steps of pasteur's experiment

3.1 Spontaneous Generation

Learning objectives.

By the end of this section, you will be able to:

• Explain the theory of spontaneous generation and why people once accepted it as an explanation for the existence of certain types of organisms
• Explain how certain individuals (van Helmont, Redi, Needham, Spallanzani, and Pasteur) tried to prove or disprove spontaneous generation

Clinical Focus

Barbara is a 19-year-old college student living in the dormitory. In January, she came down with a sore throat, headache, mild fever, chills, and a violent but unproductive (i.e., no mucus) cough. To treat these symptoms, Barbara began taking an over-the-counter cold medication, which did not seem to work. In fact, over the next few days, while some of Barbara’s symptoms began to resolve, her cough and fever persisted, and she felt very tired and weak.

• What types of respiratory disease may be responsible?

Jump to the next Clinical Focus box

Humans have been asking for millennia: Where does new life come from? Religion, philosophy, and science have all wrestled with this question. One of the oldest explanations was the theory of spontaneous generation, which can be traced back to the ancient Greeks and was widely accepted through the Middle Ages.

The Theory of Spontaneous Generation

The Greek philosopher Aristotle (384–322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation , the notion that life can arise from nonliving matter. Aristotle proposed that life arose from nonliving material if the material contained pneuma (“spirit” or “breath”). As evidence, he noted several instances of the appearance of animals from environments previously devoid of such animals, such as the seemingly sudden appearance of fish in a new puddle of water. 1

This theory persisted into the 17th century, when scientists undertook additional experimentation to support or disprove it. By this time, the proponents of the theory cited how frogs simply seem to appear along the muddy banks of the Nile River in Egypt during the annual flooding. Others observed that mice simply appeared among grain stored in barns with thatched roofs. When the roof leaked and the grain molded, mice appeared. Jan Baptista van Helmont , a 17th century Flemish scientist, proposed that mice could arise from rags and wheat kernels left in an open container for 3 weeks. In reality, such habitats provided ideal food sources and shelter for mouse populations to flourish.

However, one of van Helmont’s contemporaries, Italian physician Francesco Redi (1626–1697), performed an experiment in 1668 that was one of the first to refute the idea that maggots (the larvae of flies) spontaneously generate on meat left out in the open air. He predicted that preventing flies from having direct contact with the meat would also prevent the appearance of maggots. Redi left meat in each of six containers ( Figure 3.2 ). Two were open to the air, two were covered with gauze, and two were tightly sealed. His hypothesis was supported when maggots developed in the uncovered jars, but no maggots appeared in either the gauze-covered or the tightly sealed jars. He concluded that maggots could only form when flies were allowed to lay eggs in the meat, and that the maggots were the offspring of flies, not the product of spontaneous generation.

In 1745, John Needham (1713–1781) published a report of his own experiments, in which he briefly boiled broth infused with plant or animal matter, hoping to kill all preexisting microbes. 2 He then sealed the flasks. After a few days, Needham observed that the broth had become cloudy and a single drop contained numerous microscopic creatures. He argued that the new microbes must have arisen spontaneously. In reality, however, he likely did not boil the broth enough to kill all preexisting microbes.

Lazzaro Spallanzani (1729–1799) did not agree with Needham’s conclusions, however, and performed hundreds of carefully executed experiments using heated broth. 3 As in Needham’s experiment, broth in sealed jars and unsealed jars was infused with plant and animal matter. Spallanzani’s results contradicted the findings of Needham: Heated but sealed flasks remained clear, without any signs of spontaneous growth, unless the flasks were subsequently opened to the air. This suggested that microbes were introduced into these flasks from the air. In response to Spallanzani’s findings, Needham argued that life originates from a “life force” that was destroyed during Spallanzani’s extended boiling. Any subsequent sealing of the flasks then prevented new life force from entering and causing spontaneous generation ( Figure 3.3 ).

Check Your Understanding

• Describe the theory of spontaneous generation and some of the arguments used to support it.
• Explain how the experiments of Redi and Spallanzani challenged the theory of spontaneous generation.

Disproving Spontaneous Generation

The debate over spontaneous generation continued well into the 19th century, with scientists serving as proponents of both sides. To settle the debate, the Paris Academy of Sciences offered a prize for resolution of the problem. Louis Pasteur , a prominent French chemist who had been studying microbial fermentation and the causes of wine spoilage, accepted the challenge. In 1858, Pasteur filtered air through a gun-cotton filter and, upon microscopic examination of the cotton, found it full of microorganisms, suggesting that the exposure of a broth to air was not introducing a “life force” to the broth but rather airborne microorganisms.

Later, Pasteur made a series of flasks with long, twisted necks (“swan-neck” flasks), in which he boiled broth to sterilize it ( Figure 3.4 ). His design allowed air inside the flasks to be exchanged with air from the outside, but prevented the introduction of any airborne microorganisms, which would get caught in the twists and bends of the flasks’ necks. If a life force besides the airborne microorganisms were responsible for microbial growth within the sterilized flasks, it would have access to the broth, whereas the microorganisms would not. He correctly predicted that sterilized broth in his swan-neck flasks would remain sterile as long as the swan necks remained intact. However, should the necks be broken, microorganisms would be introduced, contaminating the flasks and allowing microbial growth within the broth.

Pasteur’s set of experiments irrefutably disproved the theory of spontaneous generation and earned him the prestigious Alhumbert Prize from the Paris Academy of Sciences in 1862. In a subsequent lecture in 1864, Pasteur articulated “ Omne vivum ex vivo ” (“Life only comes from life”). In this lecture, Pasteur recounted his famous swan-neck flask experiment, stating that “…life is a germ and a germ is life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.” 4 To Pasteur’s credit, it never has.

• How did Pasteur’s experimental design allow air, but not microbes, to enter, and why was this important?
• What was the control group in Pasteur’s experiment and what did it show?
• 1 K. Zwier. “Aristotle on Spontaneous Generation.” http://www.sju.edu/int/academics/cas/resources/gppc/pdf/Karen%20R.%20Zwier.pdf
• 2 E. Capanna. “Lazzaro Spallanzani: At the Roots of Modern Biology.” Journal of Experimental Zoology 285 no. 3 (1999):178–196.
• 3 R. Mancini, M. Nigro, G. Ippolito. “Lazzaro Spallanzani and His Refutation of the Theory of Spontaneous Generation.” Le Infezioni in Medicina 15 no. 3 (2007):199–206.
• 4 R. Vallery-Radot. The Life of Pasteur , trans. R.L. Devonshire. New York: McClure, Phillips and Co, 1902, 1:142.

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• Book title: Microbiology
• Publication date: Nov 1, 2016
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• Book URL: https://openstax.org/books/microbiology/pages/1-introduction
• Section URL: https://openstax.org/books/microbiology/pages/3-1-spontaneous-generation

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• The Theory of Biogenesis

What is Biogenesis?

An important theory in biology and molecular genetics, Biogenesis postulates the production of new living organisms from pre-existing life. Read ahead as we explore this seminal theory that changed age-old beliefs.

Biogenesis is based on the theory that life can only come from life, and it refers to any process by which a lifeform can give rise to other lifeforms. For instance, a chicken laying eggs, which hatch and become baby chicken.

Meaning of Biogenesis

The term ‘biogenesis’ comes from ‘bio’ meaning ‘life’, and ‘ genesis’, meaning ‘beginning’. Rudolf Virchow, in 1858, had come up with the hypothesis of biogenesis, but could not experimentally prove it. In 1859, Louis Pasteur set up his demonstrative experiments to prove biogenesis right down to a bacterial level. By 1861, he succeeded in establishing biogenesis as a solid theory rather than a controversial hypothesis.

What Was the Idea of Spontaneous Generation?

The belief in a spontaneous generation is age-old, quite literally. Aristotle in Ancient Greece first pronounces the idea. And consequently, the idea also came to be known as Aristotelian Abiogenesis.

The reason behind the resounding faith in this idea was perhaps the elusive and stealthy nature of the creatures attributed to it, i.e, mice, bacteria, flies, maggots, etc.

The 18th-century path-breaking invention of the microscope that allows most of these creatures, so we can observe them under the microscope and de-mystify their origin. By the time Pasteur set about to do his work in the field, macroscopic biogenesis was already accepted by the scientific community at large. He only had to confirm microscopic biogenesis to prove the hypothesis beyond doubt.

Macroscopic Biogenesis: Francesco Redi’s Experiment

Francesco Redi, as far back as 1668, had set out to refute the idea of macroscopic spontaneous generation, by publishing the results of his experimentation on the matter. Instead of his experiment , Redi had placed some rotting meat in two containers, one with a piece of gauze covering the opening, and the other without it.

He noticed that in the container without the gauze, maggots would grow on the meat itself. However, when he provided the gauze, the maggots would appear on the gauze instead of on the meat. He also observed that flies tend to lay eggs as close to a food source as possible. Thus, he surmised the possibility of macroscopic biogenesis.

Microscopic Biogenesis

Spallanzani’s experiment.

Source: Emaze

He solved this problem by drawing out all the air in the container after sealing it. After experimenting with this manner, he achieved his desired results of a broth that had not clouded with bacterial growth, in line with the theory of biogenesis.

However, his inference was countered by critics who asserted that air was indispensable to support life, therefore the lack of bacterial growth should be attributed to the lack of air, rather than the fact that bacteria spread through contamination. For almost a century since this criticism lay unchallenged.

Pasteur’s Experiment   

The caveat of Pasteur’s 1859 experiment was to establish that microbes live suspended in air, and can contaminate food and water, however, the microbes do not simply appear out of thin air. As the primary step to his experiment, Pasteur boiled beef broth in a special flask that had its long neck bent downwards and then upwards.

This interesting contraption ensured the free diffusion of air, and at the same time prevent any bacterial contamination. As long as the apparatus remained upright, the flask remained free of any bacterial growth.

Once we slant the flask, it allows the broth to pass beyond the ‘goose-neck’ bend of the flask’s neck. The broth became clouded with bacterial growth in no time. This path-breaking experiment not only silenced all the criticism based on Spallanzani’s experiment but also cemented the Law of Biogenesis.

Law of Biogenesis Vs. Evolutionary Theory

Scientist fears that the law of biogenesis opposes the theory of evolution. It has surmised that all life stems from inorganic matter from billions of years ago. However, biogenesis simply refutes the theory of spontaneous generation and delves in a matter of generational time-span, and not of what may be achieved over thousands of generations.

While the evolutionary theories take into account the lack of predators, the difference in the chemical composition of the Earth’s atmosphere during the inception of life on Earth, as well as the trial-and-error that had taken place over millions of years to bring us to the stage of life on this planet we witness now, these do not concern the law of biogenesis at all.

Whereas the evolutionary theory demonstrates how life on earth took millions of years of trial-and-error and conducive but very different atmospheric conditions, the theory of spontaneous generation had asserted that complex life could simply appear fully formed in a matter of days. This is the belief that biogenesis had successfully challenged.

Solved Question for You

Q. Who Has Propounded the Theory of Spontaneous Generation?

• Spallanzani

Ans. C. Aristotle. The idea was first propounded by Aristotle in Ancient Greece. Consequently, the idea came to be known as Aristotelian Abiogenesis.

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2.1 Spontaneous Generation

Learning objectives.

• Explain the theory of spontaneous generation and why people once accepted it as an explanation for the existence of certain types of organisms
• Explain how certain individuals (van Helmont, Redi, Needham, Spallanzani, and Pasteur) tried to prove or disprove spontaneous generation

Humans have been asking for millennia: Where does new life come from? Religion, philosophy, and science have all wrestled with this question. One of the oldest explanations was the theory of spontaneous generation, which can be traced back to the ancient Greeks and was widely accepted through the Middle Ages.

The Theory of Spontaneous Generation

The Greek philosopher Aristotle (384–322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation , the notion that life can arise from nonliving matter. Aristotle proposed that life arose from nonliving material if the material contained pneuma (“vital heat”). As evidence, he noted several instances of the appearance of animals from environments previously devoid of such animals, such as the seemingly sudden appearance of fish in a new puddle of water. [1]

This theory persisted into the 17th century, when scientists undertook additional experimentation to support or disprove it. By this time, the proponents of the theory cited how frogs simply seem to appear along the muddy banks of the Nile River in Egypt during the annual flooding. Others observed that mice simply appeared among grain stored in barns with thatched roofs. When the roof leaked and the grain molded, mice appeared. Jan Baptista van Helmont, a 17th century Flemish scientist, proposed that mice could arise from rags and wheat kernels left in an open container for 3 weeks. In reality, such habitats provided ideal food sources and shelter for mouse populations to flourish.

However, one of van Helmont’s contemporaries, Italian physician Francesco Redi (1626–1697), performed an experiment in 1668 that was one of the first to refute the idea that maggots (the larvae of flies) spontaneously generate on meat left out in the open air. He predicted that preventing flies from having direct contact with the meat would also prevent the appearance of maggots. Redi left meat in each of six containers ( Figure 2 .2 ). Two were open to the air, two were covered with gauze, and two were tightly sealed. His hypothesis was supported when maggots developed in the uncovered jars, but no maggots appeared in either the gauze-covered or the tightly sealed jars. He concluded that maggots could only form when flies were allowed to lay eggs in the meat, and that the maggots were the offspring of flies, not the product of spontaneous generation.

In 1745, John Needham (1713–1781) published a report of his own experiments, in which he briefly boiled broth infused with plant or animal matter, hoping to kill all preexisting microbes. [2] He then sealed the flasks. After a few days, Needham observed that the broth had become cloudy and a single drop contained numerous microscopic creatures. He argued that the new microbes must have arisen spontaneously. In reality, however, he likely did not boil the broth enough to kill all preexisting microbes.

Lazzaro Spallanzani (1729–1799) did not agree with Needham’s conclusions, however, and performed hundreds of carefully executed experiments using heated broth. [3] As in Needham’s experiment, broth in sealed jars and unsealed jars was infused with plant and animal matter. Spallanzani’s results contradicted the findings of Needham: Heated but sealed flasks remained clear, without any signs of spontaneous growth, unless the flasks were subsequently opened to the air. This suggested that microbes were introduced into these flasks from the air. In response to Spallanzani’s findings, Needham argued that life originates from a “life force” that was destroyed during Spallanzani’s extended boiling. Any subsequent sealing of the flasks then prevented new life force from entering and causing spontaneous generation ( Figure 2 .3 ).

• Describe the theory of spontaneous generation and some of the arguments used to support it.
• Explain how the experiments of Redi and Spallanzani challenged the theory of spontaneous generation.

Disproving Spontaneous Generation

The debate over spontaneous generation continued well into the 19th century, with scientists serving as proponents of both sides. To settle the debate, the Paris Academy of Sciences offered a prize for resolution of the problem. Louis Pasteur, a prominent French chemist who had been studying microbial fermentation and the causes of wine spoilage, accepted the challenge. In 1858, Pasteur filtered air through a gun-cotton filter and, upon microscopic examination of the cotton, found it full of microorganisms, suggesting that the exposure of a broth to air was not introducing a “life force” to the broth but rather airborne microorganisms.

Later, Pasteur made a series of flasks with long, twisted necks (“swan-neck” flasks), in which he boiled broth to sterilize it ( Figure 2 .4 ). His design allowed air inside the flasks to be exchanged with air from the outside, but prevented the introduction of any airborne microorganisms, which would get caught in the twists and bends of the flasks’ necks. If a life force besides the airborne microorganisms were responsible for microbial growth within the sterilized flasks, it would have access to the broth, whereas the microorganisms would not. He correctly predicted that sterilized broth in his swan-neck flasks would remain sterile as long as the swan necks remained intact. However, should the necks be broken, microorganisms would be introduced, contaminating the flasks and allowing microbial growth within the broth.

Pasteur’s set of experiments irrefutably disproved the theory of spontaneous generation and earned him the prestigious Alhumbert Prize from the Paris Academy of Sciences in 1862. In a subsequent lecture in 1864, Pasteur articulated “ Omne vivum ex vivo ” (“Life only comes from life”). In this lecture, Pasteur recounted his famous swan- neck flask experiment, stating that “…life is a germ and a germ is life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.” [4] To Pasteur’s credit, it never has.

• How did Pasteur’s experimental design allow air, but not microbes, to enter, and why was this important?
• What was the control group in Pasteur’s experiment and what did it show?
• K. Zwier. “Aristotle on Spontaneous Generation.” http://www.sju.edu/int/academics/cas/resources/gppc/pdf/Karen%20R.%20Zwier.pdf ↵
• E. Capanna. “Lazzaro Spallanzani: At the Roots of Modern Biology.” Journal of Experimental Zoology 285 no. 3 (1999):178–196. ↵
• R. Mancini, M. Nigro, G. Ippolito. “Lazzaro Spallanzani and His Refutation of the Theory of Spontaneous Generation.” Le Infezioni in Medicina 15 no. 3 (2007):199–206. ↵
• R. Vallery-Radot. The Life of Pasteur, trans. R.L. Devonshire. New York: McClure, Phillips and Co, 1902, 1:142. ↵

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