2. 1. The infant mouth is sterile at birth, except perhaps for a few organisms acquired from the mother’s birth canal.
2. A few hours later, the organisms from the mother’s (or the nurse’s) mouth and possibly a few from the environment are established in the mouth.
3. These pioneer species are usually streptococci, which bind to mucosal epithelium (e.g., Streptococcus salivarius).
4. The metabolic activity of the pioneer community then alters the oral environment to facilitate colonization by other bacterial genera and species. For
instance, Streptococcus salivarius produces extracellular polymers from sucrose, to which other bacteria such as Actinomyces spp. can attach (Figs 31.3
and 31.4).
5. When the composition of this complex ecosystem (comprising several genera and species in varying numbers) reaches equilibrium, a climax
community is said to exist. (Note: this is a highly dynamic system.)
6. Oral flora on the child’s first birthday usually consists of streptococci, staphylococci, neisseriae and lactobacilli, together with some anaerobes such as
Veillonella and fusobacteria. Less frequently isolated are Lactobacillus, Actinomyces, Prevotella and Fusobacterium species.
7. The next evolutionary change in this community occurs during and after tooth eruption when two further niches are provided for bacterial
colonization: the hard-tissue surface of enamel and the gingival crevice. Organisms that prefer hard-tissue colonization, such as Streptococcus mutans,
Streptococcus sanguinis and Actinomyces spp., then selectively colonize enamel surfaces, and those preferring anaerobic environments, such as
Prevotella spp., Porphyromonas spp. and spirochaetes, colonize the crevicular tissues. However, the anaerobes do not appear in significant numbers until
adolescence. For instance, only 18%–40% of 5-year olds have spirochaetes and black-pigmented anaerobes compared with 90% of 13- to 16-year olds.
8. A second childhood (in terms of oral bacterial colonization) is reached if all teeth are surgically extracted. Bacteria that colonize the mouth at this stage
are very similar to those in a child prior to tooth eruption.
9. Introduction of a prosthetic appliance at any stage changes the microbial composition once again. Growth of Candida species is particularly increased
after the introduction of acrylic dentures, while it is now recognized that the prevalence of Staphylococcus aureus and lactobacilli is high in those aged 70
or over. The denture plaque biofilm is somewhat similar to plaque biofilm on enamel surface; it may also harbour significant quantities of yeast.
3. The plaque biofilm
The plaque biofilm is a tenacious microbial community embedded
in an extracellular polysaccharide matrix, and attached to
either the soft- or hard-tissue surfaces of the mouth, comprising
living and dead bacteria and their extracellular products,
together with host compounds, mainly derived from the saliva.
4. Composition
The organisms in plaque biofilm are embedded in an organic matrix, which
comprises about 30% of the total volume. The matrix is derived from the products
of both the host and biofilm constituents. In the gingival area, proteins from the
crevicular exudate become incorporated into the plaque biofilm. This matrix acts as
a food reserve and as a cement, binding organisms both to each other and to
various surfaces.
The microbial composition of dental plaque biofilm can vary widely between
individuals; some people are so-called rapid plaque formers and others slow plaque
formers. Further, there are large variations in plaque composition within an
individual, for example:
■ at different sites on the same tooth
■ at the same site on different teeth
■ at different times on the same tooth site.
5. Distribution
Plaque biofilm is found on dental surfaces and appliances especially in the
absence of oral hygiene. In general, it is found in anatomical areas protected
from the host defences, for example, occlusal fissures, interproximally or
around the gingival crevice. Plaque samples are described in relation to their
site of origin and are categorized as supragingival:
■ fissure plaque: mainly in molar fissures
■ approximal plaque: at contact points of teeth
■ smooth surface: for example, buccal and palatal surfaces subgingival, or
appliance associated:
■ full and partial dentures (denture plaque)
■ orthodontic appliance-related plaque.
6. Microbial adherence and plaque
biofilm formation
Adherence of a microbe to an oral surface is an essential prerequisite for colonization and
biofilm formation. It is also the initial step in the path leading to subsequent infection and
invasion of tissues. There are a number of intrinsic host factors that prevent microbial
colonization on oral surfaces and these include (Fig. 31.6):
■ the mucosal barrier with constant desquamation of the epithelium that dislodges the
attached organisms from soft-tissue surfaces.
■ the dynamic salivary flow patterns in different oral niches
■ the muscular movements of the tongue and cheeks that physically dislodge the biofilms
■ the non-specific and specific defines factors (such as IgA) in saliva
■ the resident community of microbiota that offers colonization resistance’ to invading
extraneous organisms.
7. Factors affecting microbial colonization of the oral mucosa.
Plaque biofilm formation is a complex process comprising a
number of different stages:
Plaque biofilm formation
8. 1. Pellicle formation. Adsorption of host and bacterial molecules to the tooth surface forms the acquired salivary pellicle. A thin layer
of salivary glycoproteins is deposited on the surface of a tooth within minutes of exposure to the oral environment. Oral bacteria
initially attach to the pellicle and not directly to enamel (i.e., hydroxyapatite).
2. Transport. Bacteria approach the vicinity of the tooth surface prior to attachment, by means of natural salivary flow, Brownian
motion or chemotaxis.
3. Long-range interactions involve physicochemical interactions between the microbial cell surface and the pellicle-coated tooth.
Interplay of van der Waals forces and electrostatic repulsion produces a primary reversible phase of net adhesion.
4. Short-range interactions consist of stereochemical reactions between adhesins on the microbial cell surface and receptors on the
acquired pellicle. This is an irreversible phase in which polymer bridging between organisms and the surface helps to anchor the
organism, after which they multiply on the virgin surface. Doubling times of plaque biofilm bacteria can vary considerably (from
minutes to hours), both between different bacterial species and between members of the same species, depending on the
environmental conditions.
5. Coaggregation or coadhesion. Fresh bacteria now attach on to the already attached first generation of cells (also called pioneer or
initial colonizers) these may be bacteria of the same genus or different but compatible genera (Fig. 31.4).
6. Biofilm formation. The attached organisms now grow horizontally on the surface and form micro-colonies at
9. first whilst the aforementioned process continues with a resultant confluent growth and the formation of a
biofilm, which matures in complexity as time progresses. Simply defined, biofilm is a complex functional
community of one or more species of microbes encased in an extracellular polysaccharide matrix and attached
to one another or to a solid surface. The latter could be an inert surface such as tooth enamel, denture acrylic
or a plastic catheter or alternatively an organic/living surface such as a heart valve. Architecturally, the biofilm
is not a flat compact structure resembling an inert piece of concrete.
The aggregates of organisms are arranged in columns or mushroom-shaped structures interspersed with water
channels that carry metabolites and bring in nutrients (Figs 5.2 and 5.3). Thus, biofilm formation is a complex,
competitive, sequential and dynamic colonization process, and in plaque biofilms, this complexity is further
compounded due to the participation of different categories of oral bacteria. Specifically, the pioneer group of
organisms that selectively colonize the salivary pellicle during plaque biofilm formation are Gram-positive cocci
and rods. These are followed by Gram-negative cocci and rods, and finally by filaments, fusobacteria, spirils
and spirochaetes. Such an example of a natural succession of plaque flora has been elegantly demonstrated in
‘experimental gingivitis’ studies, where groups of individuals, initially subjected to meticulous oral hygiene,
were then followed up during a phase of no oral hygiene, and the freshly developing plaque flora was
monitored closely. Results of such a study are shown in Fig. 31.7. One major component of a biofilm is the
extracellular matrix. This comprises microbial polysaccharides and additional layers
10. Results from an experimental study showing the predominant groups of organisms comprising the pioneer and
the climax community of plaque. Note the relationship between the plaque index and the gingival index.
11. of salivary glycoprotein (or crevicular fluid components, depending on the site). The metabolic
products of the early plaque biofilm colonizers can radically alter the immediate environment (e.g.,
create a low redox potential suitable for anaerobes), leading to new colonizers inhabiting the
biofilm, with a resultant gradual increase in microbial complexity, biomass and thickness. As a result
of this dynamic process, the plaque biofilm mass reaches a critical size at which a balance between
the deposition and loss of plaque bacteria is established; this community is termed the climax
community (Fig. 31.8).
The molecular biology of biofilm formation is complex. Biofilm bacteria appear to maintain their
complex structure through continuous secretion of low levels of molecules called quorum-sensing
molecules (e.g., homoserine lactone, autoinducer-2) that coordinate gene expression. As the
number of organisms in the biofilm increases, there is a simultaneous, proportionate increase in the
quorum-sensing signals. These activate genes that may be related to additional extracellular
polysaccharide production, or reduction of metabolism (for bacteria at the bottom of the matrix) or
production of virulent factors, including drug-destroying genes.
Detachment
The bacteria that colonize this climax community may detach and enter the planktonic phase (i.e.,
suspended in saliva) and be transported to new colonization sites, thus restarting the whole cycle.
