Koch's Postulates: Robert Koch Demonstrates That a Particular Organism Causes a Particular Disease

views updated

Koch's Postulates: Robert Koch Demonstrates That a Particular Organism Causes a Particular Disease

Overview

Robert Koch was the first scientist to firmly establish the link between germs and disease. A doctor with a small rural practice in Germany, Koch's interest in microscopic studies led him eventually to identify the bacteria that causes the disease anthrax, which was then a common killer of sheep and cows and occasionally farmers. From there, Koch worked to develop the means by which he could prove without a doubt that these organisms were indeed to blame, the basic steps of which are now known as Koch's postulates. According to these postulates, or rules, a microbe may be proved as the cause of a disease if and only if:

1) It is found in all cases of the disease.
2) It may be isolated from the diseased body and grown artificially in a laboratory setting, creating what is called a pure culture.
3) The cultured bacteria can then be injected into a healthy animal and cause the same disease to appear in it.
4) The same bacteria can then be again found in and isolated from the newly infected lab animal.

Koch's work on anthrax was published in 1876 and touched off a revolution in medical knowledge. By the turn of the century, scientists working primarily under the tutelage of Koch and Louis Pasteur had identified most major bacilli, including those responsible for anthrax, gonorrhea, pneumonia, typhoid fever, septicemia, tuberculosis, cholera, plague, tetanus, diphtheria, and meningitis.

Background

As a young man Robert Koch (1843-1910) dreamed of working as a ship's doctor and traveling to distant and exotic places. Upon finishing his medical degree, however, he instead married and settled down to private practice in the small German town of Wollstein. Koch's work in studying bacteria began as a part time hobby after he received a microscope as a gift.

Koch began his work at a time when microbes were beginning to be studied but were still largely mysterious. The microscopic world had only been discovered to exist by Anton van Leeuwenhoek (1632-1723) a century or so earlier. Interest in this tiny world was renewed in the early nineteenth century by Louis Pasteur (1822-1895), a French chemist. In 1854 Pasteur began to study fermentation. Asked for help by local alcohol distillers who were having problems producing consistent quantities and qualities of alcohol from sugar beets, he found fermentation to be a result of the actions of active yeasts. In addition, he found that when these yeasts were crowded out by other microbes, the alcohol would not ferment properly. Along the way, he discovered that heating the alcohol to a certain temperature would kill off the offending microbes, allowing the yeasts to do their work and ensuring a consistent quality. This process of heat sterilization is now known as pasteurization.

Pasteur went on to settle an old and persistent dispute over spontaneous generation. For centuries, people had believed that many kinds of small creatures (for example, maggots found in rotting meat) developed spontaneously. The discovery of the microbial world had renewed belief in this process, for such creatures seemed far too tiny, chaotic, and ubiquitous to be generated in any other manner. Pasteur devised a test in which sterilized broth was exposed only to air filtered through cotton. Broth left standing in open air quickly teemed with microbes, but that in filtered air remained sterile. In this way, he proved that microbes did not simply appear spontaneously but were airborne.

The idea that these microbes could be responsible for diseases had been suggested but in the mid-nineteenth century was still hotly contested. Opponents believed that most diseases were caused by a corruption of the air or genetic dispositions, not by small microbes that were passed from one person to another. Those who accepted that microbes might be to blame for disease were further split between those who thought microbes were capable of transforming themselves to cause more than one disease, and those who insisted that for each disease there was a specific cause. Pasteur, a believer in the germ theory himself, gave this idea another push forward when he was called upon to help save France's silkworm industry. In 1865 an unknown disease was killing off silkworms in enormous numbers, threatening the economy of many towns. Pasteur, having helped improve the beer and wine industry through his research, was asked to apply himself to the problem of the silkworm. After several years of study he was finally able to establish that two separate parasitic organisms were to blame for the silkworm deaths. Thus Pasteur showed the importance of microbes in affecting the world and established a link between microbes and disease.

The final step was taken by Robert Koch, who emerged on the world stage in 1876 with systematic proofs that small rod-shaped bacilli were the cause of anthrax. Koch had received a microscope as a birthday gift from his wife, and he began to spend his free time between patients examining samples of all sorts under it. Eventually, he studied a drop of blood that had come from a sheep recently killed by anthrax. In addition to being deadly, anthrax was a mysterious disease that seemed to arise from nowhere. Certain fields were avoided and believed to be cursed because upon taking a healthy herd to graze in them, the farmers would find any number dead the next day. Koch found in this drop of blood some unusual rod-shaped microbes, which he correctly deduced to be the cause of anthrax. With remarkable caution, dedication, and intelligence, he set about figuring out how to prove beyond a doubt that these were indeed to blame for the disease. The process by which he proved this has become known as Koch's postulates. First, he found the tiny rods in all cases of animals he examined that had died of anthrax. Next, he reasoned that if these rods were indeed to blame for the disease, then putting them into a healthy animal should cause that animal to become ill. Using available resources, Koch carefully took a bit of fluid containing these rods and inserted it into the tail of a mouse. When the mouse began showing symptoms of anthrax and shortly died, he knew he had succeeded in passing the disease. He then dissected the mouse to see if he could still find the same little rods in the mouse's blood, which he did. Koch then continued his tests by taking a bit of fluid from the newly infected mouse and using it to infect another healthy one. In this manner, Koch continued until he had successfully infected 20 generations of mice. This multi-step process became the means by which specific bacteria were proven to cause certain specific diseases.

Impact

Koch's work brought him instant acclaim, but it also had a dramatic impact on medical science. His postulates set the method for all future identifications of specific microbes and helped to demonstrate that each disease was caused uniquely by a single microbe. Given a laboratory in Berlin and several assistants to work with in 1880, Koch began to devote himself full time to microscopic studies. His Berlin lab became something of a rival to that of Louis Pasteur in Paris, who was also continuing his microscopic studies. Pasteur had turned his attention from silkworms to human diseases, working on rabies. Unable to identify the causative agent in rabies, he nonetheless succeeded in developing a vaccine by 1885.

Koch's discovery of anthrax set off a race to discover the bacteria responsible for other diseases. In 1882 he succeeded in isolating the bacteria that causes tuberculosis (Mycobacterium tuberculosis). The next year, a severe cholera epidemic broke out in Egypt and threatened to spread across the Mediterranean Ocean to Europe. Both Germany and France sent teams to Alexandria to try and discover the responsible germ. Sadly, Louis Thuillier, one of Pasteur's team members, would contract the disease and die, a stark reminder of the hazards of working in continuous close proximity to deadly microbes. Koch headed up his own team and succeeded in isolating the cholera vibrio (Vibrio cholerae). He followed up this discovery by traveling to India, where cholera was endemic, confirming his earlier identification and discovering infected water supplies to be the cause of its transmission.

The two labs were not entirely competitive, however, and did use one another's work to build up new discoveries. The bacillus responsible for diphtheria (Corynebacterium diphtheriae) had been identified in 1883 by Theodor Albrecht Edwin Klebs (1834-1913), a scientist who studied with neither Koch nor Pasteur. Shortly thereafter, Friedrich Löffler (1852-1915), working in Koch's lab, was able to culture it. Working in Paris, Émile Roux (1853-1933) and Alexandre Yersin (1863-1943) found that the bacteria put out a toxin, which was actually responsible for causing the disease. Back in Berlin, Emile von Behring (1854-1917) picked up the work on diphtheria in an attempt to find a means of preventing it from infecting children. Assisted by the Japanese scientist Shibasaburo Kitasato (1852-1931), Behring found that blood serum from infected animals had the power to neutralize those toxins, and he therefore named it antitoxin. Antitoxin proved to be of little use in preventing diphtheria infections, as it protected for only a few weeks. It did prove useful as cure, however, as was proved in 1891. Behring first tested his antitoxin on a patient, albeit somewhat reluctantly, on Christmas night. A child who was severely ill and sure to die within days was injected with this antitoxin and made a miraculous recovery. Behring would go on to receive the Nobel Prize in medicine in 1901 for his work on serum therapy.

The idea of blood serum as an antitoxin proved useful in another disease, tetanus. At the same time that he was working with Behring on diphtheria, Kitasato also succeeded in culturing the tetanus bacillus (Clostridium tetani) and discovered that it too used a toxin. Kitasato's work on developing the tetanus antitoxin proved highly useful during World War I. Tetanus, like anthrax, resides in soils and can enter the body through open cuts or wounds. During the war, antitoxin was routinely given to all wounded men, and the death rate dropped dramatically.

Kitasato was involved in one last race to identify a microbe at the very end of the century. An outbreak of bubonic plague in Hong Kong in 1894 prompted both Kitasato (Robert Koch's student) and Alexandre Yersin (Louis Pasteur's student) to set up temporary labs and attempt to identify the bacillus. The discovery of it came simultaneously for both, and while the name of the bacillus is now Yersinia pestis, Kitasato is also acknowledged for his contemporaneous discovery.

Koch's careful studies of microbes proved their connections to disease and helped shape modern germ theory. Along with Louis Pasteur in France, Koch became a leader in the science of bacteriology. While the two men were never friends, they inspired each other to further achievements and each trained numerous students who carried on their work.

KRISTY WILSON BOWERS

PETTENKOFFER DISPUTES GERM THEORY AND LIVES TO TELL ABOUT IT

Robert Koch's work on the germ theory was not universally accepted by all scientists. When Koch announced he had isolated and identified the cholera vibrio, his findings were in particular disputed by a fellow German, Max von Pettenkoffer (1818-1901). Pettenkoffer firmly believed in the miasmatic theory of cholera, which held that putrefaction of the air caused the disease. He invited Koch to send him some of these supposed causative agents, which Koch obligingly did. Koch received the following message from Pettenkoffer:

Herr Doctor Pettenkoffer presents his compliments to Herr Doctor Professor Koch and thanks him for the flask containing the so-called cholera vibrios, which he was kind enough to send. Herr Doctor Pettenkoffer has now drunk the entire contents and is happy to be able to inform Herr Doctor Professor Koch that he remains in his usual good health. [quoted in Roy Porter, The Greatest Benefit to Mankind, W.W. Norton, 1996]

It remains a great mystery why Pettenkoffer did not become ill after swallowing the samples. Fortunately, this episode did not provide a setback to the advancement of the germ theory.

Science and Its Times: Understanding the Social Significance of Scientific Discovery