Human Microbiome and Oral Microbiology: Study Guide

Human Microbiome: Terminology and Concepts

Normal Microbiota

The normal microbiota refers to the collection of microorganisms that reside on and within the human body without causing disease under normal conditions. These microbes play essential roles in health and disease prevention.

• Resident microbiota: Microorganisms that are consistently present in a particular body site over time.

• Transient microbiota: Microbes that temporarily colonize the body but do not persist long-term.

Symbiosis

Symbiosis describes the relationship between two different organisms living in close association. Types include:

• Commensalism: One organism benefits, the other is unaffected.

• Mutualism: Both organisms benefit.

• Parasitism: One organism benefits at the expense of the other.

Opportunistic Pathogens

Opportunistic pathogens are normally harmless microbes that can cause disease when the host's defenses are compromised or when they gain access to unusual body sites.

Bacteriocins and Probiotics

• Bacteriocins: Antimicrobial peptides produced by bacteria to inhibit the growth of similar or closely related bacterial strains.

• Probiotics: Live microorganisms that, when administered in adequate amounts, confer a health benefit to the host.

Infectious vs. Disease

• Infectious: Capable of causing infection, i.e., invasion and multiplication of microorganisms in body tissues.

• Disease: A condition in which normal body function is impaired, often as a result of infection.

Benefits of the Normal Flora

• Vitamin production: Some gut bacteria synthesize vitamins (e.g., vitamin K, B vitamins).

• Competitive exclusion: Normal flora outcompete pathogens for nutrients and attachment sites.

• Microbial antagonism: Production of substances (e.g., bacteriocins) that inhibit pathogens.

• Tissue development: Normal microbiota contribute to the development of immune tissues.

• Antibody stimulation: Exposure to normal flora helps the immune system recognize and respond to pathogens.

Major Areas Colonized by Microbiota

The four major areas of the body colonized by microbiota include:

• Skin: Exposed to air, dry, acidic, and salty; contains sweat and sebaceous secretions; limits microbial growth but supports specialized residents.

• Gut (Gastrointestinal tract): Anaerobic, nutrient-rich; microbes adapt to low oxygen and bile salts.

• Oral cavity: Moist, variable pH, contains saliva with antimicrobial factors; supports diverse microbial niches.

• Other mucosal surfaces (e.g., respiratory, urogenital tracts): Each with unique secretions and environmental conditions.

Oral Cavity Microbiology

Anatomy and Microbial Niches

The oral cavity contains multiple niches, including teeth, tongue, cheeks, gingival crevice, and palate. Each niche provides distinct environmental conditions (e.g., oxygen levels, pH, nutrient availability) that select for different microbial communities.

Role of Saliva

• Mechanical action: Washes away food particles and unattached microbes.

• Antimicrobial components: Contains lysozyme (breaks down bacterial cell walls) and lactoperoxidase (produces reactive oxygen species toxic to bacteria).

Role of Breastmilk

• Provides beneficial bacteria such as Streptococcus and Lactobacillus to the infant oral cavity and gut.

Biofilms and Dental Plaque

Biofilm Structure and Development

A biofilm is a structured community of microorganisms encapsulated within a self-produced matrix of extracellular polymeric substances (EPS), adhering to a surface.

• Steps in biofilm development:

  1. Attachment to surface (often via glycoproteins on teeth)

  2. Microcolony formation

  3. Maturation (development of complex architecture)

  4. Dispersion (cells released to colonize new sites)

• Benefits of biofilm lifestyle: Protection from environmental stress, antibiotics, and host defenses; enhanced nutrient acquisition; cell-to-cell communication.

Dental Plaque

• Definition: A biofilm that forms on the tooth surface, composed of bacteria, salivary glycoproteins, and extracellular polysaccharides.

• Staining: Dental plaque can be visualized using special dyes that bind to the biofilm matrix.

Ecological Succession of Oral Plaque

• First colonizers: Early colonizers (e.g., Streptococcus species) attach to the clean tooth surface via glycoproteins.

• Second colonizers: Attach to the early colonizers, often through coaggregation.

• Third colonizers: Late colonizers, including anaerobes, establish as the biofilm matures.

The oral microbiome is dynamic and changes over time within an individual due to diet, hygiene, and other factors.

Oral Diseases

Dental Caries (Tooth Decay)

• Cariogenic diet: High in fermentable carbohydrates (e.g., sugars) promotes acid production by bacteria.

• Key bacteria: Streptococcus mutans and related species ferment sugars to produce lactic acid.

• Pathogenesis: Acid demineralizes tooth enamel (decalcification) and breaks down proteins (proteolysis).

• Treatment: Removal of decayed tissue, restoration, improved oral hygiene, dietary modification.

Periodontal Disease

• Definition: Inflammatory disease affecting the tissues and bone supporting the teeth.

• Pathogenesis: Involves inflammation, tissue and bone destruction, often initiated in the gingival crevice by anaerobic bacteria.

• Development: Progresses from gingivitis (gum inflammation) to periodontitis (destruction of supporting structures).

Methods for Studying the Oral Microbiome

• Since most oral microbes cannot be cultured, molecular methods such as 16S rRNA gene sequencing are used to identify and study microbial populations.

Additional Key Concepts

• Competitive exclusion: Normal microbiota prevent colonization by pathogens by occupying niches and resources.

• Limitations and adaptations of gut microbiome: Gut microbes face challenges such as low oxygen, bile salts, and immune defenses; they adapt via anaerobic metabolism, biofilm formation, and resistance mechanisms.

• Dysbiosis: An imbalance in the normal microbiota, often associated with disease.

• Locations in the mouth: Microbes inhabit the tongue, teeth, gingival crevice, cheeks, and palate; each site has unique environmental conditions (e.g., oxygen, pH, nutrients).

• Glycoproteins on teeth: Serve as initial attachment sites for microbial colonization and biofilm formation.

• Ecological succession: The sequential colonization and replacement of microbial species in a habitat, as seen in the development of dental plaque.

Table: Comparison of Resident vs. Transient Microbiota

Feature	Resident Microbiota	Transient Microbiota
Duration of colonization	Long-term, stable	Short-term, temporary
Adaptation to host	Well-adapted	Poorly adapted
Role in health	Often beneficial	Usually neutral or potentially pathogenic

Table: Types of Symbiosis

Type	Effect on Host	Effect on Microbe	Example
Commensalism	Neutral	Benefit	Skin bacteria
Mutualism	Benefit	Benefit	Gut bacteria synthesizing vitamins
Parasitism	Harmed	Benefit	Pathogenic bacteria

Table: Antimicrobial Components of Saliva

Component	Function
Lysozyme	Breaks down bacterial cell walls (peptidoglycan)
Lactoperoxidase	Generates reactive oxygen species toxic to bacteria
IgA antibodies	Neutralize pathogens

Equations and Formulas

• Fermentation by oral bacteria: Production of lactic acid from glucose: $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 \rightarrow 2\ \mathrm{CH}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$ (Glucose to lactic acid)

Summary

• The human microbiome consists of diverse microbial communities that play crucial roles in health and disease.

• The oral cavity is a dynamic environment with specialized niches, biofilm formation, and complex ecological succession.

• Understanding the interactions between host and microbiota is essential for preventing and treating oral diseases such as dental caries and periodontal disease.

Additional info: Some explanations and examples were expanded for academic completeness.