# Lecture 17: Host-Microbe Interactions and the Microbiome
## Microbiology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Define the human microbiome and distinguish it from the microbiota
2. Describe the major microbial communities at different body sites
3. Explain the spectrum of host-microbe relationships including mutualism, commensalism, and parasitism
4. Discuss the role of the microbiome in health, development, and disease
5. Describe methods used to study the microbiome, including 16S rRNA sequencing and metagenomics
6. Explain the concept of dysbiosis and its association with disease states
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## Lecture Content
### I. Terminology and Concepts
The study of host-associated microbial communities requires several key terms. The **microbiota** refers to the collection of microorganisms -- bacteria, archaea, fungi, viruses, and protists -- residing in or on a host. The **microbiome** technically refers to the collective genomes of the microbiota, though the term is often used interchangeably with microbiota. The **holobiont** concept considers the host organism together with all its associated microorganisms as a single biological unit. The **metagenome** encompasses all genetic material recovered from an environmental or host-associated sample.
The scale of human-microbial coexistence is remarkable. A typical adult human harbors an estimated 38 trillion bacterial cells, roughly equal to the number of human cells. The collective microbiome encodes more than 100 times more genes than the human genome, vastly expanding the metabolic and functional repertoire available to the host.
### II. Types of Host-Microbe Relationships
Host-microbe relationships span a spectrum of outcomes. In **mutualism**, both the host and the microbe benefit: gut bacteria, for example, synthesize vitamins such as K, B12, and folate while receiving nutrients and a stable habitat in return. In **commensalism**, one partner benefits while the other is unaffected, as when *Staphylococcus epidermidis* on the skin obtains nutrients without causing harm to the host. In **parasitism**, the microbe benefits at the expense of the host, as exemplified by *Mycobacterium tuberculosis* causing tissue damage.
Two additional terms capture important nuances. A **pathobiont** is a normally commensal microbe that can cause disease under certain conditions, such as immunosuppression, disrupted barriers, or dysbiosis -- *Clostridioides difficile* overgrowth after antibiotic-induced dysbiosis is a classic example. An **opportunistic pathogen** is a microbe that causes disease primarily in immunocompromised hosts. Critically, these relationships exist on a **continuum** and can shift with changes in host immunity, microbial community composition, or environmental context.
A spectrum diagram of host-microbe relationships. Left end labeled "Mutualism" (both benefit; illustration of gut bacteria producing vitamins for the host), center labeled "Commensalism" (one benefits, other unaffected; illustration of S. epidermidis on skin surface), right end labeled "Parasitism" (microbe benefits, host harmed; illustration of M. tuberculosis in lung tissue). Below the spectrum, arrows show conditions that shift the relationship toward disease: immunosuppression, antibiotic use, barrier disruption, and microbial virulence factor acquisition. A callout box defines "pathobiont" with examples.
### III. The Human Microbiome by Body Site
#### A. Gastrointestinal Tract
The gastrointestinal tract is the most densely colonized site in the human body, with approximately 10^11 bacteria per gram of colonic content. The **stomach** harbors relatively low microbial diversity due to its acidic pH, though *Helicobacter pylori* is a notable resident and pathogen. The **small intestine** shows increasing diversity distally, with *Lactobacillus*, *Streptococcus*, and *Enterococcus* among the common genera. The **colon** supports the highest bacterial density and diversity, dominated by two phyla: **Firmicutes** (including *Clostridium*, *Faecalibacterium*, and *Ruminococcus*) and **Bacteroidetes** (including *Bacteroides* and *Prevotella*). Also present are Actinobacteria (*Bifidobacterium*), Proteobacteria (*Escherichia*), and Verrucomicrobia (*Akkermansia*). The gut microbiota performs critical functions including carbohydrate fermentation to produce short-chain fatty acids (SCFAs -- butyrate, propionate, and acetate), bile acid metabolism, colonization resistance against pathogens, and education of the immune system.
#### B. Skin
The skin microbiome varies dramatically by microenvironment. **Sebaceous (oily) sites** such as the forehead and back are dominated by the lipophilic *Cutibacterium acnes*. **Moist sites** such as the axilla and groin harbor *Staphylococcus* and *Corynebacterium*. **Dry sites** such as the forearm support more diverse communities with a mix of Proteobacteria, Firmicutes, and Bacteroidetes. The skin's acidic pH (approximately 5.5), antimicrobial peptides (defensins and cathelicidins), and lipid secretions all exert selective pressure on microbial colonization.
#### C. Respiratory Tract
The upper respiratory tract supports diverse colonization by genera including *Streptococcus*, *Haemophilus*, *Moraxella*, *Corynebacterium*, and *Neisseria*. The lower respiratory tract, historically considered sterile, is now known to harbor a low-biomass microbial community. The composition of the nasopharyngeal microbiota can influence susceptibility to respiratory infections.
#### D. Urogenital Tract
The vaginal microbiome is often dominated by *Lactobacillus* species that produce lactic acid and hydrogen peroxide, maintaining a low pH of approximately 3.8--4.5. Five Community State Types (CSTs) have been defined: CST I (*L. crispatus*), CST II (*L. gasseri*), CST III (*L. iners*), CST IV (diverse anaerobes, associated with bacterial vaginosis), and CST V (*L. jensenii*). The male urogenital microbiome is less well studied but includes *Corynebacterium*, *Staphylococcus*, and *Prevotella*.
#### E. Oral Cavity
The oral cavity is the second most diverse microbial habitat after the gut, with over 700 species identified. Biofilms on teeth, known as dental plaque, harbor organisms including *Streptococcus mutans*, *Porphyromonas gingivalis*, and *Fusobacterium nucleatum*. The saliva, tongue, and mucosal surfaces each support distinct microbial communities.
An anatomical diagram of the human body showing the major microbiome sites. For each site, a magnified inset circle shows representative taxa and key features. Gut inset: bar chart of relative abundance showing Firmicutes and Bacteroidetes dominating; arrows point to functions (SCFA production, colonization resistance, vitamin synthesis). Skin inset: cross-section of skin showing sebaceous glands with C. acnes, moist fold with Staphylococcus/Corynebacterium. Oral inset: dental biofilm layers with early colonizers (Streptococcus) and late colonizers (Fusobacterium, Porphyromonas). Vaginal inset: Lactobacillus-dominated community producing lactic acid, low pH environment. Respiratory inset: nasal and pharyngeal mucosal surfaces with Streptococcus, Haemophilus, Moraxella.
### IV. Development of the Microbiome
**Colonization begins at birth**, though whether microbial colonization occurs in utero remains debated. The mode of delivery has a significant impact on initial colonization: vaginally delivered infants acquire a microbiota resembling the maternal vaginal flora (*Lactobacillus*, *Prevotella*), while cesarean-delivered infants are initially colonized by skin-associated organisms (*Staphylococcus*, *Corynebacterium*, *Propionibacterium*).
**Breast milk** provides prebiotics in the form of human milk oligosaccharides (HMOs), along with *Bifidobacterium* and immune factors such as secretory IgA. Formula-fed infants develop a different early microbiome composition. The microbiome undergoes rapid changes during the first approximately three years of life, after which it stabilizes into an adult-like configuration. The **critical window hypothesis** proposes that early-life microbial exposures shape immune development and influence disease susceptibility later in life. In aging, the microbiome tends to show reduced diversity and altered composition.
### V. Functions of the Microbiome in Health
The microbiome contributes to host health through several interconnected functions. **Metabolic functions** include fermentation of indigestible carbohydrates (dietary fiber) to produce SCFAs. Butyrate serves as the primary energy source for colonocytes and has anti-inflammatory properties that strengthen the gut barrier. Propionate feeds into gluconeogenesis in the liver, while acetate is utilized by peripheral tissues and contributes to lipogenesis. The microbiota also synthesizes essential vitamins (vitamin K and B vitamins), transforms bile acids through deconjugation and dehydroxylation, and metabolizes drugs in ways that can affect their pharmacokinetics.
**Colonization resistance** protects the host from pathogens through competition for nutrients and attachment sites, production of bacteriocins and other antimicrobial substances, and maintenance of acidic pH in certain niches, as exemplified by vaginal lactobacilli.
**Immune development and regulation** depends heavily on microbial stimulation. The microbiome educates the immune system through the development of gut-associated lymphoid tissue (GALT), induces regulatory T cells (Tregs) -- a process driven by *Bacteroides fragilis* (via polysaccharide A) and *Clostridium* clusters IV and XIVa -- stimulates IgA production, and calibrates the Th1/Th2 balance.
The **gut-brain axis** represents an active area of research. The microbiota produces neurotransmitters such as serotonin, GABA, and dopamine precursors. Vagal nerve signaling and microbial metabolites influence brain function and behavior, with emerging research linking the microbiome to mood, cognition, and neuropsychiatric disorders.
### VI. Dysbiosis and Disease
**Dysbiosis** refers to a disruption of the normal microbiome composition and/or function. It can be triggered by antibiotic use, dietary changes, infection, stress, or immune dysregulation. Dysbiosis is associated with numerous disease states. ***Clostridioides difficile* infection (CDI)** results from antibiotic-induced loss of colonization resistance, allowing toxin-producing *C. difficile* to overgrow. **Inflammatory bowel disease (IBD)** is associated with reduced microbial diversity, decreased *Faecalibacterium prausnitzii*, and increased Proteobacteria. **Obesity and metabolic syndrome** have been linked to altered Firmicutes-to-Bacteroidetes ratios (though this remains debated), reduced diversity, and altered SCFA profiles. **Allergic diseases** are addressed by the hygiene hypothesis (also called the "old friends" hypothesis), which proposes that reduced microbial exposures in early life lead to immune dysregulation and atopy. **Colorectal cancer** has been associated with enrichment of *Fusobacterium nucleatum* and genotoxin-producing *E. coli* (colibactin). Emerging associations also connect the gut microbiome to **neuropsychiatric disorders** including autism spectrum disorder, depression, and Parkinson disease.
**Fecal microbiota transplantation (FMT)** involves transferring stool from a healthy donor to restore the microbiome in patients with recurrent CDI, achieving cure rates of approximately 85--90%. FMT is also being investigated for other conditions, including IBD and metabolic syndrome.
A two-panel figure on dysbiosis. Panel A: "Healthy microbiome" -- a balanced pie chart showing normal proportions of Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria in the gut; arrows indicate beneficial functions (SCFA production, colonization resistance, immune homeostasis). Panel B: "Dysbiotic microbiome" -- the pie chart is skewed with expansion of Proteobacteria, reduction of Firmicutes diversity, and presence of C. difficile; red arrows indicate loss of colonization resistance, inflammation, and barrier disruption. Below, a flowchart shows the steps of fecal microbiota transplantation: healthy donor screening, stool processing, delivery to patient (colonoscopy, nasogastric tube, or capsules), and restoration of a diverse microbiome.
### VII. Methods for Studying the Microbiome
**Culture-based methods** have historically been limited because most microbes are difficult to grow in the laboratory -- a discrepancy known as "the great plate count anomaly." **Culturomics**, a high-throughput approach employing many different media and conditions, has substantially expanded the catalog of cultured human gut bacteria.
**16S rRNA gene sequencing** involves amplification and sequencing of hypervariable regions (V1--V9) of the bacterial 16S rRNA gene. This method identifies bacteria to the genus level and sometimes to the species level, providing taxonomic composition data but limited functional information. **Shotgun metagenomics** sequences all DNA in a sample, providing both taxonomic and functional (gene content) information at higher resolution (species and strain level), though it is more expensive and computationally intensive.
Additional approaches include **metatranscriptomics**, which uses RNA sequencing to assess active gene expression, and **metabolomics**, which measures metabolites such as SCFAs, bile acids, and tryptophan metabolites to assess the functional output of the microbiome. **Gnotobiotic ("germ-free") animal models** -- animals raised in sterile conditions that can be colonized with defined microbial communities -- allow causal experiments testing how the microbiome affects host physiology. **Bioinformatics tools** such as QIIME2, mothur, MetaPhlAn, and HUMAnN are essential for taxonomic and functional analysis of the resulting data.