Chapter 24 – Insects and the Germ Theory of Disease
Most people, if they think about entomology at all, think that it mostly entails some old scruffy guy sitting in the back of a museum sticking pins in dead insects. And, to a very limited extent, entomology does involve collecting, preserving and displaying specimens of insects. But the study of insects entails so much more than just curating insect life forms. In fact, insects and entomology have been two very important elements in discovering and elucidating key scientific theories, the foundational ideas upon which all science is based. Scientific theories are very important to the ability of scientists to make predictions and continue exploration of science even as methodologies change. In this chapter, we will investigate how insects contributed to a major scientific theory, namely the establishment of the Germ Theory of Disease. After reading both this chapter and the one that follows (How Insects Shaped Biology), you will see that insects were not just passive players in a major discovery. Rather, they were intimately involved in scientists’ ability to bring clarity to vexing problems in science. In this chapter we will look at the role that insects played in establishing how pathogenic diseases are caused and how to prevent or cure them.
Louis Pasteur
Although a number of scientists working over several centuries were important to this undertaking, the person who stands out as pivotal in the 19th century is Louis Pasteur. Pasteur was a French scientist born in 1822, just after the fall of Napoleon. Pasteur earned academic credentials in Chemistry and Microbiology. In his lengthy career, he brought clarity to many issues and extended our understanding of disease processes and how they work. For instance, Pasteur, made major progress in stemming a major complication of pregnancy, puerperal fever, a sickness that afflicts women who recently delivered a baby. Pasteur showed something that we now take for granted: if physicians wash their hands between patients and obey simple hygienic practices, the incidences of childbed fever could be greatly reduced. Pasteur also developed the first vaccine for rabies. Since rabies is a fatal disease, this advance was tremendously important. Pasteur also developed a method for processing milk and wine to remove contaminating microorganisms and reduce spoilage. In Pasteur’s honor, the process is still today referred to as pasteurization. Pasteur also made significant contributions to basic chemistry including defining the chemical phenomenon known as chirality as the ability to bend plane polarized light. Because of his many contributions, Pasteur was interred beneath the eponymous Institut Pasteur (Pasteur Institute) in Paris after his death.
Scientific Theories
Our goal in this chapter is to examine Pasteur’s role in establishing the Germ Theory of Disease and insects’ contribution to that discovery. Before we take this on, it is useful to review what we mean by a scientific theory. We do not mean, as our popular understanding of the word might suggest, something tentative or unsubstantiated by data. In science, theories are the ideas about which we are most sure, are supported by many tested hypotheses, and are able to explain large amounts of data. List 24.1 gives several well-known scientific theories. You, no doubt, have heard of some of them and recognize that they are the underpinnings of modern science.
It is important to remember that people also use the word “theory” in a non-scientific sense to convey the idea that we don’t know much about an idea. That is the common usage of the word and it needs to be distinguished from the scientific usage because it can lead to great confusion. So, to recapitulate: scientific theories, according to the National Science Foundation (NSF), are the result of repeatedly verified hypotheses, supported by many lines of evidence, capable of explaining a large number of related phenomena and are generalizable. In science, theories are NOT tentative ideas that are unsupported by data.
Returning now to our discussion of Pasteur and his role in figuring out The Germ Theory of Disease, it is useful to provide a context in which to understand his accomplishment. Prior to Pasteur’s groundbreaking work, there were still some pretty old ideas about how diseases worked. We saw this vividly when we discussed the bubonic plague and malaria. During the 14th century and several centuries after that, scientists believed in the miasmatic theory of disease transmission. That is, diseases like plague and malaria were caused by exposure to toxic vapors or miasmas that could circulate in the air. On the one hand, it was true that plague bacteria could be passed through the air and transmitted from person to person in the low percentage of people that suffered from the pneumonic form of the disease. However, the other forms of plague were not spread in this way, and malaria is only transmitted from the bites of infected mosquitoes. Because physicians did not understand that microscopic organisms were the source of these diseases, they were unable to identify the actual modes of transmission or effectively treat the illnesses they caused. One of the difficulties in making correct deductions about the nature of disease was that for centuries, there was a belief among scientists in the idea of spontaneous generation.
Basically, spontaneous generation meant that different life forms could be generated ex nihilo spontaneously out of nothing. The idea of spontaneous generation seemed to be supported by evidence. For instance, before there were good microscopes and other technologies, it appeared that mosquitoes just arose from mud or that flies could spontaneously arise from meat left out on the counter or that fireflies would just appear after morning dew or, most ominously, that diseases could suddenly appear after an invasion of swamp gas, a view that was very much in line with the miasma theory of the bubonic plague.
The Origin of Life
To take a slightly large context, we know that traditionally, there have been three additional views about how life could be generated in addition to spontaneous generation. The first, and if today’s climate is any indicator, the most persistent is special creation. This is the view that all organisms on earth were specially created by God. Today, most people will recognize that the process of fertilization of an ovum by a sperm is required but there is still much sympathy for the idea that the first humans were created directly by a creator/God.
A second prospect, advanced by scientists such as the late Francis Crick, is that life on earth came from some other planet and arrived on earth via a meteor. This is view is called panspermia. While it is intriguing to consider that life might have existed somewhere else in the universe prior to becoming established on earth, panspermia does nothing to resolve the issue of how life originally arose, so we will ignore it here.
Finally, we have the third idea and the one considered scientifically correct today, and that is biogenesis. In this view, life arose from naturalistic processes that played out over large spans of geological time. Much of the last two centuries has been spent trying to rule out which of the competing ideas could be ruled out based on evidence. Pasteur was critical to this process.
Of the four potential explanations for the origin of life, special creation has been the most persistent. Many scientists are people of faith and are not inclined to rule out the possibility that, at some level, life arose through the action of a deity. The question for those scientists is to resolve the level of that intervention. Here, to forge a religious view that is consistent with scientific principles, most scientists who believe in God will assert that God established the physical laws by which the universe operates and let those laws operate to create the world as we now know it.
There are some scientists who adopt a more biblically literal view of this issue and it is here that the arguments begin. How do we reconcile a literal view of Genesis with a science-based on the testing of hypothesis. In our experience, it can’t be done and, despite the number of times we have asked, no one has yet brought us a testable hypothesis that would allow us to determine whether God created a life through an act of divine fiat. The account of creation spelled out in the Bible may be a narrative of great beauty and it may speak to millions in a heartfelt way. But evidence to support it is not found in science. Despite the great merits of creation stories given in divine texts, they are not science and therefore have to be maintained and revered for reasons other than their scientific validity.
While those who genuinely and deeply believe in creation stories given in divine texts will not be persuaded by this argument, we do have to acknowledge that there isn’t a single creation story on which we can pin the status of being definitive. Most world religions have a story that explains how God (or the gods) brought the world and living organisms into being. We find such accounts in Native American texts, among the Babylonian cultures and in many other societies. The creation story described in Genesis is one of these accounts and shows many commonalities with other creation stories. But again, whatever their merits as metaphor, we cannot consider these stories to be scientific and it must be the criterion we use to evaluate claims that the account of creation presented in Genesis or any other divine text is literally true.
We’ve already ruled out panspermia because, even if it is true, and there is no definitive proof that it is, and it doesn’t explain the origin of life. It merely places the origin on some other planet and doesn’t resolve the fundamental issue of where life came from and how it arose.
This brings us to the third idea, namely, that of spontaneous generation. While the idea sounds fanciful today, 300 years ago, spontaneous generation seemed to explain the available data. Further, the idea of spontaneous generation reveals much about the scientific methods and the practice of science at the time. The data people could collect seemed to validate the idea that inorganic substances like mud or dew could yield living organisms. However, that would change with the advent of the modern experimental methods coupled with advances in technology, as we will see in a minute.
The final idea, biogenesis is still in play. We will have already told you that this is the view that scientists consider to be correct today. In essence, what biogenesis means is that all life arose from previously existing forms of life or omne cellula e cellula if you prefer Latin. Now, of course, this doesn’t tell you where the first living cell came from, as critics will be quick to point out. Scientists are putting that puzzle together but its full explication is beyond the scope of this discussion. It is sufficient to say that the biogenesis account places the arrival of the first reproducing cell as an accumulation of naturalistic processes that first produced a series of critical biomolecules (likely RNA and proteins) that culminated in the ability to store information in nucleotides (probably RNA) in a way that allowed for self-replication. Having cast off panspermia as being unhelpful, and special creation as being nonscientific, the task now is to decide whether spontaneous generation or biogenesis has more explanatory power for the origin of life.
Germ Theory of Disease
And that takes us to an early experiment by Francesco Redi, an Italian scientist who was among the first to challenge the idea of spontaneous generation in 1668. As you may remember from our discussion of Insectotheology and Fredrich Christian Lesser, Redi’s experiment was simple. In each of two open-mouth containers, he put a piece of meat. He left one jar completely open to the environment. The other jar was covered with a fine mesh screen, which prevented big, visible organisms like flies from having access to the meat. Only the unscreened meat teemed with maggots a few days later suggesting that flies were the source of new life forms in the meat. This was the first definitive nail in the coffin of spontaneous generation, which would be borne out by many subsequent experiments.
And now we get to Pasteur’s work on and his contributions to the question of whether or not spontaneous generation was a valid explanation for how living things arose. In essence, Pasteur did for microbes what Redi had previously done for flies nearly 200 years later (1859). Namely, Pasteur showed that if you excluded microorganisms from nutrient broth, no spoilage of the broth would occur just as excluding flies from meat meant that no maggots would infest the meat. Pasteur did this with a simple experiment involving a goose-necked flask (Figure 24.1). The flasks consisted of a normal glass receptacle that contained the broth. However, the flasks also had a long thin piece of glass attached to the receptacle that was curved to a shape that resembles a goose’s neck. This was a critical addition to the experiment because if the contents of the flask were heated up and then allowed to cool, water vapor would condense and settle into the bend in the goose neck in such a way that it plugged the tube and prevented entry of anything found in the air from getting into the receptacle where the broth was held in the flask.
Like Redi’s experiment, the design was quite simple. Pasteur set up three goose-necked flasks, all of which were filled with nutrient broth that would normally support the growth of microbes. Pasteur then boiled the broth to kill any microbes present in the broth initially. Then, he plugged one of the goose-necks with condensed water as previously described. This prevented airborne microbes from entering the flask. On the second flask, Pasteur broke the goose-neck off of the flask which meant that the second flask was now open to the surrounding air. If microbes were present in the air, they could enter the second flask and cause the contents to spoil. In the third flask, he tipped the broth so that it was able to get into the goose neck and remove the water plug.
As expected, the water plug in the first flask prevented the entry of microbes. As a result, there was no spoilage of the broth in the flask with the water-plugged gooseneck. However, the unplugged flasks, either where the neck was broken or where the plug was removed by tipping, were invaded by microbes from the air and the broth spoiled. This meant that invisible life forms circulating in the air were capable of reproducing in nutrient broth. This was the second nail in the coffin of spontaneous generation and it also had the very important function of suggesting that microorganisms that circulate in the air could be the source of human, animal and plant diseases.
Thanks to Pasteur and Redi, spontaneous generation was laid to rest and biogenesis won out. The idea that all life comes from pre-existing cells achieved that status of theory. Not only were the proponents of spontaneous generation a little upset, but there were some unhappy echoes among religious communities. Plus, Pasteur showed that there were invisible disease-causing organisms floating around in air. This had very important ripple effects in explaining modes of disease transmission and, equally as important, how to prevent that transmission. In time, the notion that invisible microscopic organisms could cause illness became known as the Germ Theory of Disease.
Pasteur took the research one step further. Consistent with Pasteur’s faith in the scientific method, Pasteur actually performed experiments with microbes (which were not visible to the eye) to show that they could cause spoilage in diverse media, which could be prevented by eliminating any existing microbes in wine and milk, and then sealing the containers to prevent other microorganisms from getting inside. Hence, the process of pasteurization was born.
Pasteur had the gift of seeing the greater impact of his work. If beverages could be contaminated by microbes, maybe humans and animals could be “contaminated” by microbes too. Instead of causing spoilage, as in the case of milk and wine, maybe the invasion of plants and animals by microbes could cause disease? IF that were the case, then maybe diseases could be prevented by keeping microbes out of the body. Further, if microbes were causing diseases in animals, then using antiseptic techniques should prevent infections of wounds in surgery and requiring physicians to wear face masks and wash their hands between patients should further reduce diseases caused by microbes. All of these ideas were enshrined in the Germ Theory of Disease—a theory for which Pasteur laid the groundwork. It led to many salutary benefits including reduction in maternal deaths following child birth and the development of vaccines from heat-killed pathogens.
Insects and Germ Theory
So how did insects contribute to this? Pasteur had already established himself as one of the leading scientists of his time and that recognition was about to bring him further acclaim. An issue arose in the French silk industry of which the French were very proud and heavily invested in its success. An entomologist, Agostino Bassi made the astonishing argument (although it wasn’t astonishing to Pasteur) that a living entity caused disease and death in silkworm cultures. Bassi specifically identified a macroscopic fungus, visible to the naked eye, as the likely culprit. As a result of Bassi’s work on the problem, the fungus in question was named after him: Beauvaria bassiana. It was one factor that was causing disease in French silkworms.
While scientists learned that the fungus could be controlled by separating visibly affected silkworms from the culture, there were clearly other organisms causing disease among silkworms. French officials were well-acquainted with Pasteur’s work on microbes and invited him to become involved in research on the problem. Pasteur’s work on the silkworm is credited with saving the silk industry in France. It was the basis for a thriving garment industry in France and very important to the economy. In fact, many families raised their own silkworms so that they could supply their own silk for clothing or for retail.
The rearing methods for silkworms predisposed the silk farms to disease. Anytime an organism is raised in highly concentrated conditions, as silkworms were and are, it makes it easy for a pathogen to find a host and spread throughout the population. A second problem was the need for very high-quality silk for the garment industry. As we previously discussed, the silkworm uses silk to make a protective cocoon that shelters the pupal stage of the insect during a very vulnerable lifestage. In industrial production, the cocoons are collected before the adult ecloses. The cocoon containing the pupa is boiled alive. The cocoons are then given to weavers to unravel. Each cocoon is produced from a single strand of silk between 1,000–1,300 feet long. It is important to take the cocoons before the adults emerge or the single strand of silk will be broken into several short and unusable strands when the adult moth emerges. To keep the culture going, a small number of cocoons are allowed to finish the lifecycle. But the majority of cocoons are harvested before the adults emerge so that the silk thread will remain intact.
Unfortunately, a fairly major disaster hit the silk industry in 1849. Specifically, a pandemic (worldwide) silkworm disease broke out. Owing to the economic consequences, the French Government requested Pasteur’s intervention in 1865 in recognition of his eminence as a microbiologist. Pasteur quickly determined that a “corspuscle” was responsible for the disease. Pasteur helped growers determine which cocoons were healthy and which were infected by the corpuscle. He helped growers isolate the healthy cocoons and prevent them from being infected. In doing so, Pasteur realized two accomplishments: he cemented the legitimacy of the Germ Theory of Disease or the idea that microorganisms could cause disease and single-handedly saved the silk industry in France.
But what exactly was killing the moths? The description corpuscle wasn’t very definitive. Pasteur wanted to know more. He observed that the disease seemed to start on the surface of the silkworms where a dusting of something that looked like pepper could be seen as the first sign of infection. He called this the pebrine form. Being a thorough scientist, he examined 1000’s of cocoons under a microscope. During these observations, he detected a “globule” which turned out to be the protozoan that was infecting the cocoons. Pasteur logically surmised that if the moths were shown to be free of the globules/corpuscles, they should produce uninfected, healthy eggs.
To be sure about this assertion, Pasteur performed some experiments. He painted mulberry leaves, the food that silkworm consume, with extracts made from either diseased or clean caterpillars. He then fed the mulberry leaves that had been painted with extracts to new, uncontaminated silkworms. Only the caterpillars that fed on the leaves painted with extract from diseased silkworms had corpuscles/globules as adult moths and laid infected eggs. This was what he predicted would happen.
Unfortunately, other experiments showed that adult moths emerging from larvae without globules could become sick. This was a very unexpected and problematic finding. There was a lot on the line because of the importance of silk to the French economy. People also had a lot of faith in Pasteur because he was so eminent. The news that silk moth adults that emerged from larvae that did not have globules and should, therefore, be disease-free was devastating both to the hopes of the silk industry and to the reputation of Pasteur. But Pasteur had a firm belief in the scientific method and the idea that when in doubt, one should experiment. So that is what he did. He ran new experiments for months. At last, Pasteur discovered that there were two diseases caused by two different microbes, one which produced globules and caused the silkworms to have a peppered or pebrine appearance and the other of which didn’t. These silkworms were flaccid, hence the name “flacherie” for this disease. These larvae also turned dark brown and died from what was essentially lethal diarrhea. The second microbe that caused flaccherie, a virus, was transmitted in silkworm frass (excrement). To prevent transmission, Pasteur recommended a simple change in the silkworm management program: in addition to screening and removing disease-ridden moths that contained pebrine globules, silkworm producers should also remove all frass from their colonies. These two measures eliminated the two different microbes that were infecting silkworms and solved the problem. Not only was the silkworm industry saved, but so was Pasteur’s well-earned reputation.
Chapter Summary
In the end, it must be acknowledged that Pasteur was not the only scientist who contributed to the development of the germ theory of disease. However, Pasteur provided much of the experimental evidence to support it and give the idea scientific validity. Secondly, Pasteur was ahead of his time in understanding that, in order to be widely accepted, scientific ideas must be explained not only to other scientists, but to nonscientists around Europe. Even science has to resonate with the public to enjoy acceptance as a legitimate scientific theory. Pasteur was very good at this element of science as well. Finally, Pasteur had an eye and talent for extracting practical applications from his experimental work, such as preventing milk and wine from spoiling and reducing maternal deaths with improved hygiene. For all of these reasons, Pasteur is recognized as the father of the Germ Theory of Disease. And an insect, the lowly silkworm, helped him to earn that distinction.
References
Chapter 24 Cover Photo: Beauvaria bassiana. Public Domain: Stefan Jaronski. Accessed via commons.wikimedia.org
Figure 24.1: Pasteur’s experiment. CC-BY-SA 4.0: Kgerow16. Accessed via https://commons.wikimedia.org/w/index.php?curid=40506737
List 24.1: Scientific theories. Data compiled by S. W. Fisher.
Additional Readings
Keim, A. and L. Lumet (2015). Louis Pasteur. CreateSpace Independent Publishing Platform, 80 pages.
Pasteur, L. (1996). Germ Theory and Its Applications to Medicine and on the Antiseptic Principle of the Practice of Surgery. Great Minds Series, Prometheus Books, 144 pages.