Miller and later researchers such as Howe discovered that Bifidobacterium and Lactobacillus, two bacteria responsible for producing lactic acid, were associated with early carious lesions on teeth; however, experimental animals did not develop caries from these exposures.
Recent work using anerobic culture and molecular cloning methods demonstrated that tooth crown and dentin caries microbiota varied depending on lesion depth. This supports ecological plaque hypothesis extended to dentin.
1. The Role of Streptococcus
Streptococcus, one of the most numerous genera found in oral microbiomes, plays a pivotal role in caries development. Kilian Clark discovered a strain of Streptococcus (later named Streptococcus mutans) from initial carious lesions capable of demineralizing tooth sections during its isolation from initial lesions in 1920s; later this strain’s colonization and production of organic acids that act on dental tissues to cause demineralization and cavities was vital in supporting his ecological plaque hypothesis which suggests changes in community structure play an integral role in caries formation over time.
At the outset of caries progression lies its colonization by pioneer species such as Streptococcus gordonii, S. sanguinis, S. oralis and Veillonella tobetsuensis; all capable of colonizing an enamel salivary pellicle-encased niche within dentin-enveloped teeth through gene expression regulation and protein production to maintain factors that keep attachment to teeth intact.
2. The Role of Lactobacillus
Studies have identified Lactobacillus as being linked with dental caries. Early oral colonization with Lactobacillus correlates with higher risk and severity of caries; additionally, Lactobacillus counts are lower among those suffering from advanced dentin caries (S-ECC) than among those without [183].
Lactobacillus species that are resistant to acid are more often isolated from caries sites, fitting well with the ecological plaque hypothesis extended to dentin caries and with the idea that the microenvironment deep within dentin contributes to disease by modulating the composition of its microbiome. Recent metatranscriptomic studies have also demonstrated that bacteria present at caries lesions have various functions; from breaking down sugars associated with decayed tooth structure through degrading sugary substances into their constituent components to producing lactic acid which promotes demineralization.
These findings could be the result of differences in biofilm sensitivity at each carious site to environmental influences such as rate of acidification, availability of nutrients substrates, presence of other microorganisms that could compete for limited resources pool and changes in pH gradient and mineral content of tooth surfaces.
3. The Role of Pseudomonas
An initial microbial colonization layer occurs on tooth enamel and dentin/root surfaces. These first colonizers, commonly referred to as pioneer species, produce cell surface ligands which interact with proteins found within teeth (e.g. pellicle-associated adhesion proteins Has and AbpA). Through such interactions between bacteria and proteins in teeth (LAPRA receptors) attachment can occur quickly on these hydroxyapatite surfaces of teeth.
Initial biofilm formation involves an extracellular polysaccharide matrix that binds bacteria cells and makes them adherent, providing structural integrity to dental plaque and outlining its physical composition [43]. This matrix defines its physical structure within biofilm.
Pseudomonas aeruginosa has been identified as one of the main causative agents for nosocomial infections among cystic fibrosis patients [44, 45]. Water samples taken from dental chair units with cellulose nitrate filters placed over tube vents were assayed for total viable bacterium (TVC) counts; Pseudomonas was the predominant contaminant found here.
4. The Role of Enterococcus
Enterococcus as an early colonizer produces abundant extracellular polysaccharides to cement itself to tooth surfaces and act as an acid reservoir, leading to infiltrative decay of dental pellicle and demineralization of enamel surfaces (dental caries).
Initial lesions may be clinically reversible. If carbohydrate challenge continues over time, however, microbial shift may occur where more acidogenic and acid-tolerant species become dominant; this community composition corresponds with ecological plaque hypothesis extended to dentin caries.
Although 16S rRNA gene-based profiling of oral microbiota can provide useful taxonomic data, systems-level analyses like metatranscriptomics and metabolomics provide more in-depth functional information that reveals how these consortia perform their various roles both healthy and diseased states, providing researchers with greater insights into the complexity of oral microbiome interactions involved with caries pathogenesis.