“Gentlemen, it is the microbes who will have the last word.” Those words spoken by Louis Pasteur a century and a half ago are unnervingly meaningful today as we confront the SARS-CoV-2 virus that is invading virtually every aspect of our life. Pasteur may be best known for formulating the germ theory of disease, but he is also one of the fathers of the science of immunology.Â
Anyone wandering through the countryside in Europe in the 1870s would notice fields speckled with dead sheep. The animals had perished from “anthrax,” the term deriving from the Greek word for coal on account of the characteristic black lesions associated with the disease. Anthrax has a long history, with some biblical scholars even suggesting that it was responsible for the plague that affected livestock in Egypt as described in the story of the Exodus.Â
Pasteur already was aware that Aloys Pollender in Germany and Pierre Rayer and Casimir Davaine in France had noted that a type of bacterium was always present in the sheep that had died from anthrax. As early as 1676 Antonie van Leeuwenhoek had observed “animalcules” through a microscope of his own design, but bacteria would not be linked with disease until 1876 when Robert Koch in Germany inoculated mice with blood taken from the spleen of animals that had died from anthrax. The mice perished, while mice inoculated with blood from the spleens of healthy animals did not. The difference was the presence of bacteria in the sick animals’ blood, eventually named Bacillus anthracis.
Clearly transmission via blood was possible, but that did not explain how sheep were getting infected. Pasteur began to ask questions, along the lines that would be asked by epidemiologists today. How come sheep were more affected in some areas than in others? What was the difference? As it turned out, the disease was more rampant where the dead sheep had been buried in fields where other animals grazed. Why was that? How could bacteria from the buried animals infect others? This is where we can reference Pasteur’s other famous quote, “chance favours the prepared mind.” The French chemist, yes, Pasteur was a chemist, noted that the soil was teeming with earthworms. Could these creatures be the infective vectors? He managed to show that this indeed was the case. The worms were transporting the bacteria to the surface and the bacterial spores then contaminated the grazing animals’ feed.
Now Pasteur’s attention turned to preventing anthrax. In 1879, he had already shown that chicken cholera could be transmitted to healthy chickens by inoculating them with a culture made from the blood of sick birds. As luck would have it, a vacation intervened, and Pasteur left a culture dish sitting idly in the laboratory. When he then used this culture to infect healthy chickens, they did not get sick. Furthermore, when these birds were then inoculated with a fresh culture, they still did not develop the disease. Pasteur concluded that leaving the culture exposed to air had somehow weakened the microbes it contained, preventing them from causing disease but rendering them capable of preventing a subsequent infection. Could this also work for anthrax?Â
Following along the lines of his chicken cholera experiments, Pasteur exposed anthrax cultures to oxygen and hoped that this would attenuate the microbes and afford protection to animals then exposed to a virulent form of the bacteria. In 1881, he carried out a historic demonstration in front of a crowd of observers. Twenty-four sheep, one goat, and six cows were inoculated with his new “vaccine,” a term Pasteur coined to honour Edward Jenner who in 1798 had shown that smallpox could be prevented by inoculation with pus from cowpox pustules.
A control group composed of the same mix of animals remained unvaccinated. All the animals were then inoculated with fresh anthrax bacilli. Two days later, the crowds gathered to see the results. Applause broke out when it became clear that all the vaccinated animals were alive. In the control group, the sheep and the goat were all dead and the cows were sick.Â
Pasteur’s attention now turned to rabies, a disease that afflicts humans as well as animals. It was clear that this was an infectious disease, but microscopic examination of blood and tissues was unrevealing. That is because rabies is not caused by a bacterium, but by a virus, a much smaller organism that would not be visible until the electron microscope was introduced in the 1930s. Still, Pasteur believed that whatever was causing the disease could somehow be modified, just as he had done with chicken cholera and anthrax, and be used to confer immunity. Eventually, he found that infecting a rabbit with blood from a rabid animal did the job. Somehow the infective agent passing through the rabbit was attenuated so that it did not cause disease. The key finding was that a vaccine prepared from the rabbit’s spinal cord successfully prevented infection. Pasteur went on to demonstrate this protection in dogs, and then in a landmark experiment treated a boy who had been bitten by a rabid dog with a series of injections that prevented the boy from developing rabies and made Pasteur into an international hero.
Pasteur’s work stimulated research into vaccines all over the world and today a variety of attenuated live virus inoculations protect us from diseases such as measles, mumps and rubella. Inactivated bacteria and viruses, or some fractions thereof, can also be used to induce immunity. Maybe some brilliant scientists will figure out a way to protect us from COVID-19 and attenuate Pasteur’s comment about microbes having the last word.
