Under the Microscope/Prof William Reville: Ask any French person to name the 10 most important of their compatriots ever and it is a sure bet that Louis Pasteur will figure prominently on the list.
Pasteur led an amazingly productive scientific life, making pioneering discoveries in medicine and chemistry and applying his science to solve problems of major economic importance to France and to revolutionise medical concepts of disease and its prevention. France is rightly proud of Pasteur. I will summarise Pasteur's scientific work in a two-part article, the second part to appear next week.
Louis Pasteur was born in 1822 in Dole, in eastern France about 400 kilometres south east of Paris, the only son of a poorly educated tanner, Jean Pasteur. In his early years at school, Louis showed little interest in academic subjects, but when science came up on the curriculum he began to show great application. Encouraged by his father, Pasteur entered the École Normale Supérieure in Paris in 1837, a prestigious university devoted to training outstanding students for university careers.
Louis received an MSc in 1845 and then began work on his PhD in chemistry, a study of crystals, then an emerging branch of chemistry. He studied tartaric acid crystals, found in sediments of fermenting wine. Crystals of para-tartaric acid are also found in the sediments. The chemical composition of these two acids had already been found to be identical. Experts had also declared that their crystals were identical.
But a mystery remained. When dissolved in water, tartaric acid rotated a beam of polarised light passing through it to the right, whereas para-tartaric acid did not rotate the light. Pasteur became convinced that the internal structure of the two acids must be different and, despite the experts, he believed this difference would show itself in the crystal form.
Pasteur found that all crystals of tartaric acid looked identical under the microscope, whereas crystals of para-tartaric acid could be divided into two very similar but not quite identical types, each a mirror image of the other - just as your left hand is a mirror image of your right hand.
Pasteur separated the two types of para-tartaric acid crystal into two heaps. He dissolved each heap separately in water and found that one type rotated polarised light to the left and the other rotated it to the right. It was now clear that organic molecules with the same chemical composition can take up unique and different shapes in space. Pasteur's observations began the new science of stereo chemistry.
These findings had a deeper meaning for Pasteur. He believed that asymmetric molecules such as he had discovered were characteristically produced by living processes. This far-sighted prediction proved correct. For example, proteins - pre-eminently important large biomolecules found in all cells - are made from small molecules called amino acids. Amino acids can occur in left-handed and right-handed forms but living organisms exclusively use only the left-handed forms.
In 1854 Pasteur was appointed professor of chemistry at the Faculty of Science in Lille, France. In 1856 he was approached by a local distillery having problems manufacturing alcohol by fermenting beetroot. The fermentation was erratic, often yielding lactic acid rather than alcohol.
Pasteur sorted out the whole problem in a few years of brilliant investigation, at the same time starting the new science of microbiology. He found that when fermentations went well, budding yeast cells were unaccompanied by other microbes. But when the fermentation produced lactic acid instead of alcohol, small rod-shaped microbes accompanied the yeast cells. He also found that fermentation produced complex organic compounds in addition to alcohol and that some of these compounds rotated polarised light.
As we know, he suspected that only living cells produced such asymmetrical molecules. He concluded and proved that living yeast cells were responsible for forming alcohol from sugar and that souring of fermentations was caused by contaminating micro-organisms.
Pasteur returned to the École Normale Supérieure in Paris in 1857 as director of scientific studies. He now turned his attention to the doctrine of spontaneous generation. The ancient Greeks believed that small animals such as worms and rodents spontaneously sprang to life from non-living matter such as rotting food. The Italian, Francesco Redi, disproved the idea of maggots spontaneously arising from meat in 1668. No maggots appeared in meat when he prevented flies from laying eggs on it. However, scientists still clung persistently to the notion of the spontaneous generation of microscopic animals.
Pasteur finally disproved this idea. He boiled broth in order to kill the microbes in it. He now allowed air to circulate over the broth, but, using ingenious devices, prevented microbes in the air from contacting the broth. No microbes arose in this broth. However, microbes flourished vigorously in the broth when they were allowed in with the air and made contact with the broth.
Pasteur had shown that life comes only from life.
William Reville is associate professor of biochemistry and director of microscopy at University College Cork