Microbiology of Caries

Is the oral cavity sterile at birth?

Yes — the human oral cavity is sterile at birth. Oral microorganisms are acquired shortly after birth.

When has S. mutans been detected in infants, and what was it associated with?

S. mutans has been detected in 3-month-old infants, associated with developmental oral nodules. A significant shift in the oral microorganisms occurs before (predentate) and after (dentate) tooth eruption.

How is an infant typically infected with Streptococcus mutans early in childhood?

By transmission from parents, playmates, or caregivers — through any behavior allowing contact with adult saliva.

List behaviors that allow transmission of S. mutans to an infant via adult saliva.

Kissing by anyone involved in the child's care, cleaning teats/pacifiers in the mother's mouth, pre-tasting the child's food, sucking on adult fingers, and sharing food or utensils.

What determines successful oral implantation (transmission) of S. mutans in an infant?

A higher S. mutans level and poor oral hygiene in the mother, along with the magnitude of the inoculum (amount of bacteria transferred to the child's mouth).

Why should control of early childhood caries start before a child is even born?

Because early infection of an infant with S. mutans is associated with high and early caries activity in childhood, so prevention should target controlling caries activity in the mother and potential caregivers ahead of time.

What is the advantage of good maternal oral hygiene for caries prevention in a child?

Lower maternal S. mutans levels and better oral hygiene reduce the bacterial inoculum transmitted to the infant, lowering the child's risk of early and high caries activity.

What are the two distinct types of indigenous (normal) oral flora, by Gram stain?

Gram-positive bacteria and Gram-negative bacteria.

Name the Gram-positive Streptococcal species that are part of normal oral flora.

S. salivarius, S. sanguis, S. mutans, and S. milleri.

Name the Gram-positive non-Streptococcal species that are part of normal oral flora.

Actinomyces spp., Corynebacterium spp., Lactobacillus spp., and Staphylococcus spp.

Name examples of Gram-negative oral flora, and what disease are they mainly associated with?

Bacteroides spp., Fusobacterium spp., Neisseria spp., Veillonella spp., and Haemophilus spp. — these are mainly associated with causing periodontal disease.

Roughly how many bacteria and bacterial species live in the human mouth?

About 100,000,000,000,000 (100 trillion) bacteria per mouth, representing more than 700 bacterial species.

What are the major microorganisms associated with dental caries in humans?

Streptococci (particularly S. mutans), Lactobacilli, Actinomyces, and more recently Bifidobacteria.

Who first recognized S. mutans as a cariogenic bacterium, and when?

J.K. Clarke, in 1924, when he isolated it from a caries lesion.

What did Bratthall (1970) define regarding Streptococcus mutans-group serotypes?

He defined 5 serotypes (a, b, c, d, e — with f, g, and h added later) based on the serological specificity of carbohydrate antigens.

Which mutans streptococci serotype/species combination causes caries on all tooth surfaces?

S. mutans serotype c.

What is the first virulence factor that allows S. mutans to colonize the tooth?

Expression of adhesin — surface antigens on the bacteria that bind to salivary glycoproteins on the tooth surface (pellicle).

What is the second major virulence factor of S. mutans, and what enzymes does it involve?

Expression of glucosyltransferase (gtf B, C, D) and fructosyltransferase (ftf), enzymes that build glucans and fructans from sucrose.

What is the third virulence factor of S. mutans?

Expression of glucan-binding proteins (GBP), which help the bacteria bind to the glucans formed on the tooth surface.

What is the fourth virulence factor of S. mutans, and what does it involve?

Acidogenicity — efficient and faster sucrose metabolism combined with production of stronger acid.

What is the fifth virulence factor related to sugar metabolism in S. mutans?

Dual metabolic systems that allow adaptation to both high and low sugar concentrations (the PTS and PMF systems).

What is the sixth virulence factor of S. mutans, called aciduricity?

The ability of the cell membrane to maintain a relatively alkaline cytoplasm under acidic conditions — i.e., the ability of the bacteria to survive and keep functioning in an acidic environment.

What are glucans, and what are the two types formed from sucrose?

Glucans are polymers of glucose formed from sucrose via glucosyltransferase; the two types are dextrans and mutans.

What is the key structural/functional difference between dextran and mutan?

Mutan is a water-insoluble glucan (predominant α-1→3 linkage) that is a major component of dental biofilm and enhances plaque formation, and is not easily metabolized by microorganisms. Dextran is generally water-soluble (α-1,6 linkages) and is easily metabolized by microorganisms for energy.

What is fructan (levan), and what is a key property of it?

Fructan (levan) is a polymer of fructose formed via fructosyltransferase from sucrose; it is very soluble and frequently metabolized by bacteria.

What is dental plaque, in terms of bacteria and EPS?

Plaque = Bacteria + Exopolysaccharides (EPS); a large portion of plaque's volume/mass is actually made up of bacteria and the extracellular polysaccharide matrix they produce.

How do Streptococci generate energy, given they lack the Krebs cycle enzymes and cytochromes?

They generate energy via anaerobic glycolysis.

What are the two main sugar-uptake systems bacteria use to metabolize glucose, and how do they differ in pH sensitivity?

The Phosphotransferase (PTS) system — high affinity, driven by phosphoenol pyruvate (PEP), operating at neutral pH — and the Proton-motive force (PMF) system — low affinity, activated by a proton motive force, operating at low (acidic) pH.

Why is the PMF system important for S. mutans specifically?

It allows S. mutans to continue transporting and metabolizing sugars under acidic conditions that are unfavorable to other, less acid-tolerant bacteria — giving it a competitive advantage in a cariogenic environment.

What are the two end-product pathways for pyruvate in bacterial sugar metabolism, and their enzymes?

Lactate dehydrogenase (LDH) converts pyruvate to lactic acid; pyruvate formate lyase (PFL) converts pyruvate to formic acid, acetic acid, and ethanol.

What defines homofermentative bacteria, and give two examples relevant to caries?

Homofermentative bacteria ferment sugars producing 90% or more lactic acid as the end product; examples are Streptococcus mutans and Lactobacilli.

What defines heterofermentative bacteria, and what acids do they produce?

They produce a mixture of metabolites/organic acids, including acetic, formic, propionic, butyric, and succinic acids.

What are the two fermentation types found among Lactobacilli species, with examples?

Homofermentative (e.g., L. casei, L. acidophilus) and heterofermentative (e.g., L. fermentum).

Under what oral conditions are Lactobacilli usually found in high numbers?

When something is wrong — such as caries, dry mouth, poor oral hygiene, or frequent sugar consumption.

What clinical stage of caries are Lactobacilli particularly associated with?

Established caries lesions and their progression, especially deep dentinal caries.

Why don't Lactobacilli initiate caries on smooth tooth surfaces?

They do not adhere well to tooth surfaces; they need retention niches such as pits/fissures, cavities, or orthodontic devices to become established.

How prevalent are Lactobacilli within total plaque flora, and when are they found in saliva?

They are only a minor fraction of plaque flora (about 1 in 10,000), but are found in saliva when open cavities are present; there is a significant correlation between caries lesions and lactobacilli counts in children and adults.

What salivary/plaque count of S. mutans or Lactobacilli was historically used as a high caries-risk threshold?

A count of ≥10^5 CFU/ml of saliva was historically used to place an individual at high caries risk.

What did a more recent study find regarding visible plaque versus S. mutans as a caries risk indicator?

The presence of visible plaque, not S. mutans specifically, was associated with caries occurrence.

What are Actinomyces species, in terms of Gram stain and morphology?

Gram-positive rods/filaments.

Give examples of facultative anaerobe and strict anaerobe Actinomyces species.

Facultative anaerobes: A. naeslundii and A. viscosus. Strict anaerobes: A. israelii and A. odontolyticus.

What acids do Actinomyces produce as glucose fermenters, and where do they predominate?

Mostly lactic acid, also acetic, succinic, and formic acids; Actinomyces spp. predominate in sub-gingival plaque.

What proportion of plaque organisms in children can Actinomyces represent, and what caries types are specific species linked to?

Up to 50% of organisms in children's plaque; A. viscosus and A. naeslundii are linked to root caries, and A. odontolyticus has been linked to early stages of caries.

What modern molecular techniques have been used to identify oral Bifidobacteria?

Pathogen-specific polymerase chain reaction (PCR), Human Oral Microbe Identification Microarray (HOMIM), Rapid Multiplexed 16S Barcode Microbiome Sequencing (RMBMS), and novel selective media containing mupirocin.

How does the detection frequency of Bifidobacteria compare to S. mutans in caries research?

Bifidobacteria have been detected in higher proportions and frequency than S. mutans in some studies, and can produce extracellular polysaccharides that allow biofilm (plaque) formation.

In which population is salivary Bifidobacteria specifically noted as caries-associated?

Older adults — salivary bifidobacteria is caries-associated in older adults, and is also found in high numbers in caries lesions of children and adults.

Which two Bifidobacteria/related species are significantly associated with caries?

Bifidobacterium dentium and Scardovia wiggsiae.

What clinical measure were salivary Bifidobacteria levels positively associated with?

The number of decayed and filled tooth surfaces (DFS).

From which oral sites have Bifidobacteria been isolated, and in which lesion types found?

Isolated from saliva, supragingival plaque, and infected dentin; found in root caries lesions in adults and in occlusal lesions in both adults and children.

What role do the collected findings suggest Bifidobacteria may play in caries?

They may play a role in the progression of occlusal caries lesions in both children and adults.