It seems astonishing that the perpetrator of these seemingly reckless efforts at rabies vaccination, more than a century ago, is universally recognised as one of the greatest scientists who ever lived. In France he enjoys a status close to sainthood.
Louis Pasteur, who died on 28 September 1895, aged 72, richly deserves his place in the pantheon of science. The ideas behind several of his fundamental discoveries lie at the heart of modern biology, while his development of vaccines against rabies and anthrax not only revolutionised our capacity to combat these diseases but laid down principles of immunisation that are used to save countless lives today. Yet recent revelations about the way Pasteur conducted his research are altering our perceptions of the hero of French, and indeed world, science.
Even in his own age, the range of Pasteur's achievements was extraordinary. Take two examples. He conducted experiments to settle the hotly contested issue of the so-called spontaneous generation of life. He placed a dilute soup of nutrients inside flasks designed to prevent airborne microbes from getting inside, heated them to kill existing microbes, and showed that living cells did not arise from these non-living materials. Microbes grew in the soup only if allowed access from the air.
Second, Pasteur's painstaking studies revealed that tartaric acid consisted of two types of crystal, mirror-images of each other. A startling discovery in its own right, this launched the discipline of stereochemistry, which deals with the "handedness" of crystals and their constituent molecules.
Despite the apparent diversity of these researches, the French chemist's career had a coherence centred on the distinctiveness of living processes. Pasteur was fascinated by the chemistry of life. His primary legacy was to show how particular microbes brought about corresponding, specific changes - whether in fermenting sugar to make alcoholic beverages or in causing disease. Applied to infections, this idea became known as specific aetiology, and is one of the central concepts of 20th-century medicine.
Pasteur also established that there need be no distinction between pure and applied science. In 1865 he saved the French silkworm industry, which was being destroyed by a disease called pebrine. Yet in working out how to prevent its transmission, he was both applying and developing the principle of specific aetiology.
Before being called in to deal with pebrine, Pasteur had established that the "lactic fermentation" (souring of milk) was caused by microbes, which he saw under the microscope. Then, after the silkworm diversion, he proved that other microbes (yeasts) were also responsible for fermentation in brewing and wine-making. Equally important, he discovered what went wrong when fermentation yielded unsatisfactory products - such as "ropey beer". Such undrinkable beers contained a particular microbe that had contaminated the process and caused it to go awry. Pasteur spotted one such organism during a visit to Whitbread's brewery in Clerkenwell, London, in 1871. Shortly after, Whitbread purchased a microscope to help to detect contamination at an early stage.
Specific aetiology was a revolutionary notion at the time and was crucial for the emergence of the germ theory of disease. It meant, for example, that the bacterium Mycobacterium tuberculosis, no other, was responsible for tuberculosis. Only Bacillus anthracis caused anthrax. And so on. A corollary to this discovery was that similarly targeted vaccines might be used to immunise people against particular diseases. Pasteur's innovation was to develop vaccines by weakening microbes so that they induced the recipient to produce specific antibodies but did not cause the disease.
Following Pasteur, other investigators extended specific aetiology in many other directions. First, they found that microbes such as the bacterium responsible for diphtheria caused disease only if they produced particular poisons that damaged the body in various ways. Then other conditions were related to the absence of specific substances - diabetes and insulin being the classic example. All of this originated with Louis Pasteur. It was he who finally swept away the ancient idea, still alive when he began his work in the mid-19th century, that diseases were attributable to imbalances between the four humours of the body.
While Pasteur's achievements were extraordinary, they have been gilded beyond belief over the years. "Here was a life, within the limits of humanity, well nigh perfect," wrote his biographer Stephen Paget in 1910. Two decades later, Paul de Kruif in Microbe Hunters and the actor Paul Muni in the Metro-Goldwyn-Mayer film The Story of Louis Pasteur portrayed the French chemist as a miracle worker. Apart from occasional suggestions by historians of science that some of his results were too good to be true, the legend of Pasteurian perfection has endured, especially in France.
The Princeton University historian Gerald Geison has now brought greater realism to the story in his book The Private Science of Louis Pasteur (Princeton University Press, pounds 24.95). He has done so by scrutinising the 10,000 or so pages of Pasteur's notebooks, which remained in the hands of the Pasteur family until recently, but can now be consulted in the Bibliotheque Nationale in Paris.
What these and other previously neglected sources show is a Pasteur who was all too human. His determination was matched by selfishness and secrecy, his vision accompanied by a tendency to evidence to fit his expectations. Pasteur did offend against the ethical standards of his day (let alone ours) in his work with rabies vaccine - though Geison invites us to judge him leniently in light of the horror surrounding the disease at that time.
Pasteur also concealed the fact that the anthrax vaccine, used to protect sheep in a dramatically successful demonstration in June 1881, was made not by his own method but by that of one of his rivals. In his work on spontaneous generation, Pasteur flouted another central principle of scientific research. He ignored those occasions when living cells did appear, against expectations, in his flasks of nutrient medium. "I did not publish these experiments," he wrote, "for the consequences it was necessary to draw from them were too grave for me not to suspect some hidden cause of error in spite of the care I had taken."
Louis Pasteur was neurotically secretive, too (taking his notebooks on holiday so no one else could consult them). And he was exceedingly difficult to work with. According to Geison, he treated even his most senior colleagues as "toilers in the Pasteurian vineyard ... whose most lasting contribution was their transmission of the Pasteurian legacy to others". Perhaps for this reason, very few of Pasteur's assistants became distinguished scientists. This is in sharp contrast to people such as the physicist Lord Rutherford in Cambridge in the Twenties and Thirties, who spawned virtual dynasties of research leaders.
The final charge against Louis Pasteur is that he was lucky, making far more than one lifetime's worth of original observations, and producing much more precise experimental results than would normally be expected. The charge is hard to deny. Yet his own, oft-quoted observation about scientific research remains the most important single clue to his own genius. "In the field of experimentation," Pasteur said, "chance favours only the prepared mind."