Koch and Pasteur — Germ Theory

Q1 — Sample Band 6 response (8 marks), annotated

Definitions and key difference: Germ theory proposes that infectious diseases are caused by specific microorganisms (pathogens) that are transmitted from an external source into a susceptible host. Miasma theory proposed that disease arose from exposure to bad air generated by decaying organic matter — the environment caused disease without an external living agent. The key difference is whether disease requires an identifiable, transmissible living agent (germ theory) or arises from environmental conditions alone (miasma theory). [1 — both defined with key difference identified]

What Snow's evidence established and its limitation: Snow's epidemiological mapping demonstrated a strong spatial association between cholera cases and proximity to the Broad Street pump — establishing correlation. His intervention (removing the pump handle) reduced cases, suggesting a causal link between the water source and disease. However, Snow's evidence was epidemiological, not experimental: he identified a probable route of transmission (contaminated water) but could not identify a specific causative agent, could not exclude that the pump water carried some non-living toxic substance consistent with miasma theory, and could not reproduce the disease in a controlled experiment. This is why the medical committee could still attribute the pattern to miasma — the pump water could plausibly have been the source of "bad air" rather than a living pathogen. [1 — what Snow established (epidemiological association); 1 — specific limitation (no causative agent identified; correlation without experimental causation)]

How Koch's postulates addressed the limitation: In 1883, Koch isolated Vibrio cholerae from cholera patients in Egypt and India, grew it in pure culture, and confirmed through postulates 1–4 that the bacterium was consistently present in all diseased individuals, absent from healthy ones, capable of causing cholera-like disease when introduced into suitable hosts, and re-isolatable from newly diseased hosts. This provided what Snow's epidemiology could not: a specific, identified living agent whose role had been confirmed by controlled experimental manipulation. Koch's approach moved the evidence for cholera from "water source associated with disease" to "this specific bacterium, transmitted via contaminated water, causes this specific disease." [1 — Koch's postulates applied to V. cholerae; 1 — how they moved beyond Snow's epidemiology (specific agent, experimental causation)]

Why miasma remained credible after Snow: Miasma theory was not irrational in the face of Snow's evidence because Snow's pattern was consistent with miasma — pump water could have carried bad air as easily as a living agent. Without a specific identifiable agent and a reproducible experimental demonstration of causation, miasma theory could accommodate Snow's findings. Koch's evidence was more decisive because it identified the causative agent by name, demonstrated that the organism alone — not contaminated water or environmental conditions — could produce the disease in a healthy host, and confirmed this through four independent experimental steps that could be repeated. [1 — why miasma remained credible (Snow's evidence compatible with miasma); 1 — what made Koch's evidence decisive (specificity, experimental causation, reproducibility)]

Judgement: Koch's investigation made the greater contribution to replacing miasma theory because it provided the first logically complete and experimentally reproducible proof that a specific living microorganism caused a specific disease — exactly what miasma theory was incapable of accommodating. Snow's contribution was essential as epidemiological groundwork — it identified the route of transmission and the likely environmental source — but it could be reinterpreted within miasma theory's framework. Koch's could not. [1 — evidence-based judgement with comparative reasoning]

Marking criteria:

• 1 mark — Defines both germ theory and miasma theory and identifies the key explanatory difference (living transmissible agent vs environmental bad air).
• 1 mark — Correctly describes what Snow's investigation established (epidemiological association between the Broad Street pump and cholera cases; removal of handle reduced cases).
• 1 mark — Identifies a specific limitation of Snow's evidence (no causative agent identified; result compatible with miasma interpretation).
• 1 mark — Correctly explains how Koch's postulates applied to Vibrio cholerae addressed that limitation (isolation, pure culture, causation in healthy host, re-isolation).
• 1 mark — Explains what Koch's approach provided beyond Snow's epidemiology (specific living agent with experimental proof of causation).
• 1 mark — Explains why miasma theory retained credibility after Snow's work (epidemiological data compatible with miasma interpretation; no identified living agent).
• 1 mark — Explains specifically what made Koch's evidence decisive (named causative agent; experimental reproducibility; four independent criteria satisfied).
• 1 mark — Reaches an explicit, evidence-based comparative judgement about which investigation made the greater contribution, with reasoning.

Q2 — Sample Band 6 response (7 marks), annotated

Overall judgement: The claim contains a defensible insight but is substantially overstated and reaches a false conclusion about how disease causation is established. [1 — evaluative judgement stated upfront]

What is defensible: DNA sequencing has transformed the speed and sensitivity with which pathogens can be detected. In cases where a pathogen cannot be cultured — such as viruses requiring living host cells, or pathogens like Treponema pallidum (syphilis) that cannot be grown reliably outside a host — sequencing can identify the organism's presence when Koch's postulates' pure-culture step is impossible. This is a genuine advance that addresses a recognised limitation of the original postulates. [1 — identifies the defensible element (sequencing overcomes culture limitations)]

What is wrong — "postulates are completely obsolete": Koch's postulates are not obsolete — the logical framework they establish remains foundational. The requirement to show that an organism is present in all diseased hosts, absent from healthy ones, capable of causing disease in a healthy host, and re-isolatable (or its molecular equivalent) is still the standard for establishing causation. Modern approaches use "molecular Koch's postulates," which apply the same logic using genome sequences instead of pure cultures. The logic has not been abandoned; the methods of applying it have evolved. [1 — correctly refutes "completely obsolete"]

What is wrong — "if you find a match, that's your cause": Finding a pathogen's genome in a sick patient's sample establishes only association (correlation), not causation. This is exactly the mistake Koch's postulates were designed to prevent. A genome sequence match shows that a particular organism's DNA is present — it does not show that the organism is active, that it produced the disease rather than being a commensal or opportunistic secondary infection, or that introducing the organism alone into a healthy host would produce the same disease. For example, the human gut contains hundreds of bacterial species whose genomes would be found in patients with many different diseases — genome presence alone cannot identify which is the cause. [1 — refutes "genome match = cause" with reasoning; 1 — correctly applies correlation vs causation distinction]

What is wrong — "any pathogen can be identified by sequencing": The claim overstates the universality of sequencing for identifying causative agents. For pathogens with very similar genome sequences to commensal organisms, or for novel pathogens with no database entry, sequencing alone cannot provide identification. Functional and epidemiological evidence is still required to determine causation. [1 — refutes universality of sequencing claim]

Defensible reformulation: Koch's postulates remain logically foundational for establishing that a specific pathogen causes a specific disease. Modern molecular techniques, particularly DNA sequencing, extend the application of Koch's logic to pathogens that cannot be cultured, but the underlying requirement — to demonstrate consistent presence in diseased hosts, absence in healthy hosts, causation in a healthy host, and reproducible recovery of the agent — has not been abandoned. Finding a genome match in a sick patient establishes correlation only; causation requires additional experimental evidence, just as Koch's original postulates required. [1 — defensible reformulation correctly maintains the causation framework and relationship to molecular approaches]

Marking criteria:

• 1 mark — States an overall evaluative judgement (e.g. "contains a defensible insight but is substantially overstated").
• 1 mark — Correctly identifies the defensible element: sequencing overcomes the pure-culture limitation for unculturable pathogens.
• 1 mark — Correctly refutes "completely obsolete": Koch's postulates' logical framework persists in molecular Koch's postulates; logic has not changed, only methodology.
• 1 mark — Correctly refutes "genome match = cause": genome presence establishes association only, not causation; commensal organisms would also match.
• 1 mark — Applies correlation-versus-causation reasoning explicitly to the genome-sequencing claim.
• 1 mark — Correctly refutes the universality claim or addresses a further flaw (novel pathogens without database entries; functional evidence still required).
• 1 mark — Reformulates the claim into a biologically defensible statement that correctly represents the relationship between molecular methods and Koch's original framework.