6 - The Human Microbiome

🦠 Microbial Symbiosis & Dysbiosis with Humans

1. 🌎 The Human Microbiome — Big Picture

Our bodies are not sterile. We live with enormous communities of microorganisms on and inside us.

“When we eat, so do they. When we travel, they come along. When we die, they consume us.” — Ed Yong

How many microbes do we have?

Humans carry approximately:

• 10¹³–10¹⁴ microbial cells

• Roughly as many bacterial cells as human cells

• Microbes contain about 100× more nonredundant genes than our own human genes.

💡 Why does this matter?

Microbes contribute genes and metabolic abilities that our human cells don't possess. Therefore, we can think of the human body and its microbiota as a combined biological system.

2. 🧫 Important Terminology

These terms are VERY important because they sound similar.

🦠 Microbiota

Microbiota = the community of living microorganisms in or on the body.

Includes:

• Bacteria 🦠

• Archaea

• Fungi 🍄

• Viruses

• Protists

📌 Think:

Microbiota = the organisms themselves

🧬 Microbiome

Microbiome = microbiota + their genes + the environment they live in.

So it includes:

Microorganisms + genetic material + surrounding environment

📌 Think:

Microbiome = the whole microbial ecosystem

🧬 Metagenome

Metagenome = all of the genetic material/DNA from all microorganisms in a particular community.

For example:

If we take all the microbes from someone's gut and collect all their DNA, that DNA represents the gut metagenome.

🔬 Metagenomic sequencing

Scientists can sequence the DNA of an entire microbial community without needing to grow every organism in the laboratory.

This was a major method used in the Human Microbiome Project.

🧠 Easy way to remember

3. 🧩 Microbial Communities Differ Between People

Not everyone has exactly the same microbes.

For example:

Person A may have one species of bacteria while Person B has another.

However, they may still have microbes performing similar metabolic functions.

Important idea:

Different microbial species can perform similar functions.

So:

Different taxa → similar metabolic pathways/functions

This helps explain why researchers don't necessarily need every person to have exactly the same bacterial species for the microbiome to function normally.

4. ❓ What Is a "Normal" Microbiota?

There is no universally accepted medical definition of a "healthy microbiota."

This is important!

We cannot simply say:

"These exact bacteria = healthy."

Instead, microbiota vary depending on:

• Body location

• Age

• Environment

• Diet

• Lifestyle

• Genetics

• Medications

• Immune system

• Other factors

Therefore, variation between people is normal.

5. 🗺 Major Microbiota Niches

Different parts of the body have different microbial communities.

The major sites are:

🧴 1. Skin

👄 2. Oral cavity

🫁 3. Respiratory tract

🚻 4. Urogenital tract

🍽 5. Gastrointestinal (GI) tract

Each location creates a different environment, so different microbes can survive there.

🧴 6. Skin Microbiota

The skin is actually a difficult environment for microbes to colonize.

Why?

• Dry 💨

• Salty 🧂

• Acidic

• Contains protective oils

• Constantly exposed to the environment

• Physical shedding of skin cells

Yet microorganisms still live there.

Factors contributing to variation in the skin microbiome. Exogenous and endogenous factors contribute to variation between individuals and over the lifetime of an individual.

🧩 Three Major Skin Microenvironments

Different parts of the skin support different microbes.

Why are there different microbes?

Because the physical and chemical conditions differ.

For example:

Oily skin → more lipids → favors lipid-metabolizing organisms

7. 🧴 Skin Structure

The skin has two major layers:

① Epidermis

Contains:

• Keratinocytes

• Squames

Squames

These are essentially dead/enucleated keratinocytes that have become strongly cross-linked.

They contribute to the protective barrier of the skin.

② Dermis

Contains:

• Sweat glands 💧

• Sebaceous glands

• Hair follicles

• Immune cells

• Subcutaneous tissues

8. 🦠 Important Skin Bacteria

Most skin bacteria are Gram-positive.

Why is this useful?

Gram-positive bacteria are generally better adapted to:

• Dry conditions

• High salt concentrations

Important examples:

🦠 Staphylococcus epidermidis

A common normal skin bacterium.

It can help prevent pathogens from colonizing the skin.

🦠 Cutibacterium acnes

Previously called:

Propionibacterium acnes

It lives particularly well in sebaceous/oily environments.

It can:

1. Break down skin oils

2. Interact with sebaceous glands

3. Contribute to inflammation

4. Be associated with acne

📌 Important:

Cutibacterium acnes is normally part of the skin microbiota, but under certain conditions it can contribute to disease.

9. 🛡 Functions of the Skin Microbiota

Think:

Skin microbiota = Barrier + Protection + Immune education

🧱 Function 1: Barrier Maintenance

Skin microbes can help maintain the physical and chemical barrier of the skin.

Some microbes can stimulate keratinocytes to:

• Strengthen tight junctions

• Improve barrier function

Some S. epidermidis and Lactobacillus strains can promote:

• Ceramide metabolism

• Lipid metabolism

• Maintenance of the skin's protective lipid barrier

They also help maintain the skin's slightly acidic environment:

Skin pH ≈ 5

🦠 Function 2: Pathogen Exclusion

Normal microbiota help prevent harmful organisms from establishing themselves.

This is called:

Colonization resistance

How does colonization resistance work?

Normal microbes compete with pathogens for:

🍽 Nutrients
📍 Attachment sites
🏠 Physical space

They can also produce substances that inhibit or kill pathogens.

For example, resident bacteria can inhibit organisms such as:

Staphylococcus aureus

Simple idea:

Good/commensal microbes occupy the available space and resources, making it harder for pathogens to move in.

🧠 Function 3: Immune Education

The microbiota constantly interact with the immune system.

Microbial molecules are detected by:

Pattern Recognition Receptors (PRRs)

These are receptors that detect characteristic microbial molecules.

They are found on immune and other cells, including:

• Dendritic cells

• Macrophages

• Epithelial cells

This continuous interaction helps "educate" or calibrate the immune system.

🧠 What does immune education mean?

The immune system learns to distinguish between:

• Normal harmless microbial signals

• Potentially dangerous situations

The skin microbiota can promote:

Regulatory T cells (Tregs)

Tregs help prevent excessive immune responses.

📌 Therefore:

Microbiota don't simply activate immunity — they also help control it.

👄 10. Oral Microbiota

The oral cavity is a complex, heterogeneous microbial habitat.

Why?

Because there are many different surfaces:

• Teeth 🦷

• Gums

• Tongue

• Cheeks

• Saliva

• Dental plaque

Each provides different conditions for microbes.

🧪 The mouth has antimicrobial defenses

Saliva contains substances such as:

Lysozyme

Breaks down bacterial cell walls.

Lactoferrin

Binds iron, making it less available to bacteria.

Secretory IgA (sIgA)

An antibody that helps prevent microbes from attaching to and invading tissues.

🤔 So why do microbes still live in the mouth?

Because the mouth also contains lots of nutrients, especially near surfaces.

Therefore:

Strong antimicrobial defenses + abundant nutrients = complex microbial community

🦷 Dental Procedures & Bacteremia

Dental procedures can sometimes cause oral bacteria to enter the bloodstream.

This is called:

Bacteremia

Bacteremia = bacteria present in the bloodstream

In certain circumstances, bacteria entering the blood can contribute to infections such as:

Subacute bacterial endocarditis

This involves infection/inflammation of the heart's inner lining and/or valves.

📌 Exam connection:

Dental procedure → oral bacteria enter blood → bacteremia → possible endocarditis

🫁 11. Respiratory Tract Microbiota

The respiratory tract can be divided into:

Upper respiratory tract

⬇

Lower respiratory tract

They have very different microbial environments.

👃 12. Upper Respiratory Tract

Includes:

• Nasal cavity

• Pharynx

• Larynx

These areas are:

Open to the outside environment

Every time we breathe, microbes enter.

Therefore, the upper respiratory tract is densely colonized.

🌬 What happens to inhaled microbes?

Microbes enter → become trapped in mucus → then are:

• Expelled

• Sneezed out

• Moved by cilia

• Swallowed

Key idea:

The upper respiratory tract doesn't try to sterilize itself.

Instead, it uses:

Trap → move → expel

🛡 Upper Respiratory Tract Defenses

① Physical defenses

Nasal hairs

Create turbulent airflow and help deposit particles onto mucus-coated surfaces.

Mucus

Produced by goblet cells.

Mucus traps microbes and particles.

Cilia

Move mucus toward the outside or throat.

Sneezing

Physically removes material.

② Chemical defenses

Mucus contains antimicrobial substances such as:

• Secretory IgA

• Lysozyme

• Lactoferrin

③ Microbial competition

Resident microbes occupy niches and compete with pathogens.

Examples include:

• Staphylococcus epidermidis

• Staphylococcus aureus in some upper respiratory sites

🧠 Overall concept

The upper respiratory tract is a:

"Permissive zone"

This means microbes can live there in relatively high numbers.

The goal is not necessarily:

❌ Eliminate every microbe

Instead:

✅ Trap and continuously remove microbes while controlling harmful organisms.

🫁 13. Lower Respiratory Tract

Includes:

• Trachea

• Bronchi

• Bronchioles

• Alveoli

Older textbooks often described the healthy lower respiratory tract as sterile.

We now know this is not strictly true.

However:

Microbial density is dramatically lower than in the upper respiratory tract.

🚨 Why must the lungs be so carefully protected?

The alveoli are responsible for:

Gas exchange

Their walls are extremely thin.

Therefore:

Too much inflammation = potentially severe damage to gas exchange.

So the lungs need a precisely controlled immune response.

🛡 Lower Respiratory Tract Defenses

1⃣ Mucus + mucociliary clearance

Mucus traps particles.

Most particles are removed before reaching the deepest parts of the lungs.

Only very small particles — approximately <5 μm — can bypass mucociliary clearance and reach the alveoli.

2⃣ Surfactant proteins

Produced by:

Type II pneumocytes

Important surfactant proteins include:

• SP-A

• SP-B

• SP-C

• SP-D

SP-A and SP-D are particularly important for immunity.

They are:

Collectins

They can bind microbes and help with their removal.

3⃣ Antimicrobial molecules

Respiratory epithelial cells can produce:

• Defensins

• Cathelicidins

They also produce cytokines such as:

• IL-25

• IL-33

These help coordinate immune responses.

4⃣ 🧹 Alveolar macrophages

These are the major immune cells in healthy alveoli.

They continuously:

• Phagocytose microbes

• Remove debris

• Monitor the environment

• Control inflammation

Why control inflammation?

Because excessive inflammation could damage the delicate alveoli.

📌 Think:

Alveolar macrophages = clean up + defend + prevent excessive inflammation

5⃣ 🧬 Dendritic Cells

Dendritic cells are also present in the respiratory epithelium.

They:

• Detect microbes

• Process antigens

• Present antigens to adaptive immune cells

• Help determine the appropriate immune response

🚻 14. Urogenital Tract Microbiota

The urogenital tract has two major zones:

🔼 Upper urogenital tract

🔽 Lower urogenital tract

They have very different microbial environments.

🔼 Upper Urogenital Tract

Normally essentially sterile.

Includes:

• Kidneys

• Ureters

• Bladder

So:

Kidneys → normally sterile
Ureters → normally sterile
Bladder → normally sterile

This is important because microorganisms in these areas can indicate infection.

🔽 Lower Urogenital Tract

The distal urethra contains normal microorganisms.

These organisms can come from:

• Periurethral skin

• Nearby vaginal microbiota

• Nearby anal microbiota

These organisms are generally commensal/harmless.

⚠ But there is an important downside

The normal microbes in the lower urogenital tract can also serve as a:

Reservoir for uropathogens

Uropathogens = microorganisms capable of causing urinary tract infections (UTIs).

So microorganisms normally living near the urinary tract can sometimes enter normally sterile areas and cause infection.

🧠 HIGH-YIELD COMPARISON

⭐ Core Concepts to Memorize

1. Microbiota ≠ microbiome ≠ metagenome

Microbiota → organisms
Microbiome → organisms + genes + environment
Metagenome → all microbial DNA

2. 🧴 Skin microbiota have 3 major roles

Barrier → Pathogen exclusion → Immune education

3. 🦠 Colonization resistance

Normal microbiota prevent pathogens from establishing themselves by:

• Competing for nutrients

• Competing for attachment sites

• Occupying physical space

• Producing antimicrobial substances

4. 👃 Upper respiratory tract

High microbial load

Main strategy:

Trap + expel

5. 🫁 Lower respiratory tract

Very low microbial load

Main strategy:

Carefully controlled immune defense

Because excessive inflammation can interfere with gas exchange.

6. 🚻 Urogenital tract

Upper tract → essentially sterile

Lower tract → normally colonized

🧠 One Big Picture

Think of the body as having different microbial neighborhoods:

🧴 Skin
→ harsh environment → specialized microbes

👄 Mouth
→ lots of nutrients → lots of microbes

👃 Upper airway
→ constantly exposed → microbes trapped and expelled

🫁 Lower airway
→ must protect delicate alveoli → very low microbial density + controlled immunity

🚻 Lower urogenital tract
→ normal commensals present

🫘 Upper urogenital tract
→ normally essentially sterile

And the overarching principle is:

The human body does not simply try to eliminate microbes. It manages microbial communities through physical barriers, chemical defenses, immune regulation, and competition between microorganisms.

🦠 Microbiology Lecture Notes: Urogenital and Gastrointestinal Microbiota

Let's simplify these notes into organized, exam-friendly sections while keeping the important details. The main idea connecting this lecture is that microbiota can protect us, support our physiology, and influence disease — but their effects depend on where they live and which organisms are present.

Part 1: 🩺 Urogenital Microbiota

1. 🌸 Vaginal Microbiome (VMB)

The vaginal microbiome consists of microorganisms that live in the vagina. In many healthy reproductive-age women, it is dominated by Lactobacillus species, which help maintain an acidic environment and protect against pathogens.

How does Lactobacillus protect the vagina?

1. Oestrogen increases

From puberty to menopause, oestrogen stimulates glycogen accumulation in vaginal epithelial cells.

2. Glycogen becomes available

Glycogen is a carbohydrate that supports Lactobacillus metabolism.

3. Lactobacillus produces lactic acid

Lactic acid helps maintain a low vaginal pH.

4. Pathogen growth is restricted

The acidic environment and other antimicrobial mechanisms make it harder for many pathogens to establish themselves.

Important correction for your notes: The normal vaginal pH in reproductive-age women is generally around 3.8–4.5, not 5.0. Also, Lactobacillus acidophilus is not the only important species; L. crispatus, L. iners, L. jensenii and L. gasseri can also be important.

Three protective mechanisms

Competitive exclusion example: If Lactobacillus occupies attachment sites on the vaginal epithelium, E. coli may have fewer opportunities to attach and establish itself.

What disrupts the vaginal microbiome?

• Antibiotics: Can kill beneficial bacteria along with the targeted pathogens.

• Douching: Can alter the vaginal environment and disrupt the normal microbial community.

• Vaginal steaming: May irritate or injure tissue and is not an evidence-based way to maintain vaginal health.

• Other factors: Hormonal changes, sexual activity and individual differences can influence microbial composition.

Disruption can allow other organisms to grow more easily, although not every change in the microbiome causes disease.

Why does life stage matter?

Before puberty

• Oestrogen levels are relatively low.

• Less glycogen accumulates in vaginal epithelial cells.

• Lactobacillus may be less dominant.

• Vaginal pH is generally less acidic.

After menopause

• Oestrogen levels decline.

• Vaginal glycogen generally decreases.

• Lactobacillus abundance may decrease.

• Vaginal pH commonly rises.

These changes can increase susceptibility to certain vaginal and urinary conditions. However, the specific risk depends on the condition: bacterial vaginosis, urinary tract infections and vulvovaginal candidiasis are distinct problems and should not be treated as interchangeable.

Exam takeaway: Oestrogen → glycogen availability → Lactobacillus activity → lactic acid → acidic vaginal environment.

2. ♂ Male Genital Microbiota

The male genital microbiome is less extensively studied than the vaginal microbiome, but it also contains diverse microorganisms.

A. Circumcision and microbial composition

Circumcision removes the foreskin.

The foreskin creates a warm, moist environment with relatively low oxygen availability. This can favour certain anaerobic bacteria.

After circumcision:

• The local environment changes.

• The abundance of some anaerobic bacteria decreases.

• The microbial community changes over time.

Some studies have associated circumcision with a reduced risk of heterosexual HIV acquisition in men. Changes in the local microbiome may contribute, but this association cannot be explained by microbiome changes alone.

Key term — anaerobic bacteria: Bacteria that can grow without oxygen, or that grow best under low-oxygen conditions.

B. What organisms are present?

Two major bacterial phyla mentioned in your notes are:

• Bacteroidetes (also called Bacteroidota)

• Firmicutes (also called Bacillota)

Age, sexual activity, hygiene and anatomical differences can all influence the microbial community.

An important unanswered question is whether some resident bacteria protect against sexually transmitted infections (STIs), have no major effect, or make colonization by a pathogen easier.

C. Shared microbiota between sexual partners

Certain bacteria associated with bacterial vaginosis (BV), including Gardnerella vaginalis, can be shared between partners.

Some may persist on the penis and contribute to the recurrence of BV in a partner.

Partner treatment can reduce BV recurrence in some circumstances, and evidence supports treating ongoing male partners in selected cases of recurrent BV under current clinical guidance.

D. Balanitis

Balanitis is inflammation of the glans (head) of the penis.

Possible contributing factors include:

• Irritation or inadequate hygiene.

• Diabetes.

• Skin conditions.

• Infection, including some fungal or bacterial infections.

It is not necessarily an STI. Management depends on the underlying cause, and the prognosis is generally good with appropriate treatment.

3. 🚽 Urogenital Microbiota and Urinary Tract Infections (UTIs)

A UTI occurs when microorganisms infect part of the urinary tract, such as the bladder or kidneys.

How common are UTIs?

Your lecture highlights that UTIs are extremely common, particularly among women.

• Escherichia coli causes approximately 80% of community-acquired UTIs.

• Around half of women experience at least one UTI during their lifetime.

• Recurrent UTIs are common.

• Risk factors include previous UTIs, sexual activity, spermicide use and postmenopausal changes.

A UTI is not the same as an STI. UTIs generally involve the urinary tract, whereas STIs are transmitted through sexual contact and may infect the genital tract or other sites.

Common uropathogens

These organisms are listed as information in your lecture, so focus on recognizing their distinguishing features.

1. Escherichia coli — the most common cause

• Causes approximately 80% of community-acquired UTIs.

• Fimbriae help it attach to the urothelium, the lining of the urinary tract.

• Can invade bladder cells and form intracellular bacterial communities, potentially contributing to recurrence.

2. Proteus mirabilis — the stone-former

• Produces urease, an enzyme that breaks down urea.

• This increases urine pH.

• Alkaline urine promotes certain urinary stones, particularly struvite stones.

• Swarming motility can help it spread along catheter surfaces.

3. Klebsiella pneumoniae — the encapsulated opportunist

• Important in healthcare-associated infections.

• Its capsule helps it resist host immune defenses.

• Some strains produce ESBLs (extended-spectrum beta-lactamases), which can make many beta-lactam antibiotics ineffective.

4. Staphylococcus saprophyticus — associated with young sexually active women

• A common cause of community-acquired UTIs in this group.

• Characteristically resistant to novobiocin in laboratory identification.

5. Candida albicans — an opportunistic fungus

• A yeast, not a bacterium.

• Can cause vulvovaginal candidiasis.

• Antibiotics, diabetes and catheters can favour Candida overgrowth or infection in susceptible settings.

• Candida detected in urine does not automatically mean a symptomatic UTI.

🧠 Quick memory trick

4. 🧫 Urogenital Microbiota and STIs

STIs are infections transmitted primarily through sexual contact. Some can be asymptomatic, meaning an infected person may have no obvious symptoms but can still transmit the infection.

Major STI pathogens

Important vocabulary:

• Intracellular bacterium: A bacterium that lives and replicates inside host cells.

• Diplococcus: A spherical bacterium that typically occurs in pairs.

• Spirochete: A spiral-shaped bacterium.

• Protozoan: A single-celled eukaryotic organism.

Why are asymptomatic infections important?

1. A person becomes infected.

2. They may not develop noticeable symptoms.

3. They may unknowingly transmit the infection.

4. The infection can persist and sometimes cause complications.

Untreated infections can cause pelvic inflammatory disease, infertility and pregnancy or neonatal complications, depending on the pathogen.

Exam takeaway: Screening is important because the absence of symptoms does not rule out an STI.

Part 2: 🍽 Gastrointestinal (GI) Microbiota

5. Overview of the GI Tract

Humans are monogastric omnivores: we have one main stomach and eat both plant and animal foods.

Our GI tract contains different microbial environments. The number and types of microbes vary along the tract because acidity, oxygen, nutrient availability and transit time differ.

1. Stomach

• Highly acidic, typically around pH 1.5–3.5.

• Most swallowed microorganisms struggle to survive.

• Certain organisms, such as Helicobacter pylori, can survive here.

2. Small intestine

• Generally has more microbes than the stomach.

• Receives bile and digestive enzymes.

• Nutrients are digested and absorbed here.

• Microbial abundance generally increases toward the lower small intestine.

3. Large intestine

• Contains the greatest concentration of gut microbes.

• Has relatively low oxygen availability.

• Supports fermentation of dietary fibre and other carbohydrates that humans cannot fully digest.

• Produces useful metabolites, including short-chain fatty acids.

Why does the gut microbiome matter?

Gut microbes influence:

• Early immune-system development.

• Digestion and energy extraction.

• Maintenance of the intestinal barrier.

• Vitamin and metabolite production.

• Resistance to pathogen colonization.

• Susceptibility to certain diseases.

Colonization begins early in life, and the microbiome develops substantially during infancy and childhood. Early microbial exposures can influence immune and metabolic development, although health outcomes also depend on genetics, environment, diet and many other factors.

Your lecture notes estimate that the large intestine contains around 101110^{11}1011–101310^{13}1013 bacteria per gram of feces.

Key idea: The GI tract is not one uniform environment. Different regions select for different microbial communities.

6. 🌎 One Species, Many Different Microbiomes

Two healthy people can have very different gut microbiomes.

For example, one person may have a greater relative abundance of Firmicutes, while another has more Bacteroidetes. Both could be healthy.

What influences gut microbiome composition?

• Diet and available nutrients.

• Age and life stage.

• Antibiotic exposure and other medications.

• Geography and environmental exposure.

• Lifestyle and health conditions.

• Interactions among the microbes themselves.

Should everyone get a gut microbiome test?

Your professor's key message is that commercial microbiome tests often cannot tell you enough to make reliable health recommendations.

Why?

1. Composition is not the same as function. Finding a bacterium does not tell us exactly what it is doing.

2. There is no single ideal microbiome. A particular bacterial ratio is not universally best.

3. Healthy people can differ substantially. Microbial diversity and abundance vary between individuals.

4. Evidence is still developing. Many test results lack validated interpretations that reliably guide treatment.

Your notes specifically mention the Firmicutes-to-Bacteroidetes ratio. There is no universally accepted ideal ratio that can reliably distinguish a healthy microbiome from an unhealthy one.

What can support a healthy gut microbiome?

• Eat a varied diet with a range of fibre-rich plant foods.

• Include different vegetables, fruits, legumes, whole grains, nuts and seeds as appropriate.

• Avoid unnecessary antibiotic use.

• Interact with natural environments and animals safely.

Your professor's “snog a farmer” comment is a humorous way of highlighting environmental microbial exposure. It is not a literal health recommendation!

Exam takeaway: Microbial diversity can be useful, but more diversity is not automatically better in every situation. The community's function and context matter.

7. 🧪 Stomach Microbiota and Helicobacter pylori

The stomach's acidity creates a strong barrier against ingested microbes.

Most organisms are killed or inhibited by the acidic environment, but some have adaptations that let them survive.

Helicobacter pylori

H. pylori is a bacterium that can colonize the stomach's mucus layer.

It survives by using several strategies:

• Urease production: Breaks down urea and generates ammonia, helping buffer the acid around the bacterium.

• Mucus penetration: Allows it to reach a more protective environment near the stomach lining.

• Persistence: It can establish long-lasting colonization.

It is associated with chronic gastritis and peptic ulcers, and it increases the risk of certain stomach cancers. However, not everyone infected with H. pylori develops symptoms or ulcers.

Your lecture notes state that approximately half the world's population has been infected, although prevalence varies considerably by region and population.

What happens when stomach acidity decreases?

Hypochlorhydria means reduced stomach acid secretion.

If acidity is reduced, some ingested pathogens may survive passage through the stomach more easily.

For example, Vibrio cholerae causes cholera after reaching and colonizing the small intestine. Reduced gastric acidity can increase susceptibility to infection because more bacteria survive passage through the stomach.

Four barriers to gastric pathogens

1. Physical barrier

• Thick mucus gel layer.

• Tight junctions between epithelial cells.

• Rapid gastric emptying limits the time microbes remain in the stomach.

2. Chemical barrier

• Hydrochloric acid creates a very low pH.

• Pepsin breaks down proteins, including many microbial proteins.

• Lysozyme contributes additional antimicrobial activity.

3. Immune barrier

• Antimicrobial peptides such as defensins.

• Pattern-recognition receptors, including Toll-like receptors (TLRs).

• Local inflammatory responses to harmful microbes.

4. Microbial barrier

• Resident microbes compete with invaders for nutrients and attachment sites.

• Some produce antimicrobial substances.

• Microbes interact with and stimulate host immune defenses.

Memory trick: Physical = blocks, chemical = kills or damages, immune = recognizes and responds, microbial = resident microbes compete.

8. 🦠 Composition of the Gut Microbiota

Your notes identify three major bacterial phyla:

• Firmicutes (Bacillota)

• Bacteroidetes (Bacteroidota)

• Proteobacteria (Pseudomonadota)

These include many organisms with different metabolic and physiological properties.

Other members of the gut community can include archaea, fungi, yeasts, viruses and other microorganisms.

Phylum versus species

Think of biological classification as a hierarchy:

Phylum

A broad group

Genus

A more specific group

Species

A more specific type of organism

For example, two people can both have Firmicutes but carry different species and strains within that phylum.

Therefore, similar phylum-level profiles do not necessarily mean two people have the same microbiome or the same microbial functions.

Part 3: 🛡 How the Gut Keeps Microbes Under Control

9. The Gut-Associated Lymphoid Tissue (GALT)

The gut needs to solve a difficult problem: it must tolerate harmless food molecules and beneficial microbes while responding to dangerous pathogens.

This is the job of the intestinal barrier and immune system, including the gut-associated lymphoid tissue (GALT).

GALT includes immune structures and cells distributed throughout the intestinal lining and underlying tissues.

Important cells and their functions

How are gut microbes monitored?

The immune system must distinguish between threats and harmless exposures.

There are three mechanisms in your notes.

1. Enterocyte sampling — pinocytosis

Enterocytes can take up small amounts of material from the intestinal lumen. This allows some microbial and food-derived antigens to become available to immune cells beneath the epithelium.

Remember: Pinocytosis means “cell drinking.”

2. Dendritic cells — antigen presentation

Dendritic cells capture antigens and present them to T cells. Depending on the antigen and surrounding signals, they can help activate immune responses or promote tolerance.

Remember: Dendritic cells help the immune system decide how to respond.

3. M cells — transcytosis

M cells take up material from the gut lumen, transport it across the cell and release it near immune cells in structures such as Peyer's patches.

Remember: Transcytosis means transporting material across a cell.

What is immune tolerance?

Immune tolerance means the immune system avoids mounting an unnecessary damaging response against a particular antigen.

For example, the gut usually needs to tolerate food proteins and many harmless resident microbes while remaining capable of responding to pathogens.

However, tolerance is not the same as ignoring everything: the immune system continues to monitor the intestinal environment.

10. 🧱 The Four Gut Barriers

The intestinal barrier is not just a wall. It is a coordinated system.

1. Microbial barrier

Beneficial resident microbes compete with pathogens for nutrients and attachment sites. Some also produce substances that inhibit other microbes.

2. Chemical barrier

Host antimicrobial substances, including defensins, and microbial products inhibit invading organisms. Secretory IgA antibodies also help control microbes at mucosal surfaces.

3. Physical barrier

Mucus, epithelial cells and tight junctions limit microbial access to underlying tissues.

4. Immune barrier

GALT and other innate and adaptive immune mechanisms detect threats, coordinate responses and help maintain tolerance to harmless antigens.

Example: What happens when a pathogen enters the gut?

Imagine a pathogen arrives after contaminated food is eaten.

1. Stomach acid kills or inhibits many of the ingested organisms.

2. Those that survive encounter mucus and the intestinal epithelium.

3. Resident microbes compete with the pathogen for nutrients and attachment sites.

4. Antimicrobial substances and immune defenses provide additional protection.

5. If the pathogen successfully establishes an infection, the immune system can mount a stronger response.

The key is that multiple barriers work together. No single barrier needs to do everything.

Part 4: ⚡ What the Gut Microbiota Does for Us

11. Five Major Functions

Your gut microbiota provides several benefits beyond simply living in the intestine.

1. Energy extraction

Gut microbes break down certain carbohydrates that our own digestive enzymes cannot fully digest, including some plant fibres and mucins.

Their metabolic products can be absorbed and used by the host.

2. Physiological barrier

Microbes can help maintain epithelial integrity and stimulate protective mucus production. They also help prevent microbial invasion.

3. Immune development

Gut microbes influence the development of immune cells, help regulate inflammation and promote appropriate immune tolerance.

4. Metabolite and vitamin production

Microbes produce metabolites, including short-chain fatty acids, and some contribute to vitamin production. They also transform bile acids.

5. Xenobiotic metabolism

Gut microbes can chemically modify substances entering the body, including some medications and other foreign compounds.

These changes can increase, decrease or otherwise alter a compound's activity.

What are mucins?

Mucins are large glycoproteins that form an important part of mucus. Some gut microbes can use mucin-associated carbohydrates as a nutrient source.

What are bile salts?

Bile acids are produced by the liver and help with fat digestion. Gut microbes can chemically modify them, affecting the pool of bile acids and their interactions with host physiology.

What are xenobiotics?

Xenobiotics are compounds foreign to the body, such as many drugs, pollutants and dietary chemicals.

Gut microbes may transform these compounds before or after they are absorbed, potentially influencing drug efficacy or toxicity.

12. 🔥 Gut Microbiota as an Energy Source

Humans cannot digest all the carbohydrates found in food. Some dietary fibres reach the large intestine largely undigested.

Gut bacteria can ferment these carbohydrates and produce metabolites that the body can use.

Step-by-step process

Dietary fibre and other carbohydrates

Not fully digested by human enzymes

Gut bacteria ferment carbohydrates

Short-chain fatty acids (SCFAs)

Mainly acetate, propionate and butyrate

Host absorbs and uses these products

For energy, cellular functions and intestinal health

Why are short-chain fatty acids important?

• Acetate: Can be used in various metabolic pathways.

• Propionate: Can contribute to metabolism, including processes in the liver.

• Butyrate: An important energy source for many cells lining the colon and helps support intestinal barrier function.

The exact amounts depend on diet, microbial composition and other factors.

Germ-free mice experiment

Your lecture describes an experiment comparing germ-free mice with mice that have a normal microbiota.

• Germ-free mice: Raised without microorganisms.

• Conventional mice: Raised with a typical community of microorganisms.

Germ-free mice may need more food to achieve comparable body weight under certain experimental conditions because they lack microbial contributions to energy extraction and metabolism.

Interpretation: Gut microbes can help the host obtain energy from food. They do not simply add calories in every situation; their effects depend on the diet, microbial community and host physiology.

Your lecture also notes that microbial metabolism may contribute a meaningful proportion of daily energy intake. Treat the specific percentage as an estimate rather than a universal value, because it varies substantially between individuals and diets.

13. 🛡 Gut Microbiota as a Microbial Barrier

Resident gut microbes can protect against pathogens through three major mechanisms.

1. Direct inhibition

Some resident bacteria produce antimicrobial substances, toxins or metabolites that inhibit other bacteria.

Example: A resident bacterium releases a substance that prevents a pathogen from growing efficiently.

2. Nutrient and receptor competition

Resident microbes consume nutrients and occupy attachment sites, making it harder for invading organisms to establish themselves.

Example: If beneficial bacteria already use a particular nutrient, an invading pathogen may struggle to obtain enough of it.

3. Immune stimulation

Resident microbes influence the production of mucus, IgA antibodies and other immune defenses.

Example: Microbial signals help the host strengthen its defenses against invading pathogens.

Salmonella mouse experiment

Your notes describe mice given Salmonella orally under different microbiome conditions.

The precise outcome depends on the experimental design and pathogen.

What does the experiment demonstrate?

When the normal gut microbiota is present, it can make pathogen colonization more difficult. When the microbiota is depleted or less complex, some pathogens gain an advantage.

This is an example of colonization resistance.

Antibiotics can sometimes disrupt colonization resistance because they remove protective organisms along with the organisms being targeted.

14. 🧬 Gut Microbiota and Systemic Immune Development

The gut microbiota influences not only the intestine but also the immune system throughout the body.

How does this work?

1. Resident microbes interact with the intestinal epithelium.

2. Microbial molecules are detected by host pattern-recognition receptors.

3. These signals influence the development and activity of immune cells.

4. Immune cells and signalling molecules help regulate local and systemic immune responses.

5. These interactions contribute to immune maturation and the balance between tolerance and inflammation.

Local immunity refers to defenses at a particular site, such as the intestinal lining.

Systemic immunity refers to immune functions throughout the body.

The gut microbiota can influence both, although the effects depend on the microbes, host and context.

Part 5: 🧠 High-Yield Exam Review

These are the most important connections to understand rather than simply memorize.