Normal Microbiota: Classification, Distribution, and Host Relationships

Normal Microbiota

Definition and Overview

Normal microbiota refers to the collection of microorganisms that colonize various surfaces of the human body, commonly known as the normal flora. These microbes typically do not cause disease under normal circumstances in healthy individuals.

• Microorganisms include bacteria, fungi, and sometimes viruses and protozoa.

• Colonization occurs on both external and internal body surfaces.

Main Groups of Microbiota

• Resident Microbiota: Microbes that are consistently present at specific body sites. They are introduced at birth and persist throughout life, living with permanence and stability.

• Transient Microbiota: Microbes that are temporarily present on body surfaces. They do not permanently colonize and are often removed by host defenses or competition.

Key Point: Attachment to the host surface is necessary for colonization. Without attachment, microbes cannot establish residence or cause infectious disease.

Distribution of Microbiota on Body Surfaces

External Surfaces

• Conjunctiva (eye surface)

• Skin

Internal Surfaces

• Mucous membranes lining the respiratory tract, digestive tract, and urogenital tract

• Other moist surfaces such as the mouth, throat, and nose

Representative Microbial Species by Region

• Skin: Staphylococcus aureus, Staphylococcus epidermidis

• Nose/Throat: Streptococcus pneumoniae, Staphylococcus aureus

• Oral Cavity: Various species belonging to genus Streptococcus, genus Lactobacillus, and other bacterial and fungal species

• Intestinal Tract: Escherichia coli, various bacterial species belonging to genus Lactobacillus

• Vagina: Various bacteria belonging to genus Lactobacillus, genus Clostridium, and various fungal species

Additional info: Bacteria belonging to genus Lactobacillus are found from the upper respiratory tract down to the vagina. Various fungal species are present on skin, mouth, and vagina.

Functions and Benefits of Resident Microbiota

Microbial Antagonism

Most resident microbiota benefit the host by preventing colonization by pathogens, a phenomenon known as microbial antagonism.

• Compete for nutrients and attachment sites

• Produce substances that inhibit or kill pathogens

Probiotics

Probiotics are beneficial bacteria that can restore the resident microbiota population, especially on the surface of the digestive tract during or after antibiotic therapy.

• Lactobacillus acidophilus is a well-known probiotic and is also called lactic acid bacteria.

• Probiotics help maintain a healthy balance of intestinal microbiota.

Transient Microbiota

Characteristics

• Temporarily present on body surfaces

• Removed by host defenses or competition

• Most are harmless, but some can be pathogenic (e.g., Staphylococcus aureus)

Opportunistic Pathogens and Disease

Opportunistic Infections

Resident microbiota can cause disease under certain conditions, becoming opportunistic pathogens.

• Immunocompromised hosts (e.g., AIDS patients) are at higher risk for opportunistic infections, such as pneumonia caused by species of genus Pneumocystis.

• Resident microbiota may gain access to deeper tissues via broken skin or mucous membranes.

Symbiotic Relationships

Types of Relationships

The relationship between resident microbiota and their host can be classified as:

• Commensalism: One organism benefits, the other is unaffected. Example: Microorganism receives nutrients and residence; host is not affected.

• Mutualism: Both organisms benefit. Example: Microbiota synthesize vitamins for the host.

• Parasitism: One organism benefits at the expense of the other. Example: Pathogenic bacteria causing disease.

Summary Table: Resident vs. Transient Microbiota

Feature	Resident Microbiota	Transient Microbiota
Duration of Colonization	Permanent, stable	Temporary, short-term
Attachment	Firmly attached to body surfaces	Weakly attached or not attached
Role in Health	Protective, beneficial	Usually harmless, sometimes pathogenic
Removal	Resistant to removal	Removed by host defenses or competition

Key Equations and Concepts

• Microbial Antagonism: $\text{Microbial Antagonism} = \text{Competition for nutrients} + \text{Production of inhibitory substances}$

• Colonization Requirement: $\text{No attachment} \rightarrow \text{No residence} \rightarrow \text{No infectious disease established}$

Examples and Applications

• Example: Lactobacillus acidophilus on the intestinal surface helps prevent colonization by pathogens and aids in digestion.

• Application: Use of probiotics to restore normal microbiota after antibiotic therapy.