What you need to know
- 6.12Evolutionary race: pathogen evasion mechanisms supporting selection on host and pathogen.
- 6.15hospital-acquired infectionAn infection caught in a hospital or other health-care setting, often by antibiotic-resistant bacteria such as MRSA.: causes that shaped antibiotic-prescription and hospital-practice codes.
6.12 is evasion biology, not a funding essay. 6.15 is codes that follow named causes, not a political programme. Do not invent examiner phrases about “always complete the course” as a mark scheme; treat that as a taught code of practice.
By the end, you should be able to…
- Give named evasion examples (HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS. antigenA molecule, often a protein or glycoprotein on a cell surface, that the immune system recognises as non-self. change, TB intracellular survival, T-helper destruction).
- Define hospital-acquired infection and three contributory causes.
- Link those causes to prescription codes and hygiene codes.
Build your understanding
An evolutionary race, not a slogan
Pathogens and hosts both face selection. Evasion mechanisms that let a pathogen persist are evidence of that race, matched by host defences such as a diverse lymphocyte repertoire. MHC diversity is enrichment naming, not a required 6.12 term. Specification 6.12 wants named evasion, not a history of Darwin.
HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS.: error-prone reverse transcriptaseAn enzyme from retroviruses, such as HIV, that makes DNA using RNA as a template. changes 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. antigenA molecule, often a protein or glycoprotein on a cell surface, that the immune system recognises as non-self. so earlier antibodyA Y-shaped protein made by plasma cells that binds specifically to one antigen. and T-cell clones fit less well; killing T helpers also strips the host of cytokineA signalling protein released by immune cells, such as T helper cells, that stimulates other immune cells.. M. tuberculosis: waxy wall and survival inside macrophages hide it from antibody. latencyA period when a virus stays in a host cell without making new viruses or causing symptoms. in tubercleA mass of infected macrophages and other cells walled off in the lungs in tuberculosis. is concealment, not a second virus.
Hospital-acquired infection
A hospital-acquired (hospital-acquired infectionAn infection caught in a hospital or other health-care setting, often by antibiotic-resistant bacteria such as MRSA.) infection is contracted in a healthcare setting and was not present on admission. Contributory causes include poor hand hygiene, overuse or incorrect use of antibioticA substance that kills bacteria or stops them growing, without harming human cells. Antibiotics do not work on viruses. selecting resistant strains, and invasive devices or surgery that breach 6.7 barriers.
Understanding those causes produced codes of practice. Prescription codes: antibiotics for confirmed bacterial infection, at a suitable dose and course, not for viral colds. Hygiene codes: hand washing or gel between patients, isolation, sterile instruments, PPE. Completing a prescribed course is taught as reducing the chance that the most resistant survivors remain — a code, not a magic spell.
Keep 6.12 and 6.15 in their lanes
A 6.12 answer names evasion (antigenic variation, intracellular hiding, destroying T helpers). A 6.15 answer names HAI causes and the codes they produced. Hand gel is not an evasion mechanism; HIV mutationA change in the sequence of bases in DNA. is not a ward policy.
MRSAMethicillin-resistant Staphylococcus aureus: a strain of bacteria resistant to many antibiotics, often a hospital-acquired infection. as a named resistant bacterium is a useful HAI example. It is not a fourth domainThe highest rank in classification. The three domains are Bacteria, Archaea and Eukarya. and not a required extra speciesA group of organisms that can interbreed to produce fertile offspring. list.
Diagram A
NOT: hospital hand gel is not an evasion mechanism. Antigenic variation is not a ward policy.
- Error-prone reverse transcriptase changes HIV envelope antigens
- Earlier antibodies then fit less well
- Latency in a tubercle is concealment, not a second virus
- MHC diversity is enrichment naming, not required recall
- The host is also under selection; the race is not only microbial
Two traps this diagram blocks
- Do not describe hand gel as the evolutionary race.
- Do not say TB evasion means the bacterium is a virus.
From the diagram alone
Question
Explain one way HIV and one way M. tuberculosis evade the immune response. [4]
Show a model answer
HIV mutates its antigens because reverse transcriptase is error-prone, so antibodies and T-cell clones made earlier fit poorly. It also destroys T helper cells. M. tuberculosis survives inside macrophages, where antibodies do not reach it, and its waxy wall adds to that protection.
Diagram B
NOT: these codes are not the evasion mechanisms of HIV or TB. MRSA is an example, not a new domain of life.
- Hospital-acquired means absent on admission
- Antibiotics are for confirmed bacterial infection, at a suitable dose and course
- Completing the course is taught as reducing resistant survivors
- Hand washing or gel, isolation and PPE are hygiene codes
- A code is not a guarantee if the practice is ignored
Two traps this diagram blocks
- Do not explain a hospital infection by HIV antigenic variation.
- Do not say antibiotics should be used for viral colds.
From the diagram alone
Question
Explain two causes of hospital-acquired infection and one code of practice that responds to them. [4]
Show a model answer
Poor hand hygiene transfers microbes between patients, and unnecessary antibiotics select resistant strains. Devices that cross the skin are a third cause. A matching code is hand hygiene between patients, or prescribing antibiotics only for bacterial infection and completing the course.
| Point | Evidence you give | What not to substitute |
|---|---|---|
| 6.12 race | HIV antigen change; TB in macrophages; T-helper loss | Hospital funding or hand-gel alone |
| 6.15 HAI | Hygiene, antibiotic overuse, broken barriers → codes | A full microbiology of every ward organism |
Make your reasoning visible
Name the evasion
“The pathogen is clever” scores nothing until you say antigenA molecule, often a protein or glycoprotein on a cell surface, that the immune system recognises as non-self. changed or it hid in a macrophage.
Pair cause with code
Hands spread MRSAMethicillin-resistant Staphylococcus aureus: a strain of bacteria resistant to many antibiotics, often a hospital-acquired infection. → wash or gel between patients.
Key explanations
- HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS. reverse transcriptaseAn enzyme from retroviruses, such as HIV, that makes DNA using RNA as a template. makes copying errors, so 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. antigens change and existing clones fit less well.
- HIV also kills T helper cells, removing the cytokineA signalling protein released by immune cells, such as T helper cells, that stimulates other immune cells. that coordinate the response.
- M. tuberculosis has a waxy wall and survives inside macrophages, so antibodyA Y-shaped protein made by plasma cells that binds specifically to one antigen. is less effective. tubercleA mass of infected macrophages and other cells walled off in the lungs in tuberculosis. conceal it further.
- Pathogen variants that evade the response leave more offspring, so those alleles increase.
- Hosts with a wider set of lymphocyte specificities are more likely to recognise a changed antigen.
- This is 6.12. Antibiotic policy and hand hygiene are the separate hospital lesson.
- Overuse or incorrect use of antibioticA substance that kills bacteria or stops them growing, without harming human cells. Antibiotics do not work on viruses., including treating viral colds, selects resistant strains.
- Tubes, lines and surgery breach skin and mucous barriers and create a route of entry.
Avoid this misconception
The claim: Antibiotic resistance is the evolutionary race, so hospital hygiene is off-syllabus.
The correction: Resistance is one selected pathogen trait and belongs with HAI codes (6.15). The 6.12 race also includes immune evasion such as antigenic variation and intracellular survival.
Why do doctors say “finish the course” and refuse antibiotics for a cold?
Bacteria in an infection vary. An antibiotic kills the most susceptible first, so the survivors are more likely to be resistant. That selection is why prescribing and hospital hygiene follow codes of practice.
What the model shows
One patient’s infection under different courses, a cold treated with an unnecessary antibiotic, and a hospital ward with four codes of practice.
What to look for
The share of partly resistant bacteria after each course, and how each code changes the number who catch MRSA.
What it tells us
Stopping early or under-dosing selects resistant bacteria. Antibiotics do nothing to viruses but still select resistance. Hand hygiene, isolation, careful prescribing and sterile devices cut hospital-acquired infections.
Be careful what you claim
Fictional bacteria and illustrative probabilities.
Try it in the model: press Play with the full course, then stop the course on day 5.
Antibiotic resistance and hospital infection
Treat one fictional patient’s infection with different antibiotic courses and watch which bacteria survive, then run a hospital ward with and without codes of practice. All bacteria and numbers are fictional.
What you see
- Patient: a 21-day calendar showing antibiotic days and how the patient feels, a sample of the infection with susceptible (green) and partly resistant (orange) bacteria, and a log-scale graph of their numbers. For a cold, the virus and the resistant share of gut bacteria.
- Ward: twelve beds, each patient clear, carrying MRSA or infected, with catheters, antibiotics and side rooms shown. The graph averages 60 fictional months; the card pairs each cause with its code of practice.
How to use it
- Patient: choose the infection, whether to prescribe, the dose and the day the course stops, then play or drag Day.
- Ward: turn the four codes of practice on or off, then play or drag Day. Another month gives a new random run.
What it shows
- How antibiotic use selects resistant bacteria, and why dose and course length matter (6.15).
- Why antibiotics are not used for viral infections.
- Causes of hospital-acquired infections and the codes of practice for prescribing and hygiene that follow from them (6.15).
Limits of the model
- All bacteria, numbers and probabilities are fictional and simplified. Real resistance can also come from new mutations and from genes passed between bacteria.
- Each day on the ward is a random run of a simple model. Real wards differ, and the averages are illustrative.
- The evolutionary race (6.12) is about evasion such as antigen change in HIV and TB surviving in macrophages. It is covered in the lesson and the questions, not in the simulation.
Guided tour · step 1 of 10
- In Patient, just below these steps, keep Stop on day on day 10 (full course).
- Press Play, or drag Day past 14.
The antibiotic kills susceptible bacteria fastest. A few partly resistant ones need more time, but a full course brings numbers low enough for phagocytes and the rest of the immune system to clear them.
Patient
Questions
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Give two pathogen evasion mechanisms that support an evolutionary race.
Show suggested answer
HIV antigenic variation (and T-helper destruction); M. tuberculosis surviving inside macrophages / waxy wall. (Other valid named mechanisms are acceptable.)
Practice question
Retrieval and explanation · suggested answer
What is a hospital-acquired infection?
Show suggested answer
An infection contracted in hospital or another healthcare setting that the patient did not have on admission.
Practice question
Retrieval and explanation · suggested answer
How has over-prescription of antibiotics shaped a code of practice?
Show suggested answer
Unnecessary use selects resistant strains, so codes restrict antibiotics to bacterial infections, with a suitable dose and course, and not for viruses.
Practice question
Retrieval and explanation · suggested answer
List three hospital-practice codes that reduce spread.
Show suggested answer
Hand hygiene between patients; isolation of infected patients; sterilising instruments / using PPE. (Any three valid.)
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A student answers “evolutionary race” with only “staff must wash hands”. Rewrite two sentences that would actually hit 6.12, then one that correctly uses hand-washing as 6.15.
Show a hint
Evasion first; vector second.
Explore the reasoning
6.12: HIV envelope antigens change so memory antibodies fail; TB remains inside macrophages, away from antibody. 6.15: because staff hands can carry resistant bacteria such as MRSA between patients, codes require washing or gel before and after each contact.
Recall this lesson
Lesson recall questions
Question 1 of 5: How does hiding inside host cells support the idea of an evolutionary race?
Evolutionary race
Practice question
Retrieval and explanation · suggested answer
How does hiding inside host cells support the idea of an evolutionary race?
Lesson: 6B 5: Antibiotic Resistance
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
WHO: resistance is selected, not invented in a ward cupboard
6B · Antibiotic ResistanceThe WHO antimicrobial-resistance fact sheet treats resistance as a selected bacterial trait driven by misuse and overuse of antimicrobials, with infection prevention as part of the response. That…Specification 6.15Read & use the scienceClose article
Why this matters
The WHO antimicrobial-resistance fact sheet treats resistance as a selected bacterial trait driven by misuse and overuse of antimicrobials, with infection prevention as part of the response. That is 6.15’s logic: understand the cause, write a code. It is not a ranking of countries and not a prescription for any reader. A code of practice joins two mechanisms: limiting opportunities for transmission and reducing selection pressure from unnecessary drugs. Neither a clean surface nor a narrow prescription alone solves every hospital infection.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.15 know how an understanding of the contributory causes of hospital-acquired infections has led to codes of practice regarding antibiotic prescription and hospital practice that relate to infection prevention and control
Use the science
Why is “bacteria become resistant because they want to” a weaker 6.12/6.15 sentence than selection language?
Explore a possible response
Resistance is a heritable trait that survives when antibiotics kill competitors. Intention is not a mechanism; selection is.
Explore the source
The hospital pharmacist as an evolution manager
6B · Antibiotic ResistanceCDC describes antibiotic stewardship teams that review prescribing, monitor resistance patterns and give feedback to clinicians. A pharmacist on such a team is using evolutionary biology: unnecessary exposure…Specification 6.15Read & use the scienceClose article
Why this matters
CDC describes antibiotic stewardship teams that review prescribing, monitor resistance patterns and give feedback to clinicians. A pharmacist on such a team is using evolutionary biology: unnecessary exposure selects bacteria already carrying resistance traits, while infection-control measures reduce opportunities for those bacteria to spread. Stewardship is not simply “never use antibiotics”; an appropriate drug can be lifesaving. The role joins data, clinical judgement and clear communication. The specification asks for contributory causes of hospital-acquired infection and resulting codes of practice, so an exam answer should link the cause to the particular measure rather than list a generic poster.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.15 know how an understanding of the contributory causes of hospital-acquired infections has led to codes of practice regarding antibiotic prescription and hospital practice that relate to infection prevention and control
Use the science
Why might a hospital combine prescription review with hand hygiene rather than choose only one?
Explore a possible response
Prescription review reduces selection pressure; hand hygiene reduces transmission. The measures interrupt different steps in the problem.