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NSWBiologySyllabus dot point

Inquiry Question 1: How are diseases transmitted?

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

A focused answer to the HSC Biology Module 7 dot point on Pasteur and Koch. Covers Pasteur's swan-neck flask experiment, Koch's anthrax and tuberculosis work, the four Koch's postulates, and the impact of germ theory on modern medicine.

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

NESA wants you to describe the experimental work of Pasteur and Koch, list Koch's four postulates accurately, and evaluate the impact of germ theory on modern medicine. This is a high-value dot point that appears in 6 to 9 mark extended response questions.

The answer

Before the 1860s, most physicians believed disease was caused by miasma (bad air) or spontaneous generation. The work of Louis Pasteur and Robert Koch established germ theory, the principle that specific microorganisms cause specific diseases.

Pasteur's work

Swan-neck flask experiment (1859)
Pasteur boiled nutrient broth in glass flasks with long, curved necks. The broth remained sterile indefinitely because airborne microbes settled in the curve of the neck before reaching the liquid. When he broke the necks or tilted the flasks so that broth contacted the trapped microbes, the broth quickly grew cloudy with microbial growth.
Conclusion
Life does not arise spontaneously. Microorganisms in broth come from other microorganisms in the air. This disproved spontaneous generation and supported germ theory.
Vaccines
Pasteur developed attenuated (weakened) vaccines for chicken cholera (1879), anthrax (1881) and rabies (1885), founding modern immunisation.
Pasteurisation
He showed that gentle heating of wine, beer and milk killed spoilage microbes without destroying the product. Pasteurisation of milk dramatically reduced food-borne tuberculosis.

Koch's work

Anthrax (1876)
Koch isolated Bacillus anthracis from infected sheep, cultured it on the cut surface of a potato, injected the pure culture into healthy mice, observed identical disease, and re-isolated the same bacterium. This was the first time a specific microbe was definitively linked to a specific disease.
Tuberculosis (1882) and cholera (1883)
Koch identified Mycobacterium tuberculosis and Vibrio cholerae, developing acid-fast staining to visualise the slow-growing tuberculosis bacterium.
Techniques
Koch's lab developed solid agar plating (suggested by Fanny Hesse), pure culture isolation, and improved staining methods. These techniques remain standard in microbiology laboratories.

Koch's postulates

Koch's four criteria for proving that a specific microbe causes a specific disease:

  1. The microorganism must be present in every case of the disease and absent from healthy hosts.
  2. The microorganism must be isolated from a diseased host and grown in pure culture.
  3. The cultured microorganism must reproduce the disease when introduced into a healthy susceptible host.
  4. The microorganism must be re-isolated from the experimentally infected host and shown to be identical to the original.

Limitations of the postulates

Asymptomatic carriers
Some pathogens (Salmonella Typhi, Mycobacterium tuberculosis) are present in healthy carriers, breaking postulate 1.
Unculturable pathogens
Viruses cannot be grown without host cells, and many bacteria (e.g. Treponema pallidum, the syphilis pathogen) are difficult to culture. This breaks postulate 2.
Ethics
Postulate 3 requires deliberately infecting a healthy host, which is not ethical in humans. Animal models, organoids and molecular Koch's postulates (linking specific genes to disease) now supplement the originals.
Multiple pathogens or host factors
Some diseases require co-infection or specific host susceptibilities.

Examples in context

Example 1. Pasteur and the Australian wine industry. Although best known for medicine, Pasteur's 1857 work was originally commissioned by French winemakers worried about wine spoilage. Pasteur showed that fermentation was caused by living yeast (Saccharomyces cerevisiae) and spoilage by contaminating bacteria, and that gentle heating ("pasteurisation") at 60 degrees C for 30 minutes killed spoilage bacteria without destroying flavour. Today every commercial Hunter Valley winery uses descendants of Pasteur's pasteurisation principles to stabilise wine before bottling, and Dairy Australia mandates pasteurisation of all retail milk under the Food Standards Code. The Pasteur Institute's continuing influence is visible in microbiology labs across Australia, including the National Centre for Immunisation Research at Westmead.

Example 2. Applying Koch's postulates to Helicobacter pylori and peptic ulcers. Until 1982, peptic ulcers in Australian patients were attributed to stress and spicy food. Perth pathologists Barry Marshall and Robin Warren proposed that the bacterium Helicobacter pylori caused ulcers. To satisfy Koch's postulates, they (1) isolated H. pylori from biopsies of ulcer patients, (2) cultured it on chocolate agar, and (3) needed to show it caused disease in a healthy host. Marshall famously drank a flask of cultured H. pylori, developed gastritis within days, then cultured the bacterium from his own gastric biopsy. Antibiotic treatment cured him. Marshall and Warren received the 2005 Nobel Prize, and antibiotic ulcer treatment is now standard in Australian general practice.

Try this

Q1. State Koch's four postulates and explain why each is necessary to establish a microbe as a cause of disease. [4 marks]

  • Cue. (1) Microbe present in all diseased hosts not healthy. (2) Isolated in pure culture. (3) Causes the same disease when introduced into a healthy host. (4) Re-isolated from the new host.

Q2. A new respiratory pathogen is suspected in a NSW outbreak. Describe how an Australian Communicable Diseases Intelligence team would apply a modified version of Koch's postulates to identify the pathogen, given that the suspect organism cannot yet be cultured. [3 marks]

  • Cue. Use molecular Koch's postulates: detect pathogen DNA/RNA in patients but not controls, sequence it, develop diagnostic test, demonstrate causation via animal model or natural history.

Q3. Evaluate the impact of Pasteur and Koch on modern medicine. (a) Identify one specific contribution of each. (b) Describe one limitation of Koch's postulates. (c) Justify whether their work or that of contemporary molecular microbiologists has had greater impact. [2+2+3 marks]

  • Cue. (a) Pasteur: germ theory, pasteurisation, vaccines. Koch: pure culture, postulates, agar. (b) Some pathogens cannot be cultured; carriers exist without disease. (c) A reasoned judgement linking historical foundations to modern molecular detection.

Exam-style practice questions

Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2024 HSC5 marksMilk pasteurisation (heating to approximately 70°C) was gradually introduced in America from the early 1900s. A graph shows the number of disease outbreaks related to raw (unpasteurised) and pasteurised milk in America from 1900–1975 (raw-milk outbreaks fall sharply, especially after 1945; pasteurised-milk outbreaks stay low). Explain the trends observed in the graph. In your response, refer to the role of Pasteur's work in pasteurisation.
Show worked answer →

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.

2021 HSC2 marksA scientist followed Koch's postulates to confirm a bacterium was causing diarrhoea in pigs. A flowchart showed: 'Bacteria isolated from a pig with diarrhoea' → [box] → 'Bacteria fed to a healthy pig (Pig 2)' → [box] → 'Bacteria isolated from Pig 2 are compared with bacteria from Pig 1'. Complete the two empty boxes to show the steps taken by the scientist.
Show worked answer →

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.

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