1.6.3: What Causes Diseases?
- Explain the key points of cell theory and the individual contributions of Hooke, Schleiden, Schwann, Remak, and Virchow
- Explain the key points of endosymbiotic theory and cite the evidence that supports this concept
- Explain the contributions of Semmelweis, Snow, Pasteur, Lister, and Koch to the development of germ theory
While some scientists were arguing over the theory of spontaneous generation, other scientists were making discoveries leading to a better understanding of what we now call the cell theory. Modern cell theory has two basic tenets:
- All cells only come from other cells (the principle of biogenesis).
- Cells are the fundamental units of organisms.
Today, these tenets are fundamental to our understanding of life on earth. However, modern cell theory grew out of the collective work of many scientists.
The Origins of Cell Theory
The English scientist Robert Hooke first used the term “cells” in 1665 to describe the small chambers within cork that he observed under a microscope of his own design. To Hooke, thin sections of cork resembled “Honey-comb,” or “small Boxes or Bladders of Air.” He noted that each “Cavern, Bubble, or Cell” was distinct from the others (Figure \(\PageIndex{1}\)). At the time, Hooke was not aware that the cork cells were long dead and, therefore, lacked the internal structures found within living cells.
Despite Hooke’s early description of cells, their significance as the fundamental unit of life was not yet recognized. Nearly 200 years later, in 1838, Matthias Schleiden (1804–1881), a German botanist who made extensive microscopic observations of plant tissues, described them as being composed of cells. Visualizing plant cells was relatively easy because plant cells are clearly separated by their thick cell walls. Schleiden believed that cells formed through crystallization, rather than cell division.
Theodor Schwann (1810–1882), a noted German physiologist, made similar microscopic observations of animal tissue. In 1839, after a conversation with Schleiden, Schwann realized that similarities existed between plant and animal tissues. This laid the foundation for the idea that cells are the fundamental components of plants and animals.
In the 1850s, two Polish scientists living in Germany pushed this idea further, culminating in what we recognize today as the modern cell theory. In 1852, Robert Remak (1815–1865), a prominent neurologist and embryologist, published convincing evidence that cells are derived from other cells as a result of cell division. However, this idea was questioned by many in the scientific community. Three years later, Rudolf Virchow (1821–1902), a well-respected pathologist, published an editorial essay entitled “Cellular Pathology,” which popularized the concept of cell theory using the Latin phrase omnis cellula a cellula (“all cells arise from cells”), which is essentially the second tenet of modern cell theory. 1 Given the similarity of Virchow’s work to Remak’s, there is some controversy as to which scientist should receive credit for articulating cell theory. See the following Eye on Ethics feature for more about this controversy.
Rudolf Virchow, a prominent, Polish-born, German scientist, is often remembered as the “Father of Pathology.” Well known for innovative approaches, he was one of the first to determine the causes of various diseases by examining their effects on tissues and organs. He was also among the first to use animals in his research and, as a result of his work, he was the first to name numerous diseases and created many other medical terms. Over the course of his career, he published more than 2,000 papers and headed various important medical facilities, including the Charité – Universitätsmedizin Berlin, a prominent Berlin hospital and medical school. But he is, perhaps, best remembered for his 1855 editorial essay titled “Cellular Pathology,” published in Archiv für Pathologische Anatomie und Physiologie , a journal that Virchow himself cofounded and still exists today.
Despite his significant scientific legacy, there is some controversy regarding this essay, in which Virchow proposed the central tenet of modern cell theory—that all cells arise from other cells. Robert Remak, a former colleague who worked in the same laboratory as Virchow at the University of Berlin, had published the same idea 3 years before. Though it appears Virchow was familiar with Remak’s work, he neglected to credit Remak’s ideas in his essay. When Remak wrote a letter to Virchow pointing out similarities between Virchow’s ideas and his own, Virchow was dismissive. In 1858, in the preface to one of his books, Virchow wrote that his 1855 publication was just an editorial piece, not a scientific paper, and thus there was no need to cite Remak’s work.
By today’s standards, Virchow’s editorial piece would certainly be considered an act of plagiarism, since he presented Remak’s ideas as his own. However, in the 19 th century, standards for academic integrity were much less clear. Virchow’s strong reputation, coupled with the fact that Remak was a Jew in a somewhat anti-Semitic political climate, shielded him from any significant repercussions. Today, the process of peer review and the ease of access to the scientific literature help discourage plagiarism. Although scientists are still motivated to publish original ideas that advance scientific knowledge, those who would consider plagiarizing are well aware of the serious consequences.
In academia, plagiarism represents the theft of both individual thought and research—an offense that can destroy reputations and end careers. 2 3 4 5
- What are the key points of the cell theory?
- What contributions did Rudolf Virchow and Robert Remak make to the development of the cell theory?
The Germ Theory of Disease
Prior to the discovery of microbes during the 17 th century, other theories circulated about the origins of disease. For example, the ancient Greeks proposed the miasma theory, which held that disease originated from particles emanating from decomposing matter, such as that in sewage or cesspits. Such particles infected humans in close proximity to the rotting material. Diseases including the Black Death, which ravaged Europe’s population during the Middle Ages, were thought to have originated in this way.
In 1546, Italian physician Girolamo Fracastoro proposed, in his essay De Contagione et Contagiosis Morbis , that seed-like spores may be transferred between individuals through direct contact, exposure to contaminated clothing, or through the air. We now recognize Fracastoro as an early proponent of the germ theory of disease, which states that diseases may result from microbial infection. However, in the 16th century, Fracastoro’s ideas were not widely accepted and would be largely forgotten until the 19th century.
In 1847, Hungarian obstetrician Ignaz Semmelweis (Figure \(\PageIndex{4}\)) observed that mothers who gave birth in hospital wards staffed by physicians and medical students were more likely to suffer and die from puerperal fever after childbirth (10%–20% mortality rate) than were mothers in wards staffed by midwives (1% mortality rate). Semmelweis observed medical students performing autopsies and then subsequently carrying out vaginal examinations on living patients without washing their hands in between. He suspected that the students carried disease from the autopsies to the patients they examined. His suspicions were supported by the untimely death of a friend, a physician who contracted a fatal wound infection after a postmortem examination of a woman who had died of a puerperal infection. The dead physician’s wound had been caused by a scalpel used during the examination, and his subsequent illness and death closely paralleled that of the dead patient.
Although Semmelweis did not know the true cause of puerperal fever, he proposed that physicians were somehow transferring the causative agent to their patients. He suggested that the number of puerperal fever cases could be reduced if physicians and medical students simply washed their hands with chlorinated lime water before and after examining every patient. When this practice was implemented, the maternal mortality rate in mothers cared for by physicians dropped to the same 1% mortality rate observed among mothers cared for by midwives. This demonstrated that handwashing was a very effective method for preventing disease transmission. Despite this great success, many discounted Semmelweis’s work at the time, and physicians were slow to adopt the simple procedure of handwashing to prevent infections in their patients because it contradicted established norms for that time period.
Around the same time Semmelweis was promoting handwashing, in 1848, British physician John Snow conducted studies to track the source of cholera outbreaks in London. By tracing the outbreaks to two specific water sources, both of which were contaminated by sewage, Snow ultimately demonstrated that cholera bacteria were transmitted via drinking water. Snow’s work is influential in that it represents the first known epidemiological study, and it resulted in the first known public health response to an epidemic. The work of both Semmelweis and Snow clearly refuted the prevailing miasma theory of the day, showing that disease is not only transmitted through the air but also through contaminated items.
Although the work of Semmelweis and Snow successfully showed the role of sanitation in preventing infectious disease, the cause of disease was not fully understood. The subsequent work of Louis Pasteur, Robert Koch, and Joseph Listerwould further substantiate the germ theory of disease.
While studying the causes of beer and wine spoilage in 1856, Pasteur discovered properties of fermentation by microorganisms. He had demonstrated with his swan-neck flask experiments (link) that airborne microbes, not spontaneous generation, were the cause of food spoilage, and he suggested that if microbes were responsible for food spoilage and fermentation, they could also be responsible for causing infection. This was the foundation for the germ theory of disease.
Meanwhile, British surgeon Joseph Lister (Figure \(\PageIndex{5}\)) was trying to determine the causes of postsurgical infections. Many physicians did not give credence to the idea that microbes on their hands, on their clothes, or in the air could infect patients’ surgical wounds, despite the fact that 50% of surgical patients, on average, were dying of postsurgical infections. 11 Lister, however, was familiar with the work of Semmelweis and Pasteur; therefore, he insisted on handwashing and extreme cleanliness during surgery. In 1867, to further decrease the incidence of postsurgical wound infections, Lister began using carbolic acid (phenol) spray disinfectant/antiseptic during surgery. His extremely successful efforts to reduce postsurgical infection caused his techniques to become a standard medical practice.
A few years later, Robert Koch (Figure \(\PageIndex{5}\)) proposed a series of postulates (Koch’s postulates) based on the idea that the cause of a specific disease could be attributed to a specific microbe. Using these postulates, Koch and his colleagues were able to definitively identify the causative pathogens of specific diseases, including anthrax, tuberculosis, and cholera. Koch’s “one microbe, one disease” concept was the culmination of the 19th century’s paradigm shift away from miasma theory and toward the germ theory of disease. Koch’s postulates are discussed more thoroughly in How Pathogens Cause Disease .
- Compare and contrast the miasma theory of disease with the germ theory of disease.
- How did Joseph Lister’s work contribute to the debate between the miasma theory and germ theory and how did this increase the success of medical procedures?
Key Concepts and Summary
- Although cells were first observed in the 1660s by Robert Hooke, cell theory was not well accepted for another 200 years. The work of scientists such as Schleiden, Schwann, Remak, and Virchow contributed to its acceptance.
- The miasma theory of disease was widely accepted until the 19th century, when it was replaced by the germ theory of disease thanks to the work of Semmelweis, Snow, Pasteur, Lister, and Koch, and others.
Footnotes
- 1 M. Schultz. “Rudolph Virchow.” Emerging Infectious Diseases 14 no. 9 (2008):1480–1481.
- 2 B. Kisch. “Forgotten Leaders in Modern Medicine, Valentin, Gouby, Remak, Auerbach.” Transactions of the American Philosophical Society 44 (1954):139–317.
- 3 H. Harris. The Birth of the Cell . New Haven, CT: Yale University Press, 2000:133.
- 4 C. Webster (ed.). Biology, Medicine and Society 1840-1940 . Cambridge, UK; Cambridge University Press, 1981:118–119.
- 5 C. Zuchora-Walske. Key Discoveries in Life Science . Minneapolis, MN: Lerner Publishing, 2015:12–13.
- 6 T. Embley, W. Martin. “Eukaryotic Evolution, Changes, and Challenges.” Nature Vol. 440 (2006):623–630.
- 7 O.G. Berg, C.G. Kurland. “Why Mitochondrial Genes Are Most Often Found in Nuclei.” Molecular Biology and Evolution 17 no. 6 (2000):951–961.
- 8 L. Sagan. “On the Origin of Mitosing Cells.” Journal of Theoretical Biology 14 no. 3 (1967):225–274.
- 9 A.E. Douglas. “The Microbial Dimension in Insect Nutritional Ecology.” Functional Ecology 23 (2009):38–47.
- 10 J.M. Jaynes, L.P. Vernon. “The Cyanelle of Cyanophora paradoxa : Almost a Cyanobacterial Chloroplast.” Trends in Biochemical Sciences 7 no. 1 (1982):22–24.
- 11 Alexander, J. Wesley. “The Contributions of Infection Control to a Century of Progress” Annals of Surgery 201:423-428, 1985.