Preventive Dentistry
Microbiological aspect of dental caries
Lec.10 Dr.Jihan Abdulhussein
• Dental caries is the result of the metabolic activities of bacteria in microbial
communities on teeth termed dental biofilms (often referred to as dental
plaque).
• Dental plaque (biofilm) is a dense, nonmineralized, highly organized
complex mass of bacterial colonies in a gel-like intermicrobial matrix. The
matrix protects the bacteria from the defensive cells of the body (neutrophils,
macrophages, and lymphocytes). It adheres firmly to the acquired pellicle and
to the teeth, calculus, and restorations.
• Resident microflora: Acquisition of the resident microflora occurs from birth
and is a natural process, during which all environmentally exposed surfaces of
the body become colonized.
• Once established, the resident microflora has a diverse composition,
consisting of a wide range of Gram-positive and Gram-negative bacterial
species, as well as yeasts and other types of microorganism. In addition, the
composition of the oral microflora will change as the biology of the mouth
alters over time.
Benefits of the resident microflora
The resident microflora contributes directly and indirectly to the normal
development of the host, and functions as part of the innate host defenses by
acting as a barrier (colonization resistance) to permanent colonization by
transient organisms, some of which are potentially pathogenic. Mechanisms
involved in colonization resistance by resident organisms include:
• Saturation of microbial attachment sites
• More effective competition for essential nutrients
• Creation of conditions unfavorable to the growth of invading microbes
• The production of inhibitory factors, e.g. bacteriocins and hydrogen
peroxide.
Transmission of cariogenic flora
Oral infection in children is related to frequent contact with large numbers of
bacteria from the mother (“cuddle effect”) and other caregivers within the
discrete period between 18/19 and 31 months (the “window of infectivity”).
But the presence of caries improves that children aged 9–10 months are also
already infected and S. mutans was detected in 25% of one sample of pre-
dentate children.
When the number of S. mutans in a mother is reduced permanently, her
child’s mouth is colonised at a late stage or not at all. Bacteria newly
introduced in the mouth are more likely to get established in children than in
adults with their highly colonisation-resistant, optimally adapted flora. In
children tooth eruption creates conditions favourable for colonisation. In 19–
31-month-old children a new bacterial colony is established relatively easily.
Oral environment
Adherence is a key ecological determinant for oral bacteria to survive and
persist. The mouth is unique in the human body in possessing non-shedding
surfaces (teeth) for microbial growth, leading to extensive biofilm formation
(dental plaque). In contrast, desquamation ensures that the bacterial load is
relatively light on mucosal surfaces.
Acquired pellicle is an amorphous layer that forms over exposed tooth
surfaces, restorations and dental calculus. It forms within minutes after all
external material has been removed from the tooth surfaces with an abrasive.
It is composed primarily of glycoproteins from the saliva that are selectively
adsorbed by the hydroxyapatite of the tooth surface. Although, pellicle
performs a protective function, acting as a barrier to the acids, it also serves
the initial site of attachment to the bacteria and begins the first stage of
biofilm development
Oral habitats
Buccal mucosa, dorsum of the tongue, teeth, crevicular epithelium, and
prosthodontics, and orthodontics appliances.
Factors modulating microbial Growth
a-anatomical factor
b-saliva
c-gingival crevicular fluid
d-microbial factors
e-miscellaneous
f-nutrition of oral bacteria
Teeth do not provide a uniform habitat for microbial growth which is optimal
for the growth of many micro-organisms.
Saliva has a profound influence on the ecology of the mouth for example, its
ionic composition promotes its buffering properties and its ability to
remineralize (i.e. repair) enamel.
In addition, the organic components (glycoproteins and proteins) can
influence the establishment and selection of the oral microflora by either
coating oral surfaces, and adaptive immunity (e.g. sIgA) and so can directly
inhibit some exogenous micro-organisms.
Saliva plays other roles in regulating the growth and metabolic activity of the
oral microflora. Saliva helps to maintain the pH in the oral cavity at values
around 6.75-7.25 and the temperature at around 35-36°C.
The tongue has a highly papillated surface providing protection in the crypts
to fastidious bacteria including obligate anaerobes. The tongue acts as
reservoir for many species that are commonly found in dental plaque. So the
biological and physiological properties of each site result in only a subset of
the organisms (often 20-30 distinct types) being able to predominate at an
individual site.
A carbohydrate-rich diet increases the acid production and growth rate of
many oral bacteria.
Plaque development
• Soon after polishing a tooth, a biofilm of negatively charged salivary
glycoproteins, the salivary pellicle, is seen to adhere to the tooth. Salivary
bacteria (
∼2.108/mL) are attracted towards this pellicle, and the sparse,
reversible colonisation of the pellicle with a few species soon stabilizes
microcolonies develop, which produce an interbacterial matrix that encloses
other bacteria which do not have a capacity to adhere to the pellicle.
• A biofilm community comprises bacterial microcolonies, an extracellular
slime layer, fluid channels, and a primitive communication system. The
bacteria cluster together to form sessile, mushroom-shaped microcolonies that
are attached to the surface at a narrow base. Different microcolonies may
contain different combinations of bacterial species. Bacteria in the center of a
microcolony may live in a strict anaerobic environment, while other bacteria
at the edges of the fluid channels may live in an aerobic environment
Clinically: plaque presents as a transparent film and therefore, difficult to
visualize. It can be detected with an explorer by passing the explorer over the
tooth surface near the gingival margin to collect plaque, which makes it easier
to see. Plaque disclosing solutions that stains the invisible plaque is used for
easy detection of plaque. It stains the plaque and makes it visible to the eyes.
These solutions disclose the extent and location of the plaque.
• The complexity of the plaque microbial population strongly depends on:
• The salivary properties,
• Crevicular fluid,
• Mechanical factors,
• The substrate and
• Other plaque-related factors (e.g. age of the plaque).
It takes a few days for newly formed plaque to become cariogenic.
Composition of plaque
• Already at birth, the mouth is colonised by bacteria. The oral epithelium is in
a state of continuous replacement; thus, bacteria that need to adhere to stable
surfaces to multiply (e.g. S. mutans) only colonise the oral cavity permanently
after tooth eruption, then more complex plaque that forms contains bacteria
associated with caries.
• Bacteria comprise 70% of plaque; the remainder 30% is composed of
intercellular material derived chiefly from the bacteria, salivary proteins and
epithelial cells, also plaque fluid with calcium and phosphate, and rarely, food
remnants.
Virulence factors
The amount of acids, the rate of production and the time for which they
remain on the tooth surface co-determine the cariogenicity of plaque. In the
presence of sugar, plaques with larger numbers of mutans streptococci quickly
produce much acid
The plaque hypothesis to explain role of dental biofilm bacteria in the etiology
of dental caries
• The specific plaque hypothesis proposed that, only a single or very small
number of species of organisms comprising the resident plaque micro-flora
were actively involved in disease. This proposal has been easy to promote
because it focused efforts on controlling disease by targeting preventive
measures and treatment against a limited number of organisms, such as by
vaccination or gene therapy or by antimicrobial treatment
• non-specific plaque hypothesis considered that disease is the outcome of the
overall activity of the total plaque microflora, so not just those that make acid,
but also species that produce alkali or consume lactate need to be considered.
Thus, a heterogeneous mixture of microorganisms could play a role in disease.
Explination: of non-specific theory
1- There is extreme variation in supra-gingival plaque
2- Other plaque bacteria have the same biochemical characteristics
3- Bacteria associated with caries other than mutans streptococcus and
lactobacillus
4- The presence of lactate-consuming species (e.g. Veillonella)
5- The production of alkali to raise the local pH (e.g. by ammonia
production by S. salivarius and S. sanguinis).
This has a great significance for caries prevention, since implicit in the
hypothesis is the concept that disease can be controlled by targeting the
putative pathogens (mutans streptococci and other acidogenic/aciduric
species) through interference with the factors that are driving the deleterious
shifts in the balance of the microflora. Identification of such critical control
points (e.g. mechanical biofilm removal, saliva stimulation and/or dietary
control) can lead to the selection of appropriate caries-preventive strategies
that are tailored to the needs of individual patients.
• The ecological plaque hypothesis proposes that the organisms associated
with disease may also be present at sound sites, but at levels too low to be
clinically relevant. Disease is a result of a shift in the balance of the resident
microflora driven by a change in local environmental conditions. In the case
of dental caries, repeated conditions of low pH in plaque following frequent
sugar intake (or decreased sugar clearance following low salivary secretion)
will favor the growth of acidogenic and aciduric species, and thereby
predispose a site to caries.