End of the Debate, Louis Pasteur, Swan Necked Flask Experiment, Spontaneous origin, Microbial Life

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End of the Debate

Irritated by continuous advocacy in favour of spontaneous generation even by nineteenth century scientists, Louis Pasteur (1861) conducted series of experiments to prove that if the solutions are made microbe-free by boiling and they are provided with microbe-free air (the "vital force" for spontaneous generation), they do not show any sign of spontaneous origin of microbial life in them. In his swan-necked flask experiment, he took various type of broths (yeast water, sugared yeast water, urine, sugar beet juice etc.) in long-naked flasks and, then, softened the neck of the flasks under a flame and drew it out in the shape of 'S' looking like the neck of the swan.

The broths of these flasks were boiled until they steamed through the necks, and then cooled. The broths so treated in the flasks did not decay, and there were no signs of microorganisms in them after days, weeks and even months though they were open to free-air. Pasteur's unique swan-necks of the flasks trapped airborne microorganisms before they could reach the broth and flourish in it. The broths in the flasks open to free air but free of microbes for very long periods, therefore, definitely discredited the doctrine of spontaneous generation.

Pasteurs Experiment With the Swan Necked Flask

1	Dust and Microorganisms Enter and Trapped in Bend	6	Open End
2	Sterile Broth	7	Flask Tipped So Microorganism Laden Dust Contacts Sterile Broth
3	Broth Colled Slowly	8	Short Time
4	Long Time	9	Microorganisms Grow in BRoth
5	Broth Remains Sterile for Many Years

Despite Pasteur's successful demonstrations against spontaneous generation, attempts to repeat his experiments occasionally failed because, after some time, existence of microbes was evident in some broths of swan-necked flasks. This created doubt in the minds of many. But, this problem was soon solved by John Tyndall, an English physicist, in the year 1877. He explained that bacteria exist in two forms: Heat-labile forms (thermolabile) which could be killed by exposure to high temperatures, and heat-resistant forms which could not be killed by continuous boiling of the broth and, after the broth has cooled, they resulted in microbial growth in such broths.

He further stated that if such broths are subjected to intermittent boiling (discontinuous boiling) on successive occasions, a process now popular as tyndallization, the heat-resistant forms of bacteria will be killed and the broths become completely free of them, and do not show any microbial growth. It so happens because the first boiling kills vegetative cells of bacteria but endospores remain as such. The endospores now germinate in
cooled broth and produce new bacteria cells Which are killed during further boiling and so on. In this way, Tyndall validated Pasteur's results and helped ending the debate on abiogenesis versus biogenesis.

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Louis Pasteur (1822-1895) : Founder of Microbiology

Pasteur's contributions to the science of microbiology are many and great. The diversity of the fields in which he used his talents is astounding and the credit of a sound and scientific beginning of microbiology goes to him. It was he who discovered that fermentation of solutions was caused by microorganisms and not, as most scientists then assumed, by a set of spontaneous chemical reactions.

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He firmly discredited the spontaneous generation doctrine by his Swan necked flask experiment; strongly supported germ theory of diseases by proving destructive silkworm disease due to the activities of protozoan parasite; discovered the existence of life in the absence of free oxygen (anaerobic growth); showed that heating could be used to kill microbes in broths (pasteurization); discovered antirabies vaccine; suggested methods to control pebrine disease of silk worms and isolated the germ responsible for chicken cholera and rabies.

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