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Home›Biology›Microbes

How germs were discovered

In 1668 Francesco Redi put meat in six jars, left two open, covered two with gauze and sealed two, and only the open jars grew maggots in the meat.

Life from nothing?

For a very long time people believed that living things could arise from non-living matter. Aristotle wrote about fish appearing in new puddles, and in the 1600s the Flemish scientist Jan Baptista van Helmont offered a recipe for mice: rags and wheat kernels left in an open container for three weeks. In reality the rags and grain were simply good food and shelter for mice. This idea is called spontaneous generation.

Experiments slowly took the idea apart. Francesco Redi’s jars of 1668 showed that maggots come from fly eggs, not from meat. Microbes were harder. In 1745 John Needham boiled broth briefly, sealed it, and saw it turn cloudy with microbes, which he took as proof of spontaneous generation; he probably did not boil it long enough. Lazzaro Spallanzani ran hundreds of careful experiments in which heated, sealed flasks stayed clear unless they were opened to the air. Needham replied that the long boiling had destroyed a “life force”.

BackgroundThe swan-neck flasks. The Paris Academy of Sciences offered a prize to settle the question. Louis Pasteur, a chemist who had been studying fermentation and wine spoilage, took it on. In 1858 he drew air through a gun-cotton filter and found the cotton full of microorganisms. Then he made flasks with long, bent necks, boiled broth in them and left them open. Air could move in and out, so any “life force” could reach the broth, but dust and microbes settled in the bends of the neck. The broth stayed sterile as long as the neck was intact, and spoiled soon after the neck was broken off. Pasteur won the Alhumbert Prize in 1862, and in a lecture in 1864 he said spontaneous generation would never recover from the blow of this simple experiment.
Pasteur’s experiment (simplified). A swan-neck flask: air gets in, but dust and microbes are trapped in the bend, so the boiled broth stays clear. B the same flask with its neck broken off: microbes fall in and grow.

From spoilage to disease

If microbes spoiled food, could they also cause disease? Several people found evidence before anyone could name the microbes. In 1847 the Hungarian obstetrician Ignaz Semmelweis noticed that mothers in hospital wards run by doctors and medical students, who came straight from autopsies, died of fever after childbirth at a rate of 10 to 20 percent, compared with about 1 percent in wards run by midwives. When he had the doctors wash their hands in chlorinated lime water, the death rate fell to the midwives’ level. Many doctors still ignored him.

In London, the physician John Snow thought cholera spread through water, not through bad air. When cholera broke out around Broad Street in Soho on 31 August 1854, he checked death records and talked to families, and found that most of the dead had lived near the Broad Street pump and drunk from it. He took his findings to the local leaders, and the pump handle was removed on 8 September 1854. A CDC report on the 150th anniversary calls his study a model for modern epidemiology. The cholera bacterium itself, Vibrio cholerae, was isolated only in 1883.

In 1867 the British surgeon Joseph Lister, who knew the work of Semmelweis and Pasteur, began spraying carbolic acid during operations, and wound infections after surgery fell sharply. Robert Koch then set out a series of postulates for showing that a particular microbe causes a particular disease, and with them he and his colleagues identified the causes of anthrax, tuberculosis and cholera. Long before any of this, in 1796, the English doctor Edward Jenner had shown that people who had caught cowpox were protected from smallpox, the start of vaccination. On 8 May 1980 the 33rd World Health Assembly declared the world free of smallpox.

  • 1668: Redi: maggots come from flies, not meat.
  • 1745: Needham’s cloudy broth.
  • 1796: Jenner: cowpox protects against smallpox.
  • 1847: Semmelweis: handwashing cuts deaths after childbirth.
  • 1854: Snow: the Broad Street pump handle is removed.
  • 1862: Pasteur wins the Alhumbert Prize.
  • 1867: Lister uses carbolic acid in surgery.
  • 1883: Vibrio cholerae is isolated.
  • 1980: Smallpox declared eradicated.
This unit is history. It describes what these scientists did and found; it does not give medical advice.
Key points
  • Spontaneous generation: life from non-living matter, believed from Aristotle on.
  • Redi (1668), Spallanzani and Pasteur’s swan-neck flasks (prize 1862) disproved it.
  • Semmelweis (1847) and Snow (1854) showed disease spreads by hands and water.
  • Lister (1867), Koch’s postulates, Jenner (1796); smallpox declared gone in 1980.
  1. What does “spontaneous generation” mean?
    • Living things arising from non-living matter
    • Cells dividing into two
    • Plants making food from light
    • Viruses infecting bacteria
  2. In Redi’s experiment, which jars had maggots in the meat?
    • Only the open jars
    • Only the sealed jars
    • All six jars
    • Only the gauze-covered jars
  3. Why did Pasteur’s broth stay clear in the swan-neck flasks?
    • Microbes were trapped in the bent neck while air could still get in
    • The flasks were sealed airtight
    • The broth contained poison
    • There was no air
  4. Redi’s experiment was in 1668 and Pasteur won the Alhumbert Prize in 1862. How many years apart?
  5. What did Semmelweis ask doctors to do?
    • Wash their hands in chlorinated lime water between patients
    • Boil all broth
    • Remove a pump handle
    • Spray carbolic acid
  6. The 1854 Broad Street outbreak began on 31 August and the pump handle came off on 8 September. How many days later?
  7. What did John Snow conclude about cholera?
    • It spread through contaminated water from a pump
    • It spread through bad air
    • It was caused by a virus in the air
    • It came from meat
  8. What did Joseph Lister begin using in 1867?
    • Carbolic acid spray during surgery
    • Penicillin
    • Swan-neck flasks
    • Electron microscopes
  9. Jenner showed the protection of cowpox in 1796, and smallpox was declared gone in 1980. How many years apart?
  10. Put these events in time order.
Sources for this course (15)
  • OpenStax Microbiology 1.1 What Our Ancestors Knew (fermented drink from rice, honey and fruit in Neolithic China by 7000 BC, Saccharomyces cerevisiae in bread, Leeuwenhoek 1675 "animalcules" in rain water, Golden Age of Microbiology 1857 to 1914; openstax.org/books/microbiology/pages/1-1-what-our-ancestors-knew)
  • 1.3 Types of Microorganisms (about 100 µm to see with the eye, animal cell about 10 µm, bacteria about 1 µm, viruses about 100 nm, three domains, coccus/bacillus/spirillum, archaea without peptidoglycan and in extreme places such as Morning Glory pool, protists, algae, diatoms 2 to 200 µm, protozoa with cilia, flagella or pseudopods, fungi with chitin walls, more than 1000 yeast species, molds and penicillin, helminths, prions about one-tenth of a virus; .../1-3-types-of-microorganisms)
  • 3.1 Spontaneous Generation (Aristotle, van Helmont's mice recipe of 3 weeks, Redi 1668 with six jars, Needham 1745, Spallanzani, Pasteur 1858 gun-cotton filter, swan-neck flasks, Alhumbert Prize 1862, 1864 lecture; .../3-1-spontaneous-generation)
  • 3.2 Foundations of Modern Cell Theory (Semmelweis 1847, 10 to 20 percent vs 1 percent, chlorinated lime; Snow; Pasteur 1856 fermentation; Lister 1867 carbolic acid; Koch's postulates; .../3-2-foundations-of-modern-cell-theory)
  • 6.1 Viruses (acellular, obligate intracellular parasites, DNA or RNA never both, capsid, envelope, spikes, helical/polyhedral/complex, 20 nm to 900 nm, Ivanovski 1892 with the Chamberland filter of 1884, pore 0.1 µm, Beijerinck 1898 and virus = poison, Stanley 1935, host range, bacteriophages, FDA 2006 phage spray on lunch meats; .../6-1-viruses)
  • 9.1 How Microbes Grow (binary fission, FtsZ Z ring, generation time, E. coli 20 minutes, human generation 25 years, M. tuberculosis 15 to 20 hours, M. leprae 14 days, 2^48 = 281,474,976,710,656 after 24 h at 30 minutes, Nn = N0 2^n, lag/log/stationary/death, budding, fragmentation, Epulopiscium; .../9-1-how-microbes-grow)
  • Royal Society: Philosophical Transactions 12:821-831, published 25 March 1677, Leeuwenhoek's Dutch letter of 9 October 1676 "concerning little animals" (doi.org/10.1098/rstl.1677.0003)
  • Royal Society archive EL/L1/22 (letter to Oldenburg dated at Delft, 18 pages, partial translation by Oldenburg)
  • Royal Society blog "Celebrating Leeuwenhoek", 2023 (first letter via de Graaf, Phil. Trans. 19 May 1673; 190 letters in fifty years; died 26 August 1723 aged 90)
  • N. Lane 2015, Phil. Trans. R. Soc. B 370:20140344 (upwards of 1,000,000 creatures in a drop of pepper-water; letter read aloud over several sessions)
  • Lens on Leeuwenhoek, letter L-040 (Oldenburg printed less than half; Hooke succeeded later in 1677)
  • CDC MMWR 53(34):783, 2004, 150th Anniversary of John Snow and the Pump Handle (31 August 1854, handle removed 8 September 1854, Vibrio cholerae isolated 1883; cdc.gov/mmwr/preview/mmwrhtml/mm5334a1.htm)
  • CDC History of Smallpox and Smallpox timeline (Jenner 1796; 33rd World Health Assembly declared the world free of smallpox on 8 May 1980; cdc.gov/smallpox/about/history.html)
  • American Chemical Society, Discovery and Development of Penicillin landmark (Fleming 1928 at St Mary's Hospital London, paper June 1929, Florey and Chain at Oxford from 1939; acs.org/education/whatischemistry/landmarks/flemingpenicillin.html)
  • Fleming 1929, Br J Exp Pathol 10:226. Sender, Fuchs and Milo 2016, PLoS Biol 14:e1002533 (3.8 × 10^13 bacteria and 3.0 × 10^13 human cells in a 70 kg reference man, ratio about 1.3, bacteria about 0.2 kg).

Checked: 2026-10-17