The Germ Theory of Disease

Before the time Louis Pasteur presented to the public his amazing experiments proving the existence of microbes everywhere and their ability to contaminate non-living things (e.g. nutrient solutions), people had no idea that many kinds of diseases are associated to microbes. They discovered some treatments for diseases by trial and error, but without knowledge about the ability of bacteria, fungi, and protozoans to cause illnesses.

The discovery of Pasteur of the ability of yeasts (microscopic fungi) to convert sugars to alcohol in the absence of air (a.k.a. the process of fermentation) was the first link between the activity of microbes and chemical and physical changes in organic materials. Scientists hypothesized that since microorganisms have the ability to modify non-living organic matter, they may also have the ability to do the same in living plants and animals – that the modifications (whether physical or chemical) they made to these organisms are the causes of disease. They later called this hypothesis as the germ theory of disease (Madigan 2006).

People had a hard time accepting the theory because for centuries they believed that people get sick as a punishment for their crimes or misdeeds. When the whole community became sick, they usually blamed the disease on demons appearing as foul odors from the sewage canals or on toxic aerosols from swamps. People could not also imagine what the scientists described as “invisible” microbes floating in the air that can readily infect animals and plants; that the invisible microbes could attach themselves in clothing and beddings and can be transmitted to humans and cause disease. Fortunately, scientists were able to gather the information required to back up the germ theory (Serafini 1993; Madigan 2006).

• In 1865, Louis Pasteur was able to prove that the new silk worm disease affecting the silk industry was caused by a protozoan. He formulated a way to identify afflicted silkworm moths so that they would be destroyed and would not infect others. With Pasteur’s ingenuity and expertise, he saved the collapsing silk industry in Europe. He proved that the “invisible” protozoans cause disease to the silk worms. Note that the “invisibility” of the protozoans is due to their very tiny sizes (a few micrometers) which can only be seen under a microscope (Madigan 2006)

• English surgeon Joseph Lister used the germ theory of disease to promote his idea that disinfection is necessary to prevent the spread of diseases caused by microorganisms (Ingraham 2002). He claimed that doctors must disinfect their hands before surgically operating on a patient because their hands may be carrying microbes that could cause infection to the patient. They should also disinfect their hands after the treatment so that they would not spread any microbe they acquired during the surgical operation. He promoted carbolic acid (phenol solution) as the disinfectant, knowing that it effectively kills bacteria. When surgeons found out that the phenol solution has dramatically reduced the incidence of infections and deaths to patients, more doctors began using it until it became a standard operating procedure.

• The German physician Robert Koch (Ingraham 2002; Madigan 2006) used the germ theory of disease to prove that the disease anthrax, which kills cattle and sheep, is caused by a bacterium. The bacterium was later given the scientific name: Bacillus anthracis. He drew blood samples from anthrax-infected sheep and cattle, then isolated and cultured the blood into nutrient broths. After a few days of incubating the bacterial culture, he injected samples of it into several healthy animals. Amazingly, the animals became sick and died. He then isolated and cultured the bacteria found in the blood of these dead animals. He found out that the bacteria isolated from these dead animals were the same to the bacteria isolated from the dead sheep and cattle. This experiment proved that microbes can be transmitted from one organism to another; it also verified the notion that some diseases can be communicable — not only anthrax, but also others. Koch later on established his postulates (popularly known as Koch’s Postulates), which are a sequence of experimental procedures for directly associating a specific microorganism to a specific disease. These postulates will be discussed in part 4 of this series

• Serafini, Anthony. 1993. The Epic History of Biology. Plenum Press.
• Madigan, Michael. 2006. Brock Biology of Microorganisms. Upper Saddle River, N.J.: Prentice Hall/Pearson Education.
• Ingraham, John. 2002. Introduction to Microbiology. Pacific Grove, Calif.: Brooks/Cole Pub.