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

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.

Worked example

Why did it take decades to confirm Helicobacter pylori as the cause of peptic ulcers?

In the 1980s Barry Marshall hypothesised that ulcers were caused by H. pylori, against the consensus that stress and acid were the cause. To satisfy postulate 3, Marshall drank a culture of H. pylori, developed gastritis, and recovered the bacterium from his own stomach. This dramatic application of Koch's postulates earned a Nobel Prize in 2005 and revolutionised peptic ulcer treatment from antacids to antibiotics.

Common traps

Stating only two or three postulates. All four are required for full marks. Memorise them in order.

Confusing the two scientists. Pasteur disproved spontaneous generation and developed vaccines. Koch isolated specific pathogens and gave us the postulates and culture techniques.

Saying Pasteur's vaccines used killed pathogens. Pasteur pioneered attenuated (weakened) vaccines. Killed-pathogen vaccines came later.

Failing to evaluate. "Evaluate" questions require a judgement. State whether the impact was large or limited, and justify with examples (modern antibiotics, vaccination, sterile surgery, epidemiology).

In one sentence

Pasteur's swan-neck flask experiment disproved spontaneous generation and established germ theory, while Koch's work on anthrax and tuberculosis and his four postulates provided the experimental framework that links specific microbes to specific diseases, founding modern medical microbiology.