Microbiology, Immunity and Forensics
Edexcel International A-Level Biology· Statements 6.1–6.20· 45 min read
1. Microbial Culturing and Growth Measurement★★★☆☆⏱ 10 min
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Aseptic technique
Procedures to prevent contamination of cultures, including flaming inoculation loops, sealing agar plates, and working within the updraft of a lit Bunsen burner.
Bacterial growth can be measured using four key methods: total cell counts (using a haemocytometer), dilution plating (counting colony forming units, CFU), dry mass measurement, and turbidity (measuring cloudiness of liquid culture with a colorimeter). The standard bacterial growth curve has four phases: lag (cells adapt to new conditions), exponential (unlimited resources, maximum growth rate), stationary (growth = death rate, resource limitation), and death (toxic waste builds up, death exceeds growth).
A 1mL sample of E. coli culture is diluted 1:10,000, and 0.1mL of the diluted sample is plated, producing 42 colonies. Calculate the number of CFU per mL of the original culture.
- 1
- Calculate total dilution factor: dilution × volume plated = 10,000 × (1 / 0.1) = 100,000
- 2
- Multiply colony count by dilution factor: 42 × 100,000 = 4.2 × 10⁶ CFU/mL
Exam tip:
Always use a log10 scale for the y-axis when plotting bacterial growth curves; marks are awarded for correct phase order and axis labelling.
2. Pathogen Structure and Non-Specific Immunity★★☆☆☆⏱ 8 min
Pathogen
Disease-causing organism, including bacteria and viruses.
Bacteria are prokaryotic cells with cell walls, DNA, and ribosomes. Viruses are non-living, consisting of nucleic acid (DNA/RNA) surrounded by a protein capsid; some (HIV, Ebola) have a lipid envelope, while others (TMV, λ phage) do not. Viruses follow either lytic (immediate replication and host lysis) or latent (integrate into host genome, remain inactive for months/years, e.g. HIV) life cycles.
Pathogens enter the body via cuts, ingestion, inhalation, or sexual contact. Physical and chemical barriers include skin, stomach acid, and commensal flora. Non-specific immune responses include inflammation, lysozyme (breaks down bacterial cell walls), interferon (blocks viral replication), and phagocytosis (white blood cells engulf and destroy pathogens).
Compare the structure of HIV and λ phage, and their life cycles.
- 1
- HIV is an enveloped RNA retrovirus that follows a latent life cycle, integrating its genetic material into human T cell genomes.
- 2
- λ phage is a non-enveloped DNA virus that infects E. coli bacteria, following a lytic life cycle that immediately lyses host cells to release new viral particles.
3. Adaptive Immunity and Antibiotics★★★☆☆⏱ 10 min
Antigen-presenting cell (APC)
Cell (e.g. macrophage) that engulfs pathogens and displays antigens on its surface to activate T helper cells.
Adaptive immunity involves B cells (produce memory cells and antibody-secreting plasma cells) and T cells (T helper cells activate B cells and macrophages; T killer cells destroy infected host cells; memory T cells provide long-term protection). Immunity is categorised in a 2×2 grid: natural/artificial × active/passive. Vaccination is artificial active immunity; breast milk antibodies are natural passive immunity.
Antibiotics target bacterial cells only: bacteriostatic antibiotics inhibit growth, while bactericidal antibiotics kill cells. Core Practical 14 tests the effect of antibiotics on bacterial growth using agar plates and zone of inhibition measurements. Hospital-acquired infections (e.g. MRSA) are controlled using strict hygiene codes of practice.
Classify the immunity gained from receiving an injection of pre-made antibodies against rabies.
- 1
- The immunity is artificial, as it is medically administered, not acquired via natural exposure.
- 2
- The immunity is passive, as you receive pre-made antibodies rather than producing your own immune response.
- 3
- Classification: Artificial passive immunity.
Exam tip:
Do not confuse T helper and T killer cell roles: T helper cells activate other immune cells, T killer cells directly destroy infected host cells.
4. Forensic Molecular Biology Tools★★★☆☆⏱ 8 min
Polymerase Chain Reaction (PCR)
Technique to amplify small DNA samples in three core steps: denaturation, annealing, extension.
PCR steps: 1. Denaturation (95°C, DNA strands separate), 2. Annealing (55°C, primers bind to target DNA sequences), 3. Extension (72°C, Taq polymerase synthesises new DNA strands). Gel electrophoresis separates DNA fragments by length: shorter fragments travel further through the gel matrix. DNA profiling uses gel electrophoresis to compare unique non-coding DNA patterns for identification, paternity testing, and evolutionary studies.
A forensic sample from a crime scene is compared to DNA from three suspects using gel electrophoresis. Which suspect matches the crime scene sample?
- 1
- Compare the band pattern of the crime scene sample to each suspect's band pattern.
- 2
- The suspect with an identical band pattern across all fragments is the match, as each individual has a unique DNA profile.
Exam tip:
Always list PCR steps in the correct order, with approximate temperatures, to gain full marks.
5. Forensic Time of Death Estimation★★☆☆☆⏱ 7 min
Forensic entomology
Use of insect life cycles on decomposing remains to estimate minimum time since death.
Five key methods to estimate time of death: 1. Body temperature cooling (algor mortis): body cools at ~1.5°C per hour until it reaches ambient temperature. 2. Muscle contraction (rigor mortis): muscles stiffen 2-6 hours after death, relaxes 24-36 hours later. 3. Decomposition stages: visible changes to body appearance over time. 4. Forensic entomology: age of insect larvae found on remains gives minimum time since death. 5. Ecological succession: changes in species present on remains over time.
A body is found with a core temperature of 30°C, and ambient temperature is 20°C. Estimate minimum time since death.
- 1
- Normal human body temperature is 37°C. Temperature drop = 37 - 30 = 7°C.
- 2
- Assume cooling rate of 1.5°C per hour: time since death = 7 / 1.5 = ~4.7 hours (minimum 4-5 hours).
6. Common Pitfalls
Wrong move:
Mixing up bacteriostatic and bactericidal definitions.
Why:
The terms sound similar and are frequently tested in multiple choice and short answer questions.
Correct move:
Use mnemonic: STATIC = stops growth (cells remain alive), CIDAL = kills cells (cide = kill).
Wrong move:
Plotting bacterial growth curves on a linear y-axis.
Why:
Exponential growth produces a J-curve on linear scale that hides phase transitions and makes growth rate calculations impossible.
Correct move:
Always use a log10 scale for the number of bacterial cells on growth curve plots.
Wrong move:
Classifying vaccination as passive immunity.
Why:
Students often confuse active and passive immunity definitions.
Correct move:
Vaccination introduces antigen that triggers your body to produce its own antibodies and memory cells = artificial active immunity; passive immunity is receiving pre-made antibodies.
Wrong move:
Confusing T helper and T killer cell roles.
Why:
Both are T lymphocytes but have distinct non-overlapping functions.
Correct move:
T helper cells activate B cells and macrophages; T killer cells destroy infected host cells directly.
Wrong move:
Including restriction endonucleases in PCR explanations.
Why:
Restriction enzymes are part of Unit 5 genetic engineering content, out of scope for this topic.
Correct move:
Only describe the three core steps of PCR: denaturation, annealing, extension.
Wrong move:
Stating all viruses have lipid envelopes.
Why:
Only some viruses have envelopes; TMV and λ phage are non-enveloped, HIV and Ebola are enveloped.
Correct move:
7. Quick Reference Cheatsheet
Concept | Key Details | Exam Recall Prompt |
|---|---|---|
Aseptic Technique | Flame loops, seal plates, work near Bunsen burner | List 3 aseptic steps for culturing |
Growth Curve Phases | Lag → Exponential → Stationary → Death | Order the 4 bacterial growth phases |
Immunity Types | Natural active: infection; Artificial active: vaccine; Natural passive: breast milk; Artificial passive: antibody injection | Classify immunity from given scenario |
PCR Steps |
| List 3 PCR steps in order |
Antibiotic Types | Bacteriostatic: inhibits growth; Bactericidal: kills bacteria | Distinguish between antibiotic modes of action |
Time of Death Methods | Body temp, rigor mortis, decomposition, forensic entomology, succession | Name 3 methods to estimate time of death |
8. Frequently Asked
What is the difference between active and passive immunity?
Active immunity occurs when your own immune system produces antibodies and memory cells in response to an antigen, providing long-term protection. Passive immunity occurs when you receive pre-made antibodies from another source, providing short-term protection with no memory cell production.
Why is a log scale used for bacterial growth curves?
Bacterial growth is exponential, so a linear y-axis produces a steep J-curve that hides distinct growth phases. A log10 scale converts exponential growth to a straight line, making it easy to identify lag, exponential, stationary and death phases, and calculate growth rate constants.
Going deeper
What's Next
Now that you have mastered all content for this topic, you should move on to practicing past paper questions for Unit 4 WBI14 to test your understanding of calculations and application questions. This topic accounts for ~35% of Unit 4 marks, so focus on growth rate calculations, immunity classification, and DNA profiling interpretation which are frequent high-mark questions. Next, you can review Topic 5 content on energy flow and ecosystems to complete your Unit 4 preparation, then move on to Unit 5 content for the full A Level Biology course.