spontaneous generation- needham- history of microbiology

Spontaneous generation is the hypothesis that some vital force contained in or given to

organic matter can create living organisms from inanimate objects. Spontaneous generation was

a widely held belief throughout the middle ages and into the latter half of the 19th century.

In fact, some people still believe in it today. The idea was attractive because it meshed

nicely with the prevailing religious views of how God created the universe. There was a

strong bias to legitimize the idea because this vital force was considered a strong proof of

God’s presence in the world. Many recipes and experiments were offered in proof. To create

mice, a recipe called for dirty underwear and wheat grain to be mixed in a bucket and left

open outside. In 21 days or less, you would have mice. The real cause may seem obvious from a

modern perspective, but to the proponents of this idea, the mice spontaneously arose from the

wheat kernels.
Another often-used example was the generation of maggots from meat that was left in the open.

The failing here was revealed by Francesco Redi in 1668 with a classic experiment. Redi

suspected that flies landing on the meat laid eggs that eventually grew into maggots. To test

this idea he devised the experiment shown in Figure 1-20. Here he used three pieces of meat.

One piece of meat was placed under a piece of paper. The flies could not lay eggs onto the

meat and no maggots developed. The second piece was left in the open air, resulting in

maggots. In the final test, a third piece of meat was overlaid with cheesecloth. The flies

were able to lay the eggs into the cheesecloth and when this was removed no maggots

developed. However, if the cheesecloth containing the eggs was placed on a fresh piece of

meat, maggots developed, showing it was the eggs that “caused” flies and not spontaneous

generation. This helped to end the debate about spontaneous generation for large organisms.

However, spontaneous generation was so seductive a concept that even Redi believed it was

possible in other circumstances.
Figure 1-20 The Redi experiment

Using several pieces of meat, paper and cheesecloth, Francesco Redi produced compelling

evidence against the theory of spontaneous generation. One of the strong points of this

experiment was its simplicity, which allowed others to easily reproduce it for themselves.

See the text for details of the experiment.
The concept and the debate were revived in 1745 by the experiments of John Needham. It was

known at the time that heat was lethal to living organisms. Needham theorized that if he took

chicken broth and heated it, all living things in it would die. After heating some broth, he

let a flask cool and sit at a constant temperature. The development of a thick turbid

solution of microorganisms in the flask was strong proof to Needham of the existence of

spontaneous generation. Lazzaro Spallanzani later repeated the experiments of Needham, but

removed air from the flask, suspecting that the air was providing a source of contamination.

No growth occurred in Spallanzani’s flasks and he took this as evidence that Needham was

wrong. Proponents of spontaneous generation discounted the experiment by asserting that air

was required for the vital force to work.
It was not until almost 100 years later that the great French chemist Louis Pasteur, pictured

in Figure 1-5, put the debate to rest. He first showed that the air is full of microorganisms

by passing air through gun cotton filters. The filter trapped tiny particles floating in the

air. By dissolving the cotton with a mixture of ether and alcohol, the particles were

released and then settled to the bottom of the liquid. Inspection of this material revealed

numerous microbes that resembled the types of bacteria often found in putrefying media.

Pasteur realized that if these bacteria were present in the air then they would likely land

on and contaminate any material exposed to it.
Figure 1-5 Louis Pasteur

The French microbiologist Louis Pasteur. Drawing by Tammi Henke
Pasteur then entered a contest sponsored by The French Academy of Sciences to disprove the

theory of spontaneous generation. Similar to Spallanzani’s experiments, Pasteur experiment,

pictured in Figure 1-6, used heat to kill the microbes, but left the end of the flask open to

the air. In a simple, but brilliant modification, the neck of the flask was heated to melting

and drawn out into a long S-shaped curve, preventing the dust particles and their load of

microbes from ever reaching the flask. After prolonged incubation the flasks remained free of

life and ended the debate for most scientists.
Figure 1-6 The swan neck flask experiment

Pasteur filled a flask with medium, heated it to kill all life, and then drew out the neck of

the flask into a long S shape. This prevented microorganisms in the air from easily entering

the flask, yet allowed some air interchange. If the swan neck was broken, microbes readily

entered the flask and grew.
A final footnote on the topic was added when John Tyndall showed the existence of

heat-resistant spores in many materials. Boiling does not kill these spores and their

presence in chicken broth, as well as many other materials, explains the results of Needham’s

experiments.
While this debate may seem silly from a modern perspective, remember that the scientists of

the time had little knowledge of microorganisms. Koch would not isolate microbes until 1881.

The proponents of spontaneous generation were neither sloppy experimenters nor stupid. They

did careful experiments and interpreted them with their own biases. Detractors of the theory

of spontaneous generation were just as guilty of bias, but in the opposite direction. In

fact, it is somewhat surprising that Pasteur and Spallanzoni did not get growth in their

cultures, since the sterilization conditions they used would often not kill endospores. Luck

certainly played a role. It is important keep in mind that the discipline of science is

performed by humans with all the fallibility and bias inherent in the species. Only the

self-correcting nature of the practice reduces the impact of these biases on generally held

theories. Spontaneous generation was a severe test of scientific experimentation, because it

was such a seductive and widely held belief. Yet, even spontaneous generation was overthrown

when the weight of careful experimentation argued against it. Figure 1-21 lists important

events in the spontaneous generation debate.

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