Background
The achievements of Louis Pasteur (1822-1895)
GCSE texts cannot provide more than a brief summary of highlights of Pasteur's work. Yet his achievements were phenomenal and appear all the more remarkable when considered in the context of the contemporary scientific climate. His contribution is a monument to the application of an enquiring mind, dogged perseverance, very careful recording of his observations and results, the scientific method. The main points, which unfold in a remarkably 'evolutionary' sequence, are reviewed below. teachers may feel it a worthwhile and rewarding exercise for students to research Pasteur's work from the many available 'library' sources, possibly as an extended homework topic.
During the 1880s, Pasteur concerned himself with problems experienced during the fermentation of alcohol by fermentation of beet sugar. Yeasts had been described by earlier workers, but their role in fermentation was unappreciated. Microscopic examination of the brew by Pasteur revealed the presence of dividing yeast cells in the vats and he demonstrated that, if the yeasts were killed by boiling, no fermentation occurred. Pasteur realised that yeasts were the cause of fermentation and not just incidental contaminants. turning to vats in which fermentation was not proceeding properly, Pasteur showed that very few yeast cells were present but instead, innumerable, much smaller, rod-shaped structures predominated. The vats produced lactic acid instead of alcohol. To confirm that the rods were living organisms, Pasteur inoculated a nutrient medium with a tiny quantity of the material from a problem vat. The rods multiplied very rapidly. He was able to establish that the process of fermentation was due to the activities of living microorganisms and that the products depended upon the types of organism involved. Pasteur then showed that the spoilage of various types manufacture was due to the presence of various kinds of microorganism in addition to the desired yeasts on grape skins. He found that the contaminants could be killed by heating the wine to 55 °C without adversely affecting the quality of the wine. This was the first use of pasteurisation.
Using his now famous swan-neck flasks, Pasteur proceeded to show that microorganisms can come from the air, simultaneously disproving the ten widely held belief in spontaneous generation. Broth was boiled in the flasks, sterilising the system. As the broth cooled, air entered, but the microorganisms settled out and were trapped on the moist walls of the curved neck. The broth remained sterile. However, tipping a flask so that the broth contacted the neck wall introduced contaminants with the result that the broth soon deteriorated after becoming densely populated by rapidly multiplying micro-organisms. Fracturing the swan-neck to expose the broth to air had a similar effect.
Subsequently, diseases of both animals and humans attracted Pasteur's attention. He showed that pébrine, a disease of silkworms which was ravaging the silk industry, was caused by a microorganism and that it was infectious. He then investigated anthrax, a disease which was severely affecting farm animals and humans. The microbial nature of anthrax had already been established by Robert Koch, but Pasteur extended the investigations. He showed, example, that when a culture of anthrax bacilli was passed through a filter fine enough to exclude the bacteria, the filtrate alone did not infect animals into which it was injected, whilst injection of the bacteria themselves did precipitate the disease. Pasteur and Koch together firmly established the 'germ theory' of human disease.
Pasteur went on to demonstrate the scientific basis for vaccination, partly developed by Jenner (Core syllabus content). Pasteur observed that chickens which had been previously injected with material from old, cultures of cholera bacteria were protected from infection by subsequent injection using virulent bacteria from fresh cultures. Soon, this technique was being used throughout France to protect chickens from cholera.
Pasteur made similar findings about anthrax. He prepared a series of strains of the bacteria in order of decreasing virulence, by holding the cultures for increasing lengths of time at 42 °C. He injected farm animals with a low-virulence strain which they survived, then after 12 days injected a higher-virulence strain which they again survived. After a month, he was able to inject a fully-virulent strain which they again survived.
Pasteur then applied the principle of using attenuated (weakened) strains of a pathogen to induce immunity in animals, and subsequently in humans, to rabies. In this case, attenuated strains were produced by drying samples of infected tissue (brain and spinal cord) for increasing lengths of time, rather than by the heat treatment used for the anthrax bacilli. Despite dealing with a viral pathogen he could not see, his efforts were successful. This use of attenuated strains of pathogens was to find enormous application in the years to come.
In summary, Pasteur: