Investigate the work of Pasteur and Koch and evaluate the impact of their work on the understanding of infectious disease, including Koch's postulates

Full marks (5) need a thorough link between Pasteur's work and the graphed trends. Key points from the guidelines:

• Pasteur's work disproved spontaneous generation, showing microbes (not the milk itself) cause disease/spoilage — so organisms in milk can cause the outbreaks.
• Pasteur showed heating kills microbes; heating milk to ~70°C kills many bacteria, so pasteurised milk causes fewer outbreaks.
• Trend: outbreaks from pasteurised milk are generally much lower than from raw milk, and raw-milk outbreaks decreased significantly after 1945 (also partly because fewer people drank raw milk).
• Remaining pasteurised-milk outbreaks may be due to problems during pasteurisation, storage or transport.

Marker feedback: link Pasteur's work directly to the trends, use specific data, and explain both trends (raw and pasteurised), not just one.

Source: NESA 2024 HSC Biology examination and marking guidelines.

2 marks for completing both boxes correctly with the matching Koch's-postulate steps. Box 1 (after isolating bacteria from the sick pig): the isolated bacteria are grown/cultured in pure culture. Box 2 (after the cultured bacteria are fed to the healthy Pig 2): Pig 2 develops the same disease (diarrhoea). This satisfies Koch's postulates: the microbe is found in the diseased animal, isolated and cultured, causes the same disease when introduced to a healthy animal, then is re-isolated and shown to be the same microbe. One mark for some relevant information (one box correct). Marker note: apply biological knowledge to the specific situation in the flowchart. Source: NESA 2021 HSC Biology examination and marking guidelines.

1 mark - the conclusion. Pasteur concluded that microorganisms in the broth come from other microorganisms in the air, not from the broth itself; life does not arise from non-living matter.

1 mark - the idea disproved. This disproved spontaneous generation (the belief that living things arise spontaneously from non-living material), and supported germ theory.

The mark hinges on naming spontaneous generation AND the correct conclusion. An answer that only describes the flask without stating what it proved caps at 1 mark.

1 mark - postulate 1

The microorganism must be present in every case of the disease and absent from healthy hosts.

1 mark - postulates 2 and 3

The microorganism must be isolated from a diseased host and grown in pure culture, and the cultured microorganism must reproduce the disease when introduced into a healthy susceptible host.

1 mark - postulate 4

The microorganism must be re-isolated from the experimentally infected host and shown to be identical to the original.

All four steps, in the correct order, are required. A response giving only two or three postulates cannot reach full marks.

1 mark - Pasteur. Pasteur disproved spontaneous generation (swan-neck flask), and developed pasteurisation and attenuated vaccines, establishing that microbes cause disease and spoilage.

1 mark - Koch. Koch linked specific microbes to specific diseases (anthrax, tuberculosis), developed pure-culture and staining techniques, and formulated Koch's postulates.

The discriminator is general germ theory / vaccines (Pasteur) versus specific-microbe-to-specific-disease proof and the postulates (Koch). Blurring the two caps at 1 mark.

1 mark - found in the diseased host

Koch observed and isolated Bacillus anthracis from sheep that had died of anthrax (the microbe was present in the diseased animal).

1 mark - pure culture

He grew the bacterium in pure culture (on the cut surface of a potato), free of other organisms.

1 mark - reproduce the disease

He injected the pure culture into healthy mice, which then developed the same disease (anthrax).

1 mark - re-isolation

He re-isolated the same bacterium from the experimentally infected mice, identical to the original.

This sequence is Koch's postulates applied in practice; it was the first definitive link of a specific microbe to a specific disease. Full marks need all four stages tied to anthrax, not a generic list.

1 mark - the setup

Pasteur boiled (sterilised) nutrient broth in flasks with long curved (swan) necks left open to the air.

1 mark - the control of the variable

The curved neck let air reach the broth but trapped airborne microbes (and dust) in the bend, so the only thing excluded was the microbes, not the air itself.

1 mark - the result

The broth in intact swan-neck flasks remained sterile indefinitely, showing nothing in the air alone caused growth.

1 mark - the manipulation

When he broke the neck or tilted the flask so broth contacted the trapped microbes, the broth rapidly grew cloudy with microbial growth.

1 mark - why the design matters

Because air still reached the sterile broth, the design ruled out the counter-argument that excluding air prevented spontaneous generation; growth only appeared once microbes reached the broth, proving microbes (not the broth or air) were the source of life.

Band 6 answers explicitly identify that admitting air while excluding microbes is what isolates the variable and defeats the "vital force in air" objection.

Award up to 4 marks for two genuine limitations, each explained against the relevant postulate.

Limitation 1 - asymptomatic carriers (2 marks). Postulate 1 requires the microbe to be absent from healthy hosts, but some pathogens (e.g. Salmonella Typhi, Mycobacterium tuberculosis) are carried by healthy carriers who show no disease, so postulate 1 is broken.

Limitation 2 - unculturable pathogens (2 marks). Postulate 2 requires growth in pure culture, but viruses cannot be grown without host cells and some bacteria (e.g. Treponema pallidum) are very hard to culture, so postulate 2 cannot be satisfied for them.

Accept also: postulate 3 is unethical to apply in humans (deliberate infection), and some diseases need co-infection or host susceptibility. Each limitation must be tied to the specific postulate it breaks; a vague "they do not always work" caps at 1 mark.

"Evaluate" requires a judgement about the size and significance of the impact, supported by specific contributions and a recognition of limits. A Band 6 response reaches a reasoned conclusion, not just a list.

Their contributions (2-3 marks)

Pasteur disproved spontaneous generation (swan-neck flask), establishing germ theory; he developed pasteurisation (reducing food-borne disease such as milk-borne tuberculosis) and attenuated vaccines (chicken cholera, anthrax, rabies), founding immunisation. Koch proved that specific microbes cause specific diseases (anthrax, tuberculosis, cholera), and gave microbiology its core tools: pure culture on solid agar, staining, and Koch's postulates.

The downstream impact (2-3 marks)

Germ theory reframed medicine: it underpins antiseptic and aseptic surgery, sanitation and public health, antibiotics, modern vaccination and epidemiology. The postulates remain the logical basis for establishing causation (e.g. confirming new pathogens), and pasteurisation and vaccination still save large numbers of lives.

Judgement, including limits (1-2 marks)

A supported conclusion: their impact was transformative and foundational - it shifted medicine from miasma theory to a microbial, evidence-based science. Acknowledge limits: the postulates do not fit every pathogen (viruses, carriers, unculturable microbes), so modern molecular methods extend rather than replace them. An answer that lists achievements without an explicit, justified judgement, or that ignores the limitations, caps below full marks.

Target a sequenced response that recognises which postulate fails and applies modern (molecular) Koch's postulates.

Identify the problem (1-2 marks)

The bacterium grows poorly, so postulate 2 (isolation in pure culture) - and therefore postulates 3 and 4 that depend on it - cannot be reliably met by classical methods alone.

Molecular evidence of association (2-3 marks)

Use molecular Koch's postulates: detect the pathogen's DNA/RNA (e.g. by PCR or sequencing) in diseased patients but not in healthy controls, show that the amount of pathogen correlates with disease, and identify the genes responsible for virulence.

Demonstrate causation and reversibility (1-2 marks)

Reproduce the disease in an animal model or organoid/cell-culture system, show that removing or inactivating the virulence gene removes the disease, and confirm treatment (e.g. antibiotics) resolves the illness - mirroring the logic of postulates 3 and 4 without classical pure culture.

Full marks need: the specific postulate that fails, a molecular detection-and-association step, and a causation/reversibility step. A purely classical answer that ignores the culture problem cannot reach the top band.