-10 Microbes are discovered to cause disease

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  • Ignaz Semmelweis showed that child-bed fever was spread by physicians and could be prevented by careful hand washing with chloride of lime.
  • Louis Pasteur, while working on sour wine, discovered that unwanted microbes were infecting the wine. He correctly deduced that infectious disease was caused by similar infections with harmful microbes.
  • Robert Koch was the first to isolate a disease-causing microbe, Bacillus antrhacis. In the process he developed techniques and standard protocols for defining the cause of a disease.

It was long suspected that living things were the agents of disease. In volume 6 of his epic poem De Rerum Natural (On the Nature of the Universe) written sometime around 50 B.C., Titus Lucretius Carus speculates about invisible atoms causing disease. This was only one idea among many and some thought that an imbalance in humors caused illness, while others felt that supernatural forces were at work. The prevailing theory held by most doctors of the 19th century was that chemical toxins were carried from an ill patient to others, causing them to contract the same malady. The bacteria that were known to be present were seen as a symptom of the disease and not its cause.

Ignaz Semmelweis, pictured in Figure 1-7, a Hungarian physician working in Vienna, made the first breakthrough in the true nature of disease. He realized that asepsis in obstetrical wards could prevent the transmission of childbirth fever from patient to patient. He therefore instigated a policy for all attending physicians to wash their hands with chloride of lime (a mixture of calcium chloride hypochlorite, CaCl(OCl); calcium hypochlorite, Ca(OCl)2; and calcium chloride, CaCl2) between patients. This innovation dropped the mortality rate from 18% to 2.4%.

Ignaz Semmelweis

Figure 1.7. Ignaz Semmelweis. Ignaz Semmelweis. Drawing by Tammi Henke

Ignaz became a vigorous proponent of his ideas, but the Hungarian doctor's efforts were opposed by many who could not accept that physicians themselves could be responsible for spreading bacterial infection. Ridicule of his idea caused him to move from Vienna to Pest, Hungary and ultimately played a role in a nervous breakdown. Ironically Semmelweis died from an infection that he contracted during a surgery he performed, while recovering from his nervous breakdown.

Before his death he published his ideas in a paper The Cause, Concept, and Prophylaxis of Childbed Fever in 1861. Although the poor writing of the paper contributed to the obscurity of his ideas, the work was ignored for 17 years, which raises an interesting point about the culture of science. Radical ideas, even those that are correct and can save lives, are sometimes ignored. It takes time to overcome the dogma of the day. The personalities involved and the negative light it might throw on past practices play a large role in the rate of acceptance of a new idea.

In a seemingly unrelated event, Louis Pasteur found something interesting while working on wine souring, a problem where wine fermentations produce a sour taste and very little alcohol. Fermentation of alcoholic beverages was thought to be a simple chemical reaction. Heating, common in most beverage preparations, was thought to cause the breakdown of sugar into alcohol. Pasteur was asked to help out the wine industry in France because wine souring was pushing it close to ruin. His work on wine-making revealed that the process of converting sugar to alcohol is actually performed by various yeast strains. He then showed that the wine was going bad because a contaminating microbe was generating lactic acid instead of alcohol from the sugar. This idea was controversial, but gained credibility when Pasteur solved the problem by heating the wine and killing the contaminant. The heating process was named pasteurization in his honor and is still widely used today. In a brilliant step of generalization, Pasteur realized that souring of wine and infectious disease shared a common thread in that they both might involve infection by a microorganism. His suggestion that microbes cause disease became known as the germ theory of disease.

Joseph Lister became aware of Semmelweis's work and together with Pasteur realized the true nature of disease. He then recognized that he could use this idea to help his surgery patients. At this time, major injuries, broken bones or surgery would often result in infection of the damaged area, sometimes leading to amputation or death. Lister found he could greatly reduce the number of microorganisms on wounds and incisions by using bandages treated with phenic acid, a compound that killed microorganisms (phenic acid, now known as phenol, is the active ingredient in Listerine). During surgery he began the practice of spraying the wound with a fine mist of phenic acid to kill microbes. These practices greatly reduced the rate of infection and mortality of surgery patients, lending further credence to the germ theory of disease.

In 1876 Robert Koch provided definitive proof of the germ theory by isolating the cause of anthrax and showing it to be a bacterium. From this came the development of Koch's Postulates, a set of rules for the assignment of a microbe as the cause of a disease:

  1. The specific organism should be shown to be present in all cases of animals suffering from a specific disease, but should not be found in healthy animals.
  2. The specific microorganism should be isolated from the diseased animal and grown in pure culture on artificial laboratory media.
  3. This freshly isolated microorganism, when inoculated into a healthy non-immune laboratory animal, should cause the same disease seen in the original animal.
  4. The microorganism should be reisolated in pure culture from the experimental infection.

The postulates are Koch's most famous contribution to science and it is a testament to the utility of these postulates that they are stilled used today to discover the cause of new emerging diseases. Koch went on to apply these principles in the study of many other diseases including tuberculosis, cholera and sleeping sickness. It should be pointed out that Koch's postulates cannot be applied to all diseases. For example if a disease-causing microbe has humans as its sole host and has a significant possibility of causing death, it would be unethical to apply this microbe to test humans as dictated by postulate 3. Also, it is not always possible to obtain a disease-causing microbe in pure culture.

While attacking the problem of disease, Koch developed the tools for obtaining pure cultures. Advances in science often come from innovations in the available technology. Robert Koch was an important microbiologist because his pioneering work in the isolation and characterization of bacterial diseases helped to identify the causes of many of the maladies plaguing humanity. Further work by other scientists then began the long road to conquering them.

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