Difficulty: Intermediate to Advanced | Prerequisites: Exam 1 and 2 material (cell structure, metabolism, genetics, regulation, viruses)
This final block moves from molecules and mechanisms to organisms in their environments and in their hosts. You start with microbial ecology, covering how microbes drive nutrient cycles and form communities in natural and built environments. Then you explore the intimate relationships between microbes and their plant, animal, and human hosts, ranging from beneficial symbioses to disease. The pathogenicity section explains how microbes cause disease at the molecular level, which sets up the immunity chapters on how hosts fight back. The block concludes with clinical microbiology (how infections are diagnosed and treated), epidemiology (how diseases spread through populations), and specific disease groups (vectorborne, soilborne, and eukaryotic pathogens). If you have been keeping up with the metabolic diversity and genetic regulation from earlier, this material will click into place; if not, review chemolithotrophy, the electron transport chain, quorum sensing, and horizontal gene transfer before proceeding.
Microorganisms are the engines of global nutrient cycling and form complex communities in every environment on Earth, including the human body. Some microbes cause disease through specific virulence factors, and hosts counter with layered innate and adaptive immune defences. Clinical microbiology applies diagnostic tools and antimicrobial agents to manage infections, while epidemiology tracks how diseases spread and how outbreaks are controlled.
Biogeochemical cycle
The global-scale movement of chemical elements (carbon, nitrogen, sulfur, iron, phosphorus) between biological, geological, and chemical reservoirs, largely driven by microbial metabolism.
Carbon cycle
The cycling of carbon between CO₂ (atmosphere), organic matter (biomass), and inorganic carbonates. Microbes contribute through photosynthesis and chemolithotrophy (CO₂ fixation), decomposition and respiration (organic C → CO₂), and methanogenesis (CO₂ or acetate → CH₄).
Nitrogen cycle
The cycling of nitrogen through its various oxidation states. Key microbial processes:
Nitrogen fixation: N₂ → NH₃ (nitrogenase enzyme, found in free-living and symbiotic bacteria)
Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (aerobic chemolithotrophs, e.g., Nitrosomonas, Nitrobacter)
Denitrification: NO₃⁻ → N₂ (anaerobic respiration, returns nitrogen to the atmosphere)
Ammonification: organic N → NH₃ (decomposition)
Anammox: NH₄⁺ + NO₂⁻ → N₂ (anaerobic ammonium oxidation, discovered relatively recently)
Nitrogenase
The enzyme complex that reduces atmospheric N₂ to NH₃. Extremely sensitive to oxygen, which is why nitrogen-fixing organisms have evolved various strategies to protect nitrogenase from O₂ (e.g., heterocysts in cyanobacteria, leghemoglobin in root nodules, respiratory protection in Azotobacter).
Biofilm
A structured community of microorganisms attached to a surface and enclosed in a self-produced matrix of extracellular polymeric substances (EPS). Biofilms are the dominant mode of microbial life in most environments. Cells in biofilms are far more resistant to antibiotics and host immune defences than planktonic (free-floating) cells.
Microbiome
The collective genomes of all microorganisms in a given environment. Often used loosely to refer to the microbial community itself (the microbiota). The human microbiome contains trillions of microbial cells, concentrated especially in the gut, skin, and oral cavity.
Symbiosis
A close and often long-term interaction between two different species. Subtypes:
Mutualism: both partners benefit
Commensalism: one benefits, the other is unaffected
Parasitism: one benefits at the other's expense
Rhizosphere
The zone of soil immediately surrounding plant roots, enriched with root exudates that support a dense and metabolically active microbial community.
Mycorrhizae
Mutualistic associations between fungi and plant roots. The fungus extends the root's absorptive surface area (especially for phosphorus), and the plant supplies the fungus with carbon. Extremely widespread: found in roughly 80% of plant species.
Root nodule symbiosis
The mutualistic association between nitrogen-fixing bacteria (rhizobia, e.g., Rhizobium, Bradyrhizobium) and leguminous plants. The bacteria fix atmospheric N₂ into ammonia within specialised root structures (nodules), and the plant provides carbon and a low-oxygen environment (via leghemoglobin).
Normal microbiota (normal flora)
The resident microbial communities that colonise the body surfaces and cavities of healthy individuals. They provide colonisation resistance (preventing pathogen establishment), synthesise vitamins, aid digestion, and prime the immune system.
Opportunistic pathogen
A microorganism that is part of the normal microbiota or the environment and causes disease only when the host's defences are compromised (e.g., immunosuppression, breach of skin or mucosal barriers, disruption of normal flora by antibiotics).
Pathogenicity
The ability of a microorganism to cause disease.
Virulence
The degree or measure of pathogenicity. A highly virulent pathogen causes severe disease; a less virulent one causes milder disease.
Virulence factor
A molecule produced by a pathogen that contributes to its ability to cause disease. Examples include adhesins, invasins, toxins, capsules, siderophores, and secretion systems.
Adhesin
A surface molecule (often a protein on the tip of pili or fimbriae) that allows a pathogen to bind specifically to host cell receptors. Adhesion is typically the first step in infection.
Exotoxin
A protein toxin secreted by a pathogen (usually Gram-positive but also some Gram-negatives). Often highly specific in action. Types:
Cytotoxins: kill or damage host cells (e.g., diphtheria toxin inhibits protein synthesis)
Enterotoxins: affect the intestinal lining (e.g., cholera toxin causes massive water secretion)
Neurotoxins: affect nerve function (e.g., botulinum toxin blocks acetylcholine release, tetanus toxin blocks inhibitory neurotransmitter release)
Superantigens: non-specifically activate large numbers of T cells, causing a cytokine storm (e.g., toxic shock syndrome toxin, TSST-1)
Endotoxin
The lipid A component of lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. Released when cells lyse. Triggers fever, inflammation, and in severe cases, septic shock. Unlike exotoxins, endotoxin is not a single protein but a structural component of the cell wall.
AB toxin
A common toxin architecture: the B (binding) subunit attaches to a host cell receptor, and the A (active) subunit enters the cell and carries out the enzymatic damage. Examples include cholera toxin, diphtheria toxin, and Shiga toxin.
Type III secretion system (T3SS)
A needle-like apparatus in Gram-negative bacteria that injects effector proteins directly into the host cell's cytoplasm. Functions like a molecular syringe. Used by Salmonella, Shigella, Yersinia, Pseudomonas, and others.
Koch's postulates
The four criteria (covered in Exam 1) for establishing a causal link between a microorganism and a disease. Limitations: some pathogens cannot be cultured, some cause disease only in humans (no animal model), some healthy people carry the pathogen asymptomatically.
Innate immunity
The non-specific, immediate host defences present from birth. Includes physical barriers (skin, mucous membranes), chemical barriers (lysozyme, defensins, stomach acid, complement), cellular defences (phagocytes: neutrophils, macrophages; natural killer cells), and the inflammatory response. Does not improve with repeated exposure.
Adaptive immunity
The specific, acquired immune response that develops after exposure to an antigen. Mediated by lymphocytes (T cells and B cells). Features specificity (each lymphocyte recognises one antigen), diversity, memory (faster and stronger response on re-exposure), and self/non-self discrimination.
Antigen
Any molecule (usually a protein or polysaccharide) that can be specifically recognised by the adaptive immune system. The specific region of an antigen that is recognised by an antibody or T cell receptor is called an epitope.
Antibody (immunoglobulin)
A Y-shaped protein produced by B cells (plasma cells) that binds specifically to an antigen. Functions include neutralisation (blocking pathogen attachment), opsonisation (marking for phagocytosis), and complement activation. Five classes: IgG, IgM, IgA, IgE, IgD.
T cell
A lymphocyte that matures in the thymus. Types:
Helper T cells (CD4⁺): recognise antigen presented by MHC class II on antigen-presenting cells; secrete cytokines that coordinate the immune response
Cytotoxic T cells (CD8⁺): recognise antigen presented by MHC class I on infected cells; kill those cells directly
Regulatory T cells: suppress immune responses to prevent autoimmunity
B cell
A lymphocyte that matures in the bone marrow and produces antibodies. Upon antigen stimulation (and usually T cell help), B cells differentiate into antibody-secreting plasma cells and memory B cells.
MHC (major histocompatibility complex)
Cell-surface proteins that present antigen fragments to T cells. MHC class I is on all nucleated cells and presents intracellular antigens to CD8⁺ T cells. MHC class II is on antigen-presenting cells (macrophages, dendritic cells, B cells) and presents extracellular antigens to CD4⁺ T cells.
Complement system
A cascade of serum proteins that can be activated by three pathways (classical, lectin, alternative). Functions: opsonisation (C3b coating), inflammation (C3a, C5a recruit phagocytes), and direct lysis of pathogens via the membrane attack complex (MAC).
Inflammation
A localised tissue response to infection or injury. Cardinal signs: redness, heat, swelling, pain. Mediated by histamine, prostaglandins, cytokines. Vasodilation and increased vascular permeability bring phagocytes and complement to the site of infection.
Toll-like receptors (TLRs)
Pattern recognition receptors on innate immune cells that detect conserved microbial structures (PAMPs, pathogen-associated molecular patterns) such as LPS (TLR4), flagellin (TLR5), and bacterial DNA (TLR9). Activation triggers signalling cascades that initiate inflammation and activate adaptive immunity.
Vaccine
A preparation that stimulates adaptive immunity against a pathogen without causing the disease. Types: live attenuated, inactivated (killed), subunit/recombinant, toxoid, mRNA, conjugate. The goal is to generate immunological memory so that a subsequent encounter with the real pathogen produces a rapid, strong secondary response.
Antibiotic
A substance that kills or inhibits the growth of bacteria. Originally the term referred to naturally produced compounds (e.g., penicillin from Penicillium), but it is now used broadly to include synthetic and semisynthetic agents. Major targets: cell wall synthesis (beta-lactams, vancomycin), protein synthesis (aminoglycosides, tetracyclines, macrolides, chloramphenicol), nucleic acid synthesis (fluoroquinolones, rifampin), metabolic pathways (sulfonamides, trimethoprim), and cell membrane integrity (polymyxins).
Antibiotic resistance
The ability of bacteria to survive exposure to an antibiotic that would normally kill or inhibit them. Mechanisms: enzymatic inactivation (e.g., beta-lactamase destroys penicillin), target modification (e.g., altered penicillin-binding proteins in MRSA), efflux pumps (pump the drug out), reduced permeability (loss of porins), and bypass pathways. Resistance genes spread via horizontal gene transfer (plasmids, transposons, integrons).
MIC (minimum inhibitory concentration)
The lowest concentration of an antimicrobial agent that inhibits visible growth of a microorganism after overnight incubation. Used to guide clinical dosing.
Epidemiology
The study of the distribution, determinants, and frequency of disease in populations. Concerned with how, where, and why diseases occur and how they can be controlled.
Epidemic
A disease occurrence that is significantly above the expected (endemic) level in a population. A pandemic is an epidemic that has spread across multiple countries or continents.
Endemic
The constant, expected level of a disease in a population or region.
Reservoir
The habitat in which a pathogen normally lives, grows, and multiplies. Reservoirs can be human (carriers), animal (zoonotic), or environmental (soil, water).
Vector
An organism (typically an arthropod) that transmits a pathogen from one host to another. Biological vectors (the pathogen replicates or develops within the vector, e.g., Anopheles mosquito for malaria) vs. mechanical vectors (passive carriage, e.g., flies carrying enteric bacteria on their legs).
Zoonosis
A disease that is transmissible from animals to humans. Examples: rabies, Lyme disease, plague, hantavirus, avian influenza.
Nosocomial infection (healthcare-associated infection, HAI)
An infection acquired in a healthcare setting. Common culprits include MRSA, Clostridioides difficile, vancomycin-resistant enterococci (VRE), and Gram-negative bacteria with extended-spectrum beta-lactamases (ESBL).
Eukaryotic pathogen
A disease-causing eukaryotic organism: fungi (e.g., Candida, Aspergillus, Cryptococcus), protozoa (e.g., Plasmodium for malaria, Trypanosoma for sleeping sickness, Giardia for giardiasis), helminths (parasitic worms). Treatment is more challenging because eukaryotic pathogens share many cellular processes with their human hosts, limiting drug targets.
Carbon cycle:
CO₂ fixation: autotrophs (phototrophs, chemolithotrophs) convert inorganic CO₂ into organic carbon
Decomposition: heterotrophs break down organic matter, releasing CO₂
Methanogenesis (archaea): produces CH₄ from CO₂ + H₂ or from acetate, in anoxic environments (wetlands, ruminant guts, rice paddies)
Methanotrophy: aerobic bacteria (and some anaerobic archaea) oxidise CH₄, the only known biological sink for this potent greenhouse gas
Nitrogen cycle:
Nitrogen fixation (nitrogenase): only prokaryotes can do this; free-living (Azotobacter, cyanobacteria) or symbiotic (Rhizobium in legume nodules)
Nitrification: two-step aerobic process, NH₃ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter); important in soil fertility and wastewater treatment
Denitrification: anaerobic respiratory process, NO₃⁻ → NO₂⁻ → NO → N₂O → N₂; returns fixed nitrogen to the atmosphere; performed by diverse facultative anaerobes
Anammox: anaerobic oxidation of ammonium coupled to nitrite reduction; discovered in wastewater treatment reactors, now known to be globally significant
Sulfur cycle: sulfate reduction (SO₄²⁻ → H₂S, anaerobic respiration), sulfur oxidation (H₂S → S⁰ → SO₄²⁻, chemolithotrophy), and assimilatory sulfate reduction (incorporating sulfur into amino acids)
Microbial communities and interactions:
Syntrophy: a metabolic partnership in which one organism's waste product is another's substrate; neither can thrive alone. Common in anaerobic degradation of organic matter
Competition, predation, and parasitism also shape community structure
Microbes are ubiquitous in buildings, hospitals, water systems, food production facilities
Water treatment: coagulation, filtration, chlorination or UV disinfection to remove pathogens; coliforms (e.g., E. coli) used as indicator organisms for faecal contamination
Wastewater treatment: primary (physical settling), secondary (biological: activated sludge, trickling filters; microbial communities oxidise organic matter), tertiary (nutrient removal, advanced disinfection)
Food microbiology: food preservation methods (refrigeration, canning, drying, fermentation, chemical preservatives) and food spoilage organisms; foodborne pathogens (Salmonella, E. coli O157:H7, Listeria, Staphylococcus aureus enterotoxin, Clostridium botulinum)
Rhizobium-legume symbiosis:
Molecular dialogue: plant root exudates (flavonoids) induce nod genes in rhizobia; Nod factors trigger root hair curling and nodule formation
Infection thread delivers bacteria into root cortical cells, where they differentiate into bacteroids
Leghemoglobin maintains low O₂ concentration to protect nitrogenase while allowing bacterial respiration
Ecologically and agriculturally crucial: legume crops reduce the need for synthetic nitrogen fertiliser
Mycorrhizal symbiosis: arbuscular mycorrhizae (AM) penetrate root cells; ectomycorrhizae form a sheath around roots. Both types extend nutrient absorption, especially phosphorus
Plant pathogens: Agrobacterium tumefaciens (crown gall disease) transfers T-DNA from the Ti plasmid into plant cells, a natural genetic engineering mechanism that has been harnessed for creating transgenic plants
Rumen symbiosis: ruminants (cattle, sheep) harbour dense anaerobic microbial communities in their rumen. Bacteria, protozoa, and fungi ferment cellulose into volatile fatty acids (acetate, propionate, butyrate) that the animal absorbs as its primary energy source. Methanogens in the rumen produce CH₄ as a byproduct
Human gut microbiome:
Dominated by Firmicutes and Bacteroidetes
Functions: digestion of complex polysaccharides, vitamin synthesis (K, B12, biotin), immune system maturation, colonisation resistance against pathogens
Dysbiosis (imbalance in gut microbiota) is associated with inflammatory bowel disease, obesity, and susceptibility to C. difficile infection
Insect symbionts: Buchnera in aphids provides essential amino acids the host cannot synthesise; Wolbachia in many insects manipulates host reproduction
Steps of infection:
Exposure and entry (portal of entry: skin, respiratory, gastrointestinal, urogenital tracts)
Adhesion to host tissues (adhesins, pili, surface proteins)
Invasion: penetration into or through host tissues (invasins, enzymes like hyaluronidase, collagenase)
Evasion of host defences (capsule resists phagocytosis, protein A of S. aureus binds IgG Fc region, antigenic variation, intracellular survival)
Damage to host (toxins, immune-mediated damage, direct tissue destruction)
Toxins:
Exotoxins: specific, potent, protein-based; can be used to make toxoids (inactivated toxins used as vaccines, e.g., tetanus toxoid)
Endotoxin (LPS/lipid A): less specific, triggers systemic inflammation; no toxoid can be made
Superantigens: bypass normal antigen processing, activate up to 20% of T cells simultaneously, causing massive cytokine release
Secretion systems: T3SS (molecular syringe), T4SS (e.g., Agrobacterium T-DNA transfer, Helicobacter pylori CagA injection), T6SS (contact-dependent killing of competing bacteria)
Pathogenicity islands: large chromosomal regions acquired by horizontal gene transfer that carry clusters of virulence genes. Often have different G+C content from the rest of the chromosome, flanked by mobile genetic elements
Innate immunity (first and second lines of defence):
Physical barriers: skin (dry, acidic, shedding), mucous membranes (mucus trapping, ciliary action), normal microbiota (colonisation resistance)
Chemical defences: lysozyme (in tears, saliva), defensins (antimicrobial peptides), stomach acid, bile salts
Cellular defences:
Phagocytes: neutrophils (first responders), macrophages (resident in tissues, also APCs), dendritic cells (key antigen-presenting cells that bridge innate and adaptive)
Natural killer (NK) cells: kill virus-infected and tumour cells without antigen-specific recognition
Pattern recognition: TLRs and other pattern recognition receptors detect PAMPs (LPS, flagellin, peptidoglycan, viral dsRNA, CpG DNA)
Complement system: three activation pathways converge on C3 → C3a + C3b; C3b opsonises, C3a/C5a recruit phagocytes, MAC lyses cells
Inflammation: vasodilation, increased permeability, phagocyte recruitment; fever (systemic response mediated by pyrogens and prostaglandins acting on the hypothalamus)
Adaptive immunity:
Humoral immunity (antibody-mediated):
B cells recognise antigen via surface immunoglobulin (BCR)
With T helper cell assistance, B cells differentiate into plasma cells (secrete antibodies) and memory B cells
Antibody functions: neutralisation, opsonisation, complement activation, agglutination
Five classes: IgM (first produced, pentamer, complement activation), IgG (most abundant in serum, crosses placenta), IgA (mucosal immunity, in secretions), IgE (allergy, parasites), IgD (B cell surface, signalling)
Cell-mediated immunity:
CD4⁺ helper T cells: recognise antigen on MHC II; Th1 cells activate macrophages and cytotoxic T cells; Th2 cells stimulate B cells and antibody production
CD8⁺ cytotoxic T cells: recognise antigen on MHC I; kill infected host cells by releasing perforin and granzymes
Clonal selection: each lymphocyte bears receptors for one specific antigen; when that antigen appears, the matching lymphocyte is selected, proliferates (clonal expansion), and differentiates into effector and memory cells
Primary vs. secondary response: the primary response is slow (days), dominated by IgM; the secondary response (on re-exposure) is faster, stronger, and dominated by IgG, due to memory cells
Vaccines work by priming the adaptive immune system to generate memory without causing disease
Diagnostic methods:
Culture-based: isolation on selective and differential media (e.g., MacConkey agar for Gram-negative enterics, blood agar for haemolysis patterns)
Microscopy: Gram stain, acid-fast stain (for mycobacteria), fluorescence microscopy
Biochemical tests: catalase, oxidase, coagulase, sugar fermentation profiles
Molecular: PCR, real-time (quantitative) PCR, 16S rRNA sequencing for identification, MALDI-TOF mass spectrometry (rapid protein fingerprinting)
Immunological: ELISA, lateral-flow immunoassays (rapid antigen tests), serotyping
Antimicrobial targets and agents:
Cell wall synthesis: beta-lactams (penicillins, cephalosporins, carbapenems) bind penicillin-binding proteins (PBPs); vancomycin binds D-Ala-D-Ala of peptidoglycan precursors
Protein synthesis (30S): aminoglycosides (cause misreading), tetracyclines (block A-site)
Protein synthesis (50S): macrolides (erythromycin), chloramphenicol, lincosamides (clindamycin)
Nucleic acid synthesis: fluoroquinolones (inhibit DNA gyrase/topoisomerase IV), rifampin (inhibits RNA polymerase)
Metabolic pathways: sulfonamides and trimethoprim (sequential block of folate synthesis)
Cell membrane: polymyxins (disrupt outer membrane of Gram-negatives), daptomycin (depolarises Gram-positive membrane)
Resistance mechanisms:
Enzymatic inactivation: beta-lactamases, aminoglycoside-modifying enzymes
Target alteration: altered PBPs (MRSA), modified ribosomes (macrolide resistance)
Efflux pumps: pump tetracyclines, fluoroquinolones, and other drugs out of the cell
Reduced uptake: loss of outer membrane porins (Gram-negatives)
Target bypass: vancomycin resistance in enterococci (van genes change D-Ala-D-Ala to D-Ala-D-Lac)
Antibiotic stewardship: using antibiotics only when necessary, at appropriate doses and durations, to slow the development and spread of resistance
Epidemiological terminology: incidence (new cases per time), prevalence (total cases at a point in time), morbidity, mortality, case fatality rate
Disease transmission:
Direct: person-to-person (respiratory droplets, sexual contact, direct skin contact, transplacental)
Indirect: vehicle-borne (food, water, fomites), airborne (droplet nuclei), vector-borne (arthropods)
Outbreak investigation: identify the agent, determine the source and mode of transmission, identify the population at risk, implement control measures. Epidemiological tools: case definition, attack rate, epidemic curve
Herd immunity: when a sufficient proportion of the population is immune (through vaccination or prior infection), transmission is reduced enough to protect susceptible individuals. The threshold depends on the pathogen's basic reproduction number (R₀)
Public health measures: surveillance, vaccination programmes, quarantine and isolation, sanitation and water treatment, vector control, education
Vectorborne:
Malaria (Plasmodium spp., transmitted by Anopheles mosquitoes): the most significant parasitic disease globally; complex life cycle alternating between mosquito and human host
Lyme disease (Borrelia burgdorferi, transmitted by Ixodes ticks): erythema migrans rash, joint and neurological complications if untreated
Plague (Yersinia pestis, transmitted by flea bites from infected rodents): bubonic, septicaemic, and pneumonic forms
Dengue, Zika, Chikungunya (Aedes mosquitoes): viral infections of increasing global concern
Soilborne:
Tetanus (Clostridium tetani): spores in soil; tetanospasmin toxin causes spastic paralysis
Anthrax (Bacillus anthracis): spores in soil; cutaneous, inhalation, and gastrointestinal forms
Fungal infections: Histoplasma capsulatum (found in bat and bird droppings), Coccidioides immitis (desert soils of the southwestern US)
Pathogenic fungi:
Superficial and cutaneous mycoses: dermatophytes (ringworm, athlete's foot)
Subcutaneous mycoses: sporotrichosis (Sporothrix schenckii)
Systemic mycoses: histoplasmosis, coccidioidomycosis, blastomycosis, cryptococcosis (Cryptococcus neoformans, an opportunistic pathogen especially in immunocompromised patients)
Opportunistic mycoses: candidiasis (Candida albicans), aspergillosis (Aspergillus fumigatus)
Antifungal drugs: azoles (target ergosterol synthesis), polyenes (amphotericin B binds ergosterol), echinocandins (target cell wall beta-glucan synthesis)
Pathogenic protozoa:
Plasmodium (malaria): complex life cycle, targets red blood cells; chloroquine resistance is widespread; artemisinin-based combination therapies are current standard
Trypanosoma (African sleeping sickness, Chagas disease)
Leishmania (leishmaniasis, transmitted by sandflies)
Giardia lamblia (giardiasis, waterborne)
Entamoeba histolytica (amoebic dysentery)
Toxoplasma gondii (toxoplasmosis, cat faeces as source; dangerous for immunocompromised patients and during pregnancy)
Helminths: not covered in depth in this course, but know that they are multicellular eukaryotic parasites (roundworms, tapeworms, flukes) with complex life cycles often involving intermediate hosts
R₀ (basic reproduction number): The average number of secondary infections produced by one infected individual in a fully susceptible population. R₀ > 1 means the infection will spread; R₀ < 1 means it will die out.
Herd immunity threshold: H = 1 – (1 / R₀) For example, if R₀ = 5, then H = 1 – 0.2 = 0.8, meaning 80% of the population needs to be immune to achieve herd immunity.
Complement cascade (simplified): Classical/Lectin/Alternative pathways → C3 convertase → C3 → C3a (inflammation) + C3b (opsonisation) → C5 convertase → C5a (inflammation) + C5b → MAC (C5b-C9, lysis)
Nitrogen fixation by rhizobia in legume root nodules is why farmers rotate crops with legumes to replenish soil nitrogen without synthetic fertiliser
Understanding biofilms is critical in medicine: biofilms form on catheters, prosthetic joints, and heart valves, making infections extremely difficult to eradicate with antibiotics alone
The development of antibiotic resistance is a direct consequence of the horizontal gene transfer mechanisms covered earlier in the course; MRSA, VRE, and carbapenem-resistant Enterobacteriaceae are among the most urgent clinical threats
Herd immunity calculations directly informed vaccination policy during the COVID-19 pandemic and continue to guide public health strategy for measles, polio, and other vaccine-preventable diseases
Students often think "normal flora" are purely passive bystanders. The normal microbiota actively contributes to health by competing with pathogens, producing antimicrobial substances, training the immune system, and synthesising vitamins. Disrupting them (e.g., with broad-spectrum antibiotics) can cause disease, as in C. difficile colitis
Antibiotics do not work against viruses. This is a fundamental point that students sometimes muddle when discussing antimicrobial therapy. Antiviral drugs exist but work by entirely different mechanisms (targeting viral enzymes like reverse transcriptase or neuraminidase)
Endotoxin and exotoxin are frequently confused on exams. Remember: exotoxins are secreted proteins (specific, potent, can be converted to toxoids); endotoxin is the lipid A portion of LPS (released on cell lysis, less specific, no toxoid)
Innate immunity is sometimes dismissed as "less important" than adaptive immunity. In reality, the innate system is the first to respond, and without it, the adaptive response would never be properly activated (dendritic cells bridge the two systems by presenting antigen to T cells)
⚠️ Be able to trace the steps of the nitrogen cycle and name the organism types responsible for each transformation
⚠️ Know the differences between exotoxins and endotoxin (source, chemical nature, specificity, heat stability, ability to form toxoids)
⚠️ Understand the distinction between innate and adaptive immunity, including specific cell types, receptors, and effector mechanisms
⚠️ Be able to compare humoral and cell-mediated immunity: which cells, which targets, which effector molecules
⚠️ Know the major antibiotic target categories and at least one drug example for each
⚠️ Understand herd immunity, R₀, and why vaccination coverage below the herd immunity threshold allows outbreaks
⚠️ Be able to describe the Rhizobium-legume symbiosis: signalling (Nod factors), infection thread, bacteroid differentiation, leghemoglobin, nitrogenase
⚠️ Know examples of vectorborne diseases and their vectors (malaria/Anopheles, Lyme/Ixodes, plague/flea)
True or False: Only eukaryotes can fix atmospheric nitrogen.
Fill in the blank: The lipid A component of ________ is the endotoxin of Gram-negative bacteria.
True or False: IgM is the first antibody class produced during a primary immune response.
Fill in the blank: Beta-lactam antibiotics target bacterial cell wall synthesis by binding to ________.
True or False: A pathogen with R₀ = 0.5 will cause an epidemic in a fully susceptible population.
(Answers: 1. False, only certain prokaryotes (bacteria and archaea) can fix nitrogen; no eukaryote can. 2. LPS (lipopolysaccharide). 3. True. 4. Penicillin-binding proteins (PBPs). 5. False, R₀ < 1 means each infected individual infects fewer than one other person on average, so the infection will die out.)
Q: Explain why biofilms are clinically problematic and more resistant to antibiotics than planktonic cells.
A: Biofilms are structured communities encased in an extracellular polymeric substance (EPS) matrix. This matrix physically limits antibiotic penetration. Cells deep within the biofilm are often in a slow-growing or dormant state, and most antibiotics are most effective against actively growing cells. Biofilms also harbour persister cells, which are phenotypic variants tolerant to antibiotics. Additionally, the close physical proximity of cells in a biofilm facilitates horizontal gene transfer of resistance genes.
Q: Compare and contrast the roles of CD4⁺ helper T cells and CD8⁺ cytotoxic T cells in adaptive immunity.
A: CD4⁺ helper T cells recognise antigen presented on MHC class II molecules by antigen-presenting cells. They do not kill pathogens directly; instead, they coordinate the immune response by secreting cytokines. Th1 helpers activate macrophages and cytotoxic T cells, while Th2 helpers stimulate B cell differentiation and antibody production. CD8⁺ cytotoxic T cells recognise antigen presented on MHC class I molecules on infected or abnormal host cells. They kill those cells directly by releasing perforin (forms pores) and granzymes (trigger apoptosis), eliminating the reservoir of intracellular infection.
Q: Why can endotoxin not be converted into a toxoid for use in vaccines, whereas exotoxins can?
A: Exotoxins are proteins, and their toxic activity can be destroyed by heat or chemical treatment (e.g., formaldehyde) while preserving their antigenic structure, producing a toxoid that stimulates protective antibody production. Endotoxin is the lipid A portion of LPS, a lipid-based molecule. Its toxicity is an intrinsic property of its lipid structure and cannot be separated from its antigenicity in the same way. Treating it to eliminate toxicity destroys its immunogenic usefulness.
Q: Describe the molecular dialogue between a rhizobium and a legume that leads to root nodule formation.
A: The plant root secretes flavonoids into the rhizosphere. These flavonoids are detected by compatible rhizobia and induce expression of the bacterium's nod genes. The Nod factor produced by the bacterium (a lipochitooligosaccharide) is perceived by receptors on root hair cells, triggering root hair curling and formation of an infection thread. The bacteria travel through the infection thread into the root cortex, where plant cells divide to form the nodule. Inside the nodule, bacteria differentiate into bacteroids and begin fixing nitrogen. The plant produces leghemoglobin, which binds O₂ to maintain the microaerobic environment nitrogenase requires.
Q: A hospital sees a sudden cluster of wound infections caused by a multi-drug-resistant Gram-negative bacterium. What epidemiological steps should be taken?
A: Establish a case definition (clinical and microbiological criteria). Identify all cases (active surveillance, review laboratory records). Characterise the cases by person, place, and time to look for commonalities (same ward, same surgical team, same procedure dates). Construct an epidemic curve. Investigate potential sources and modes of transmission (environmental cultures, hand hygiene compliance, surgical instrument sterilisation). Implement control measures (contact precautions, enhanced cleaning, possible cohorting of patients). Perform molecular typing (e.g., PFGE or whole-genome sequencing) on isolates to confirm they are clonally related, distinguishing a true outbreak from unrelated sporadic cases.
The nutrient cycling material (Ch. 20–21) draws directly on the metabolic diversity covered in Exam 1 (chemolithotrophy, anaerobic respiration, phototrophy). If the sulfur cycle or nitrogen cycle confuses you, revisit the redox chemistry from Ch. 3
Pathogenicity islands and virulence gene regulation (Ch. 25) connect back to horizontal gene transfer, operons, and two-component systems from the Exam 2 material
Antibiotic resistance mechanisms are a direct extension of the genetics material (mutations, plasmids, transposons, conjugation) from Ch. 11
The immune system's recognition of LPS, peptidoglycan, and flagellin only makes sense if you remember the cell structure material from Ch. 2
biogeochemical cycle, carbon cycle, nitrogen cycle, nitrogen fixation, nitrogenase, nitrification, denitrification, anammox, sulfur cycle, biofilm, EPS, microbiome, microbiota, symbiosis, mutualism, commensalism, parasitism, rhizosphere, mycorrhizae, Rhizobium, root nodule, leghemoglobin, Nod factor, rumen, gut microbiome, dysbiosis, normal flora, colonisation resistance, pathogenicity, virulence, virulence factor, adhesin, exotoxin, endotoxin, LPS, lipid A, AB toxin, superantigen, type III secretion system, T3SS, pathogenicity island, Koch's postulates, innate immunity, adaptive immunity, antigen, epitope, antibody, immunoglobulin, IgG, IgM, IgA, IgE, T cell, B cell, CD4, CD8, helper T cell, cytotoxic T cell, MHC class I, MHC class II, complement, MAC, inflammation, TLR, PAMP, vaccine, toxoid, antibiotic, beta-lactam, penicillin, vancomycin, MRSA, antibiotic resistance, MIC, efflux pump, beta-lactamase, epidemiology, epidemic, pandemic, endemic, R0, herd immunity, reservoir, vector, zoonosis, nosocomial infection, malaria, Plasmodium, Lyme disease, Borrelia, plague, Yersinia, Candida, Aspergillus, Cryptococcus, dermatophyte, antifungal, azole, amphotericin B, Brock Biology of Microorganisms Chapter 20 21 22 23 24 25 26 27 28 29 31 33, MCB 3020