The existence of microscopic organisms was discovered between 1665 and 1683 by two Fellows of the Royal Society, Robert Hooke, and Antoni van Leeuwenhoek. In Micrographia (1665) Hooke presented the first published representation of a microorganism, the micro fungus Mucor.
The Microscope and Discovery of Microorganisms
Even before microorganisms were seen, some researchers suspected their existence and responsibility for disease. Among other things, the Roman philosopher Lucretius ( about 98-55 BC) and the physician Girolamo Fracastoro (1478-1553) suspected that diseases are caused by invisible living beings.
As early as 1658, a monk named Athanasius Kircher (1601-1680) spoke of “worms” that are invisible to the naked eye in decaying bodies, meat, milk, and diarrheal secretions. Although his description was inaccurate, Kircher was the first to recognize the importance of bacteria and other microbes in disease
Robert Hooke (1635–1703) is credited with publishing the first drawings of microorganisms in the scientific literature. In 1665 he published a very detailed drawing of the fungus Mucor in his book Micrographia.
Micrographia is important not only for its exquisite drawings but also for the information about building microscopes. A design discussed in Micrographia was likely a prototype for the microscopes built and used by the amateur microscopist Antony van Leeuwenhoek of Delft.
Although he was probably not the first to see bacteria and protozoa, Antony van Leeuwenhoek, who lived in Delft, Holland from 1632 to 1723, was the first to report his observations with detailed descriptions and drawings.
Leeuwenhoek had the means and the opportunity to pursue his hobby, lens grinding, and microscope making. During his lifetime he manufactured more than 250 microscopes, which consisted of self-ground lenses in brass and silver, of which the strongest magnified around 200 to 300 times.
These microscopes bear little resemblance to today’s compound light microscopes, which achieve magnifications of 1,000 to 3,000 times.
However, the lenses on Leeuwenhoek’s microscopes were well made and Leeuwenhoek had the openness that is so important to an investigator. His descriptions of protozoa were so precise that many of the shapes he described are easy to spot today.
Discovery of Bacteria (Animalcules)
One of Leeuwenhoek’s greatest contributions to biology, the discovery of bacteria or animalcules. Leeuwenhoek carefully recorded his observations in a series of letters to the British Royal Society.
In one of the first letters of September 7, 1674, to Henry Oldenburg, Secretary of the Royal Society, he described the “very small animals” that we recognize as free-living protozoa.
On October 9, 1676, he wrote: In 1675 I discovered living beings in rainwater that had only stood for a few days in a new clay pot glazed blue on the inside. This invited me to look at this water with great attention, especially the small animals that appear to me ten thousand times less than those that can be seen in the water with the naked eye.
He described his little animals in great detail, leaving little doubt that he saw bacteria, fungi, and many types of protozoa. For example, he reported on June 16, 1675, when he examined well water in which he had put a whole pepper the day before:
I discovered an incredible number of very small animals in a tiny drop of water, and these of different types and sizes. They moved with bends like an eel always swims head forward and never tail first, yet these creatures swam both forwards and backward, although their movement was very slow.
His enthusiastic letters were read with interest by British scholars, but the importance of his discoveries was evidently not appreciated. This remarkable man’s talents and acumen is best seen by reading Dobell’s Leeuwenhoek biography.
Before Pasteur’s time, microorganisms were studied mainly to satisfy curiosity about their properties and their relationships with higher living things, without being aware of their importance in fermentation and disease.
First Observation of Anaerobic Bacteria
The popular belief is that Pasteur made the first observation of microbial life in the absence of oxygen gas. Indeed, Pasteur rediscovered the anaerobic lifestyle.
The first person to observe anaerobic microorganisms was Leeuwenhoek, who carried out a remarkable experiment detailed in his letter to the Royal Society of June 14, 1680.
He filled two identical glass tubes about halfway with crushed pepper powder to which some clean rainwater was added. With a flame, he sealed one of the pipes while the other remained open.
After several days I discovered many very small animals in the water of the open tube, of the type of divers, which have their own special movement. After five days, he opened the sealed tube, which had developed some pressure, which forced the liquid out.
He expected to see no “living creatures in this water”. In fact, however, he observed: “a species of a live animalcule that was round and larger than the largest species I said was in the other water.”
Apparently, the conditions in the sealed tube had become quite anaerobic due to the consumption of oxygen by aerobic microorganisms.
In 1913, the great Dutch microbiologist Martinus Beijerinck exactly repeated Leeuwenhoek’s experiment and identified Clostridium butyricum as a prominent anaerobic bacterium in the sealed pepper infusion tube liquid.
Beijerinck commented: We, therefore, come to the remarkable conclusion that Leeuwenhoek in his experiment with the completely closed tube undoubtedly cultivated real anaerobic bacteria and saw this, which was only to occur again after 200 years, namely around 1862 by Pasteur.
It is understandable that Leeuwenhoek, a hundred years before the discovery of oxygen and the composition of air, was unaware of the significance of his observations.
But the fact that he observed an increased gas pressure from fermentative bacteria in the closed tube and also saw the bacteria, in any case, proves that he was not only a good observer but was also able to conceive an experiment from which a conclusion could be drawn.