What you need to know
- 6.1–6.3Aseptic culture, measuring growth, closed growth curve and k.
- 6.5–6.7Bacteria versus viruses, TB and HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS. infection, routes and barriers.
Core Practical 13 has the liquid-culture method. Immunity details remain 6B.
By the end, you should be able to…
- Keep bacteria, viruses and fungi (yeast in CP13) in the correct category.
- Choose viable versus total counts, and exponential-phase numbers for k.
- Finish TB and HIV answers at named cells and a path to death.
Build your understanding
Start with the organism type
Bacterium: cell, DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code., wall. Virus: nucleic acid, capsidThe protein coat of a virus, made of capsomeres., maybe envelopeAn outer covering membrane. The nucleus and chloroplasts have a double-membrane envelope; some viruses, such as HIV, have a lipid envelope taken from the host cell.. Yeast in school broth: eukaryoticDescribes cells with a nucleus and membrane-bound organelles, such as animal and plant cells. fungus, used because it is safer than a pathogen, not because it is Mycobacterium. All numbers below are fictional.
Finish the mechanism
Aseptic: both directions. turbidityCloudiness of a liquid culture. It increases with the number of cells and can be measured with a colorimeter.: dead cells scatter. k: exponential only, log₁₀ 2 given. HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS.: CD4A receptor glycoprotein on T helper cells that HIV binds to. T helper cellA T cell that binds to an antigen-presenting cell and releases cytokines that activate B cells and T killer cells., reverse transcriptaseAn enzyme from retroviruses, such as HIV, that makes DNA using RNA as a template., provirusViral DNA, such as HIV DNA, that has been inserted into a host cell’s chromosome., opportunistic death. TB: macrophage, tubercleA mass of infected macrophages and other cells walled off in the lungs in tuberculosis., gas exchange. Barriers: skin, acid, flora — not antibodyA Y-shaped protein made by plasma cells that binds specifically to one antigen..
Make your reasoning visible
Do not import 6B
Leave plasma cells and antibiotics unless the question brings them.
Show the k line
Write the formula, substitute, give units.
Avoid this misconception
The claim: turbidityCloudiness of a liquid culture. It increases with the number of cells and can be measured with a colorimeter., CFU and k are three words for the same number.
The correction: Turbidity is optical biomass, CFU is viable count, k is doublings per unit time from exponential-phase N values.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Check your understanding (continued)
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Give two reasons for aseptic technique.
Show suggested answer
Prevent contamination of the culture; prevent escape of cultured organisms to people or the environment.
Practice question
Retrieval and explanation · suggested answer
Why incubate school plates at about 25 °C rather than 37 °C?
Show suggested answer
To reduce the risk of selecting microorganisms that grow well at human body temperature, including pathogens.
Practice question
Retrieval and explanation · suggested answer
Which specified method gives a viable count?
Show suggested answer
Dilution plating (colony-forming units).
Practice question
Retrieval and explanation · suggested answer
Name the four phases of a closed bacterial growth curve.
Show suggested answer
Lag, exponential, stationary, death.
Practice question
Retrieval and explanation · suggested answer
Fictional: N₀ = 1.00 × 10⁵, Nₜ = 8.00 × 10⁵ in 2.00 h of exponential growth, log₁₀ 2 = 0.301. Calculate k.
Show suggested answer
log₁₀(Nₜ/N₀) = log₁₀ 8 = 0.903; k = 0.903 / (2.00 × 0.301) = 1.50 h⁻¹ (3 s.f.).
Practice question
Retrieval and explanation · suggested answer
Compare HIV and λ phage on nucleic acid and envelope.
Show suggested answer
HIV: RNA, envelope. λ phage: DNA, no envelope.
Practice question
Retrieval and explanation · suggested answer
Define latency.
Show suggested answer
Viral nucleic acid remains in the host without immediately producing new virions.
Practice question
Retrieval and explanation · suggested answer
How do skin flora act as a barrier?
Show suggested answer
They occupy the surface and compete with incoming pathogens for space and nutrients.
Practice question
Retrieval and explanation · suggested answer
How can M. tuberculosis infection result in death?
Show suggested answer
Bacteria surviving in macrophages form tubercles that destroy lung tissue, so gas exchange fails, or the infection spreads.
Practice question
Retrieval and explanation · suggested answer
Why can untreated HIV infection result in death?
Show suggested answer
T helper cells are destroyed, specific immunity collapses, and opportunistic infections such as TB can kill.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Fictional CFU falls while absorbance holds, and a student still calculates k from the last two hours. Rewrite a specification-level comment.
Show a hint
Phase and method.
Explore the reasoning
The culture is leaving exponential growth; viable count is falling so those hours are not for k. Absorbance is still high because dead cells scatter light. Use two exponential-phase viable numbers, or a log plot’s straight section, then state k with units.
Recall this lesson
Lesson recall questions
Question 1 of 5: How does measuring turbidity with a colorimeter estimate microbial growth?
Measuring microbial growth
Practice question
Retrieval and explanation · suggested answer
How does measuring turbidity with a colorimeter estimate microbial growth?
Lesson: 6A 5: Measuring the Growth of Bacterial Cultures
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Principles, not a photocopied SOP
6A · Exam PracticeThe Microbiology Society’s case for studying microbes is that they are everywhere and that method matters. Exam answers should name sterile transfer, a growth measure, a phase and…Specification 6.1Read & use the scienceClose article
Why this matters
The Microbiology Society’s case for studying microbes is that they are everywhere and that method matters. Exam answers should name sterile transfer, a growth measure, a phase and a cell — not a brand of autoclave tape. CP13 is the place for the liquid-culture sequence. In an unfamiliar culture question, state the variable, the growth measure and a control before interpreting a curve. Sterile technique matters because a contaminant could change the reading and invalidate the comparison.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.1 understand the principles and techniques involved in culturing microorganisms, using aseptic technique
Use the science
Why would copying a 20-step colorimeter manual as a 6.4 answer still miss the specification?
Explore a possible response
The point is aseptic sampling over time, a named growth measure, controls and safe disposal — not a proprietary recipe.
Explore the source
- Microbiology Society: what is microbiologyMicrobiology Society
A cleanroom is aseptic technique at scale
6A · Microbial TechniquesThe FDA’s guidance for sterile drug production treats cleanroom air, equipment, operators and materials as potential sources of contamination. The principle is the same as school aseptic transfer:…Specification 6.1Read & use the scienceClose article
Why this matters
The FDA’s guidance for sterile drug production treats cleanroom air, equipment, operators and materials as potential sources of contamination. The principle is the same as school aseptic transfer: reduce the chance that an unwanted microorganism enters the culture or product. Professional teams also monitor the environment and validate the process rather than assuming that a white coat guarantees sterility. This connection is about risk control, not instructions for culturing unknown organisms. For a school practical, the teacher’s approved safety procedure remains essential. A clean-looking sample can still contain invisible contaminants.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.1 understand the principles and techniques involved in culturing microorganisms, using aseptic technique
Use the science
Why would sterilising a transfer loop be insufficient if the vessel were left open in unfiltered air?
Explore a possible response
Microorganisms or particles could enter from the air, so asepsis requires control of several contamination routes, not only the loop.
Explore the source
- Sterile drug products produced by aseptic processingUS Food and Drug Administration
Cloudy is not the same as alive
6A · Measuring the Growth of Bacterial CulturesOptical density estimates how much light a microbial suspension scatters. A published study found that stress and reduced viability changed the relationship between optical density and cell count…Specification 6.2Read & use the scienceClose article
Why this matters
Optical density estimates how much light a microbial suspension scatters. A published study found that stress and reduced viability changed the relationship between optical density and cell count in Listeria. A food microbiologist therefore calibrates an instrument for the organism and conditions being tested, and may check a culture by dilution plating. Dead cells and particles can still scatter light; a colony count instead records units able to grow on the chosen medium. Neither method is a perfect census, because clumps and growth conditions affect the result. The lesson’s methods answer different questions, even when both produce a number.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.2 understand the different methods of measuring the growth of microorganisms, as illustrated by cell counts, dilution plating, mass and optical methods (turbidity)
Use the science
If a disinfectant kills many cells without immediately clearing the suspension, how might turbidity and viable plate count differ?
Explore a possible response
Turbidity may remain high because dead cells still scatter light, while the viable colony count should fall.
Explore the source
- Optical density and cell count under stressInternational Journal of Food Microbiology · Abstract