% lesser risk of dental caries (RR = 0.60; 95% CI: 0.45-0.80; p ≤ 0.001).
• Children sharing more than one genotype with peers had a 46% higher risk of dental caries (RR = 1.46; 95% CI: 1.13-1.89; p = 0.004).
• These dose-response findings strongly support the biological plausibility and clinical significance of horizontal transmission in caries development.

VIRULENCE FACTORS OF MUTANS STREPTOCOCCI ENABLING TRANSMISSION AND COLONIZATION

1. Adhesion - S. mutans produces glucosyltransferases (GTF-B, GTF-C, GTF-D) that synthesize extracellular polysaccharides (glucans) from dietary sucrose. These glucans mediate irreversible adhesion to tooth surfaces and form the structural scaffold of the cariogenic biofilm. Fructosyltransferase (FTF) also produces fructans that serve as energy reserves.
2. Acidogenicity - S. mutans ferments a wide range of carbohydrates (glucose, sucrose, fructose, lactose) through glycolytic pathways, generating lactic acid as the primary end product. This rapid acid production decreases plaque pH below the critical level (pH 5.5) for enamel demineralization.
3. Aciduricity - A defining property that distinguishes MS from other oral streptococci is their ability to survive and continue metabolic activity at low pH (pH 4.0-5.0). This acid tolerance is mediated by F-ATPase proton pump activity, allowing S. mutans to outcompete commensal organisms at reduced pH, creating a positive feedback loop in biofilm acidification.
4. Bacteriocin production - Mutacins, bacteriocin-like substances produced by S. mutans, inhibit competing bacterial species, helping to establish and maintain dominance within the biofilm community.

RECENT EVIDENCE: ORAL MICROBIOME PERSPECTIVES AND 2026 DATA

The Infant Oral Microbiome and Parental Transmission

• The infant oral microbiome is not sterile at birth and begins to be colonized immediately, initially from birth canal, skin, and environmental exposures.
• Parental practices - particularly salivary sharing behaviors such as pre-tasting food, cleaning pacifiers orally, and mouth-to-mouth feeding - are confirmed modulators of oral microbiome composition in infants.
• Horizontal transmission from caregivers shapes the diversity and stability of the infant oral microbiome with demonstrable consequences for caries risk.
• Oral microbiome dysbiosis driven by colonization with cariogenic species and dietary sugar exposure increases the risk of early childhood caries.

2026 Evidence: Glucosyltransferase Genes and Maternal-Child Transmission

• S. mutans, S. sanguinis, and S. oralis were the predominant species detected.
• Mean S. mutans load was significantly correlated between mother and child (r = 0.42, p < 0.01 for maternal-child DMFT values).
• Six gtf gene variants (gtfD, gtfT, gtfK, gtfP, gtfR, gtfG) were variably distributed across isolates, with maternal enrichment of S. oralis gtfR.
• For the first time in a Turkish population, multispecies gtf gene carriage (including non-mutans streptococci) was documented, highlighting that cariogenic potential is not confined to S. mutans alone.
• The study underscored the importance of maternal microbial screening and family-centered preventive strategies in pediatric dentistry.

MODES OF TRANSMISSION

1. Vertical Transmission (Caregiver to Child)

• Saliva sharing through kissing on the mouth, blowing on food to cool it, pre-tasting infant food, and cleaning pacifiers orally.
• The mother is the primary reservoir in most studied populations; fathers and other primary caregivers can also be sources.
• Timing is critical - transmission during the "window of infectivity" (roughly 19-31 months) leads to establishment of MS, while avoidance during this period can keep children MS-free.

2. Horizontal Transmission (Child to Child)

• Occurs among siblings, daycare peers, and classmates through shared toys, utensils, and close oral contact.
• Demonstrated by molecular genotyping of shared S. mutans strains across unrelated children.
• Higher caries risk is associated with sharing multiple genotypes rather than a single one.

3. Transmission of Non-Mutans Cariogenic Organisms

• Lactobacillus spp. are secondary colonizers associated with established caries lesions and can similarly be transmitted between individuals.
• Non-mutans streptococci with gtf gene carriage can contribute to biofilm formation and caries progression.

CLINICAL IMPLICATIONS AND PREVENTION BASED ON TRANSMISSIBILITY

1. Maternal Caries Risk Reduction

• Xylitol chewing gum for mothers: Isokangas et al. demonstrated a ~70% reduction in dmfs scores at age 5 in children of mothers who habitually used xylitol compared with chlorhexidine or fluoride varnish groups.
• Chlorhexidine varnish applied to mothers' dentition reduces salivary MS levels.
• Treatment of active carious lesions in mothers to reduce salivary MS loads.

2. Counseling on Salivary Sharing Behaviors

• Avoiding pre-tasting infant food with the same spoon.
• Avoiding oral pacifier cleaning.
• Avoiding mouth-to-mouth kissing in high-risk families.

3. Early Microbial Risk Assessment

4. School and Daycare Settings

• Supervised toothbrushing programs.
• Avoidance of shared eating utensils.
• Fluoride programs to reduce the pathogenic potential of transmitted organisms even if colonization occurs.

FULFILLMENT OF CRITERIA FOR AN INFECTIOUS TRANSMISSIBLE DISEASE

CONCLUSION

CONSOLIDATED REFERENCES

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3. Caufield PW, Cutter GR, Dasanayake AP. Initial acquisition of mutans streptococci by infants: evidence for a discrete window of infectivity. J Dent Res. 1993 Jan;72(1):37-45. [PMID: 8418105]
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11. Manchanda S, Sardana D, Liu P, Lee GHM, Lo ECM, Yiu CKY. Horizontal transmission of Streptococcus mutans in children and its association with dental caries: a systematic review and meta-analysis. Pediatr Dent. 2021 Jan 15;43(1):E1-E12. [PMID: 33662253]
12. Olate P, Martinez A, Sans-Serramitjana E, et al. The infant oral microbiome: developmental dynamics, modulating factors, and implications for oral and systemic health. Int J Mol Sci. 2025 Aug 19;26(16):7983. [PMID: 40869304]
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ESSAY WRITING TIPS FOR MDS EXAMINATION

• Introduction (define the concept, state the thesis, ~10% of word count)
• Historical evidence (Keyes-Fitzgerald experiments, the experimental basis)
• Conceptual framework (Caufield's work, host-parasite model, window of infectivity)
• Molecular evidence (genotyping studies proving transmission routes)
• Vertical vs. horizontal transmission (as separate sub-headings)
• Virulence factors enabling colonization
• Recent updates (post-2020 evidence, microbiome perspective)
• Clinical implications and prevention
• Conclusion
• References cited paragraph by paragraph