Test 3 (lecture 2) Human Microbiome: Structure, Function, and Influencing Factors

Human Microbiome

Definition and Composition

The human microbiome refers to the collective community of microorganisms that inhabit various sites of the human body. These include bacteria, archaea, viruses, yeast, fungi, and protists, comprising thousands of different species. The term normal flora (or microflora) is often used interchangeably with microbiome.

• Microbiota: The actual organisms present in a specific environment.

• Microbiome: The collective genomes of the microbiota.

• Largely beneficial: Most members of the microbiome provide health benefits to the host.

Key Functions:

• Compete with pathogens for space and nutrients, preventing pathogen colonization.

• Produce products that inhibit pathogens (e.g., antibiotics).

• Synthesize vitamins for the host (e.g., B vitamins, vitamin K).

• Contribute to the development and training of immune tissues, helping the immune system distinguish self from non-self.

• The skin acts as an excellent barrier to microbial invasion.

Example: Staphylococcus epidermidis is a common skin commensal that inhibits pathogen growth.

Entry Points for Microbes

Microbes can enter the body through wounds on the skin, mouth, eyes, ears, and the genital-urinary tract.

Ratio of Human to Bacterial Cells

Cell Numbers and Colonization

The human body contains approximately $10^{13}$ eukaryotic cells and is associated with about $10^{13}$ bacterial cells, as well as archaea, fungi, protozoa, and viruses.

• Most skin surfaces and mucous membrane surfaces are colonized by microflora.

• Blood, lymph organs, and nervous system are generally NOT colonized by microbiome.

Example: Washing hands reduces the microbial load on the skin.

Factors Influencing the Microbiome

Individual Variation

The composition of the human microbiome varies between individuals due to several factors:

• Birthing method: Vaginal birth exposes infants to maternal microbiota, while C-section may result in different initial colonization.

• Diet: Breast milk vs. formula, and western vs. non-western diets, influence microbiome diversity.

• Age: Microbiome changes from newborn to elderly.

• Climate/environment: Moist vs. dry environments affect microbial communities.

• Genetics and gender: Host genetics and hormonal changes (e.g., puberty) impact microbiome composition.

Efforts to Alter the Microbiome:

• Probiotics: Live microbes administered to confer health benefits.

• Prebiotics: Nutrients that promote the growth of beneficial microbes.

• Phage therapy: Use of bacteriophages to target specific bacterial pathogens (experimental).

Example: Consumption of yogurt containing Lactobacillus species as a probiotic.

Microbiome of the Skin

Characteristics and Dominant Microbes

The skin microbiome serves as the first line of defense against pathogens. It is a dry, acidic, and salty environment, with conditions varying by location (e.g., armpits, privates).

• Microbial composition is influenced by weather, age, hygiene, and contact with pets.

• Dominated by Gram-positive cocci, such as Staphylococcus (especially coagulase-negative species).

• Anaerobes like Propionibacterium acnes grow in hair follicles and sebaceous fluids.

Example: Propionibacterium acnes is associated with acne development in sebaceous glands.

Skin Region	Dominant Microbes	Environmental Features
Dry areas (forearm)	Staphylococcus, Corynebacterium	Low moisture, acidic pH
Moist areas (armpit, groin)	Staphylococcus, Propionibacterium	Higher moisture, more anaerobes
Hair follicles	Propionibacterium acnes	Sebaceous fluids, anaerobic

Additional info: The skin microbiome also plays a role in modulating local immune responses and can be disrupted by excessive hygiene or antibiotic use.