Introduction to Microbiology

History of Microbiology

Golden Age of Microbiology

The Golden Age of Microbiology was a time of tremendous advancement during the mid-19th through early 20th centuries with the discovery of the role microbes play in disease and identification of many pathogen.

As early as the sixth century BCE scholars wondered whether a disease was passed from person to person via tiny agents that could not be seen. However, no definitive investigation took place for centuries because researchers lacked the tools they needed to study microorganisms, or microbes, organisms too small to be seen without magnification. Initially, hand lenses provided enough magnification to see large, previously unknown unicellular eukaryotic organisms and stimulated further refinement of lenses that eventually lead to the development of microscopes with compound lenses.

English physicist and chemist Robert Hooke paved the way for the study of cells (a word that he coined) with his book Micrographia, which was published in 1665. Hooke observed cork cells using a microscope.
Robert Hooke, a scientist and founding member of the Royal Society, published Micrographia in 1665. It was the first major published study of microscopic imagery.
Credit: Library of Congress
Around the same time, Dutch scientist Antonie van Leeuwenhoek built a microscope of his own design and used it to observe protozoa, unicellular organisms from the domain Eukarya, in water. This was the first observation made of still-living cells, as well as the first observation of living microbes. Later, van Leeuwenhoek also observed bacteria and documented his observations. His discoveries were published in the Royal Society of England's Philosophical Transactions in 1673, ushering in what is known as the Golden Age of Microbiology. Following van Leeuwenhoek's discovery, scientists flocked to the new scientific discipline of studying the tiny organisms only visible under magnification.
Antonie van Leeuwenhoek, a Dutch scientist, was the first to publish on microscopy of living microorganisms.
Credit: Metropolitan Museum of Art Purchase, Mrs. Carl L. Selden Gift, in memory of Carl L. Selden, 1987
In 1796, English physician Edward Jenner observed that survivors of cowpox were immune to smallpox infection. He took fluid from pustules on cows infected with cowpox, a virus closely related to human smallpox, and purposefully infected patients, which led to smallpox immunity. This conferred complete or partial immunity to the majority of the inoculated patients. Jenner did not identify the pathogen responsible for smallpox, but his use of inoculation to protect against infection was invaluable and resulted in his informal title as the "father of immunology." Vaccination, the technique of exposing people or animals to a form of a pathogen to develop immunity to it, is still in use today, although the methods have changed.
In 1796, Edward Jenner found that inoculating patients with cowpox provided most of them with partial or complete immunity from smallpox. His discovery eventually led to the vaccinations we use today and, because of such vaccines, there has not been a natural outbreak of smallpox in the United States in several decades.
Credit: National Library of Medicine

Foundations of the Germ Theory of Disease

The germ theory of disease, introduced by Louis Pasteur in the mid-19th century, states that infectious diseases are caused by microbes.
In the mid-19th century French biologist Louis Pasteur demonstrated that spontaneous generation—or the growth of microorganisms without the contamination of a medium, a provided nutritious substance for microbes to grow—did not occur. To demonstrate this, he boiled a liquid medium (broth) in flasks. The flasks had a long neck that formed an S shape away from the bulb of the flask and were open to the air on one end. Boiling causes water vapor to rise in the neck and then condense into liquid water in the S-bend of the neck, which prevents microbes from the air outside from moving into the broth. When he left the neck of the flasks intact, no microbial growth occurred. Only when he broke the neck or tilted the flasks, allowing microbes from the air to touch the medium, did growth occur. This demonstrated that it is not the air itself that causes contamination, but rather microbes in the air.

Pasteur's Experiment

Louis Pasteur disproved spontaneous generation. To do so, he developed an experiment in which he boiled flasks containing a medium. The condensation from boiling the medium formed water that settled in the curved neck of the flask. This prevented microorganisms from getting into the flask from the air outside. When the curved neck was left intact, no growth occurred because no microorganisms were able to enter from the outside. When Pasteur broke the neck of the flask, however, growth occurred. Had organisms spontaneously generated, Pasteur would have seen growth in the blocked flask, and their absence provided evidence that microorganisms do not spontaneously generate.
Pasteur applied his technique of boiling growth medium to other liquids as well. He knew that heating liquids killed microbes and applied this process to liquids that were lost by spoilage, such as milk, in a process now known as pasteurization. He also investigated the yeast microbes that cause fermentation, the breakdown of sugar in the absence of oxygen, in wine. When wine was heated, no fermentation occurred and only grape juice remained. This proved that yeasts are required for fermentation to occur. His discoveries were instrumental in the development of the germ theory of disease, the theory that infectious diseases are caused by microbes.
Louis Pasteur, a mid-19th century scientist, contributed much to our understanding of microbial growth and disease. Among his many notable achievements are the invalidation of spontaneous generation, the process of pasteurization, the process of fermentation, and contributions to vaccination and the relationship between microorganisms and disease.
Credit: National Library of Medicine
Also in the mid-19th century, British physician Joseph Lister developed what is known as the aseptic technique, a method for sterilizing surgical instruments and laboratory tools. In Lister’s original conception, aseptic technique also included guidelines for treating wounds, including surgical wounds. These guidelines state that bacteria should never come into contact with a surgical wound. Although the precise methods of keeping wounds free of microbes have changed, the aseptic technique remains in practice today. The most widely used sterilization methods used for laboratory tools include autoclaving, using steam under pressure, and flame sterilization, burning a metal to red hot.
Joseph Lister, a mid-19th century physician, developed a method for sterilizing surgical instruments and laboratory tools, known as the aseptic technique.
Credit: Lister, Joseph, Baron, 1827-1912; Cameron, Hector Clare, (Sir) 1843. Published Oxford The Clarendon Press. Contributor Gerstein - University of Toronto.
In 1849 English physician John Snow observed a pattern in the outbreak of cholera in London. He published his findings and suggested that the handles of water pumps had become contaminated. Later investigations uncovered the fact that many London water pumps were built near cesspits, where people dumped fecal waste. City officials, under Snow's advice, separated water sources and waste dumps, effectively ending the cholera outbreak.

Koch's Postulates

German microbiologist Robert Koch developed four rules, or postulates, that conclusively identify the bacterium that causes a particular disease.
In the mid-19th century, while Louis Pasteur and John Snow were laying the foundations of germ theory, German microbiologist Robert Koch was studying bacteria. He wanted to culture bacteria on a solid medium, and he wanted to grow pure cultures, which are cultures consisting entirely of a single species of bacterium. He first tried using potato slices as the solid medium for bacterial culturing with no success. He later turned to using a gelatinous substance made from algae called agar, which, when supplemented with salt and carbohydrates, served as an excellent solid growth medium. He was able to pick bacteria from a single colony, or spot, on a solid medium plate and then use it to grow a plate or flask of that one strain of bacteria: a pure culture. This practice remains in use today. Using these techniques, he discovered the bacteria that cause the diseases anthrax (Bacillus anthracis), tuberculosis (Mycobacterium tuberculosis), and cholera (Vibrio cholerae), providing evidence that supported the germ theory of disease.
Robert Koch, a mid-19th century microbiologist, developed a solid growth medium that allowed him to grow and study pure bacterial cultures.
Credit: National Library of Medicine
Koch was very interested in connecting a particular bacterium with the disease it caused. In 1890 he published four criteria that establish a causal relationship between a microorganism and its disease, known as Koch's postulates: 1. The microorganism must be present in all cases of the disease and absent in healthy individuals.
2. The microorganism must be able to be isolated from a host and grown in pure culture.
3. Samples from the pure culture must be able to produce the disease when inoculated into a healthy, susceptible laboratory animal.
4. The microorganism must be isolated from the laboratory animal and shown to be identical to the microorganism isolated from the original host.

Koch's Postulates

German scientist Robert Koch developed four steps, known as Koch's postulates, for identifying the microorganism responsible for causing a particular disease.
Microbiologists use Koch's postulates to identify the causal organisms associated with many diseases affecting both humans and livestock. However, many microbes, including viruses and many species of bacteria, cannot be grown outside the normal host. Because these microbes cannot be cultured and because they cannot infect a laboratory animal, microbiologists are not capable of applying Koch's postulates to them.