Lec-5 Ecology of the oral flora

Collage of Dentistry
Medical Microbiology for third stage undergraduate student

Lec-5 Ecology of the oral flora

The oral ecosystem
Ecology is the study of the relationships between living organisms and their environment. An understanding of oral ecology is essential to comprehend the pathogenesis of diseases, such as caries and periodontal disease, caused by oral bacteria.
The oral environment
The human mouth is lined by stratified squamous epithelium. This is modified in areas according to function (e.g. the tongue) and interrupted by other structures such as teeth and salivary ducts. The gingival tissues form a cuff around each tooth, and there is a continuous exudate of crevicular fluid from the gingival crevice. A thin layer of saliva bathes the surface of the oral mucosa.
The mouth, being an extension of an external body site, has a natural microflora. This commensal (or indigenous, or resident) flora exists in harmony with the host, but disease conditions supervene when this relationship is broken. The predominant dental diseases in humans (caries and periodontal disease) are caused in this manner. In addition to the commensal flora, there are others (such as coliforms) that survive in the mouth only for short periods (transient flora). These transient flora cannot get a foothold in the oral environment due to the ecological pressure, i.e. the colonization resistance exerted by the resident flora. Indeed, the latter are considered critical in defending the key portal of entry into the digestive system, by offending pathogens.
The oral ecosystem comprises the oral flora, the different sites of the oral cavity where they grow (i.e. habitats) and the associated surroundings.
Oral habitats
The major oral habitats are:
• buccal mucosa
• dorsum of the tongue
• tooth surfaces (both supragingival and subgingival)
• crevicular epithelium

• prosthodontic and orthodontic appliances, if present.
Buccal mucosa and dorsum of the tongue
Special features and niches of the oral mucosa contribute to the diversity of the flora; for instance, the cheek mucosa is relatively sparsely colonized, whereas the papillary surface of the tongue is highly colonized because of the safe refuge provided by the papillae. The papillary surface of the tongue has a low redox potential (the oxidation–reduction potential) of the environment (Eh), promoting the growth of anaerobic flora, and thus may serve as a reservoir for some of the Gram-negative anaerobes implicated in periodontal disease. Further, the keratinized and non-keratinized mucosae may offer refuge to variants of oral flora.
Teeth
The surfaces of the teeth are the only non-shedding area of the body that harbours a microbial population. Large masses of bacteria and their products accumulate on tooth surfaces to produce dental plaque, present in both health and disease. Plaque is a classic example of a natural biofilm and is the major agent initiating caries and periodontal disease. In the latter situations, there is a shift in the composition of the plaque flora away from the species that predominate in health.
A range of habitats are associated with the tooth surface. The nature of the bacterial community varies depending on the tooth concerned and the degree of exposure to the environment: smooth surfaces are colonized by a smaller number of species than pits and fissures; sub- gingival surfaces are more anaerobic than supragingival surfaces.

Fig. 1 Habitats associated with tooth surfaces and the nomenclature of the plaque derived from these habitats.
Crevicular epithelium and gingival crevice
Although this habitat is only a minor region of the oral environment, bacteria that colonize the crevicular area play a critical role in the initiation and development of gingival and periodontal disease.
Prosthodontic and orthodontic appliances
If present and not kept scrupulously clean, dental appliances may act as inanimate reservoirs of bacteria and yeasts. Yeasts on the fitting surface of full dentures can initiate Candida- associated denture stomatitis due to poor denture hygiene.
¥ Acquisition of the normal oral flora

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.

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 lost as a result of senility. Bacteria that colonize the mouth at this stage are very similar to those in a child before tooth eruption.

9. Introduction of a prosthetic appliance at this 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 years or over. The denture plaque is somewhat similar to enamel plaque; it may also harbour significant quantities
of yeast.


BOOK: Essential microbiology for dentistry 2012. Elsevier Ltd. Fourth edition, Lakshman Samaranayake. Elsevier Ltd.

Prepared by A. Prof. Dr. Zainab Khudhur Ahmad