Biology • Year 12 • Module 7 • Lesson 2
Classifying Pathogens
Build HSC Band 5–6 extended-response technique on pathogen classification, host entry adaptations, and the treatment specificity principle.
1. Stimulus-based extended response — prion disease in Australia’s cattle industry (Band 5–6)
8 marks Band 5–6
Stimulus. Bovine spongiform encephalopathy (BSE, “mad cow disease”) is caused by an abnormally folded form (PrPSc) of a normal host protein (PrPC). When a cow ingests BSE-contaminated meat-and-bone meal, PrPSc accumulates in neural tissue, causing progressive vacuolation of the brain and inevitable death. No nucleic acid has been found in purified prion preparations. BSE can be transmitted to humans as variant Creutzfeldt–Jakob disease (vCJD). Australia has maintained BSE-free status partly because it banned ruminant-to-ruminant feeding in 1996. A 2023 review of global prion disease cases noted that no antiviral, antibiotic, antifungal or anthelmintic treatment has yet achieved a clinical cure for any prion disease. The only documented reduction in human risk has come from dietary and agricultural interventions.
Q1. Analyse and evaluate, using lesson content, the biology of BSE as a pathogen and the basis for the finding that no existing drug class is effective against it. Assess whether Australia’s agricultural intervention (banning ruminant-to-ruminant feeding) is a scientifically justified response to the limitations of drug treatment.
In your answer:
- Classify BSE and compare its biological features to at least two other pathogen types covered in this lesson.
- Explain, with specific reference to molecular targets, why antibiotics, antivirals, antifungals and anthelmintics are all ineffective against prions.
- Assess the agricultural intervention as a public health strategy, using evidence from the stimulus and the lesson.
- Reach an evidence-based judgement about whether prion diseases represent a unique challenge to the “classification determines treatment” principle introduced in Card 1.
2. Source critique — evaluate a media article's scientific claim (Band 5–6)
6 marks Band 5–6
“Scientists are concerned that a new antibiotic-resistant strain of Candida auris (a fungal pathogen) is spreading through Australian hospitals. Health authorities are urging doctors to prescribe broad-spectrum antibiotics immediately to any patient who tests positive, and to combine this with antifungal therapy. Officials warn that Candida auris is particularly dangerous because, unlike most fungi, it is a prokaryote that divides by binary fission — allowing it to spread even faster than bacteria.”
— Hypothetical news article, illustrative of common media errors in reporting fungal outbreaks.
Q2. Evaluate the scientific accuracy of this article. Identify the claims that contain biological errors, explain what the correct biology is for each, and discuss how a clinician relying on this article could make a treatment decision that harms rather than helps a patient.
Q1 — Sample Band 6 response (8 marks), annotated
BSE is caused by a prion — a non-cellular pathogen consisting exclusively of a misfolded protein (PrPSc) with no nucleic acid. This distinguishes it sharply from bacteria (prokaryotic cellular organisms with peptidoglycan cell walls, DNA, and ribosomes), viruses (non-cellular but possessing DNA or RNA inside a protein capsid), and fungi (eukaryotic cellular organisms with chitin cell walls). [1 — classification with comparison to at least two other types]
No existing drug class has a rational target in a prion because a prion has no nucleic acid, no cell wall, no ribosomes and no metabolic enzymes of its own. Antibiotics (e.g. amoxicillin) target bacterial peptidoglycan synthesis or 70S ribosomes — neither present in a misfolded protein. Antivirals (e.g. oseltamivir, reverse transcriptase inhibitors) block viral enzymes or surface proteins — prions possess neither enzymes nor nucleic acid. Antifungals (e.g. fluconazole) target ergosterol in fungal membranes — prions have no membrane. Anthelmintics (e.g. mebendazole) disrupt tubulin polymerisation in worm cells — a prion is not a cell. The PrPSc protein itself is essentially a modified version of a normal host protein, so it is extremely difficult to design a drug that selectively destroys the misfolded form without harming the normal form in host neurons. [1 per drug class correctly explained — antibiotics; antivirals; antifungals; anthelmintics — up to 4 marks for this section, marked in pairs]
Australia’s 1996 ban on ruminant-to-ruminant feeding is a scientifically justified response. If BSE cannot be treated once established in an animal or human, the only rational strategy is to interrupt transmission. The stimulus confirms that ingestion of contaminated meat-and-bone meal is the primary bovine entry route. Removing this exposure pathway directly prevents the causal event rather than attempting to target the agent after infection. The stimulus also notes that Australia has maintained BSE-free status, providing real-world evidence of effectiveness, while the global case review confirms no drug has succeeded in a clinical cure. [1 — assesses the intervention as evidence-based and rationally linked to the biology of transmission]
Prion diseases do represent a unique challenge to the “classification determines treatment” principle, but they also confirm it — in a sobering way. Classification correctly predicts that no conventional drug class will work, precisely because the prion has none of the biological structures those drugs target. The principle still holds: knowing what a prion is tells you exactly which treatments will fail. The challenge is that this knowledge yields no viable treatment option — only prevention. This does not break the principle; it extends it into the conclusion that some pathogen types can only be managed by elimination of exposure rather than by pharmacological intervention. [1 — evidence-based judgement that engages specifically with the principle and reaches a non-trivial evaluative conclusion]
Marking criteria.
- 1 mark — Classifies BSE as a prion and correctly describes its defining feature (misfolded protein, no nucleic acid) compared to at least two other pathogen types (e.g. bacteria with peptidoglycan, viruses with nucleic acid).
- 2 marks (1 per pair) — Explains why antibiotics and antivirals are ineffective, citing the specific structural targets each class requires that prions do not possess.
- 2 marks (1 per pair) — Explains why antifungals and anthelmintics are ineffective, citing the specific structural targets each class requires that prions do not possess.
- 1 mark — Assesses Australia’s agricultural intervention as scientifically justified, linking the ban directly to interrupting the confirmed transmission route (ruminant ingestion of contaminated meal) in the absence of any effective treatment.
- 1 mark — Reaches an explicit, evidence-based judgement about whether prion diseases confirm or challenge the “classification determines treatment” principle, with a nuanced conclusion that the principle holds but yields only preventive rather than pharmacological strategies.
- 1 mark — Uses precise lesson terminology throughout (prion, non-cellular, peptidoglycan, nucleic acid, ergosterol, anthelmintic, misfolded protein) and sustains a logical argument across all four criteria.
Q2 — Sample Band 6 response (6 marks)
The article contains two significant biological errors that would directly mislead a prescribing clinician.
Error 1 — Candida auris described as a prokaryote that divides by binary fission: Candida auris is a fungus, which means it is a eukaryotic organism — it has a membrane-bound nucleus, mitochondria, and other membrane-bound organelles. Fungi do not divide by binary fission; binary fission is a bacterial (prokaryotic) process. Candida species reproduce by budding (asexual reproduction). [1 — identifies the prokaryote/binary fission error; 1 — correct biology: eukaryote, budding]
Error 2 — Recommending broad-spectrum antibiotics for a Candida infection: Antibiotics target bacterial structures (peptidoglycan cell walls, 70S ribosomes). Fungi have chitin cell walls (not peptidoglycan) and eukaryotic 80S ribosomes; neither is susceptible to standard antibiotics. Prescribing antibiotics for a fungal infection will not treat the infection and would promote antibiotic resistance in the patient’s bacterial microbiome. The correct treatment for Candida auris is an antifungal agent (primarily echinocandins such as caspofungin, which target the fungal cell wall by inhibiting β-glucan synthesis). [1 — identifies the antibiotic recommendation error; 1 — correct biology: chitin/eukaryotic 80S ribosome, echinocandin treatment]
Clinical harm analysis: A clinician who follows the article’s advice and prescribes broad-spectrum antibiotics instead of (or before) antifungals would be giving a drug with no mechanism of action against the actual pathogen. During that delay, Candida auris could spread further in an immunocompromised patient. Additionally, the antibiotic would kill commensal bacteria in the patient’s microbiome, potentially allowing Candida to proliferate further (disrupting competitive exclusion). The clinical harm is therefore two-fold: ineffective primary treatment, and disruption of natural microbial defences. [1 — clinical harm analysis linking both errors to a concrete patient outcome; 1 — explicitly connects the antibiotic-resistance and microbiome-disruption consequences]
Marking criteria.
- 1 mark — Identifies Error 1: the article wrongly describes Candida auris as a prokaryote that divides by binary fission.
- 1 mark — Corrects Error 1: Candida auris is a eukaryotic fungus that reproduces by budding.
- 1 mark — Identifies Error 2: the article recommends antibiotics for a fungal infection.
- 1 mark — Corrects Error 2: antibiotics target peptidoglycan / bacterial ribosomes, absent in fungi; antifungals (e.g. echinocandins) are the correct treatment.
- 1 mark — Discusses clinical harm: treatment delay from ineffective drug; may worsen outcome in immunocompromised patients.
- 1 mark — Extends the harm analysis: antibiotic use disrupts the bacterial microbiome / promotes resistance; demonstrates the real-world consequence of misclassification.