Origin of Microorganisms
The discovery of microorganisms raised an intriguing question: "Where did these microscopic forms originate?" The theory of spontaneous generation suggested that organisms, such as tiny worms, can arise spontaneously from non-living material. It was completely debunked by Francesco Redi, an Italian biologist and physician, at the end of the seventeenth century. By a simple experiment, he demonstrated conclusively that worms found on rotting meat originated from the eggs of flies, not directly from the decaying meat as proponents of spontaneous generation believed. To prove this, he simply covered the meat with gauze fine enough to prevent flies from depositing their eggs. No worms appeared.
Theory of Spontaneous Generation Revisited
Despite Redi's findings, the theory of spontaneous generation was difficult to disprove, and it took about 200 years more to refute this idea. Because the gauze used by Redi could not prevent the development of microscopic organisms, new experiments were needed to refute the theory. The typical experiment
Experiments of Pasteur
One giant in science who did much to disprove the theory of spontaneous generation was the French chemist Louis Pasteur, considered by many to be the father of modern microbiology. In 1861, Pasteur published a refutation of spontaneous generation that was a masterpiece of logic. First, he demonstrated that air is filled with microorganisms. He did this by filtering air through a cotton plug, trapping organisms that he then examined with a microscope. Many of these trapped organisms looked identical microscopically to those that had previously been observed by others in many infusions. Infusions are liquids that contain nutrients in which microorganisms can grow. Pasteur further showed that if the cotton plug was then dropped into a sterilized infusion, it became cloudy because the organisms quickly multiplied.
Most importantly, Pasteur's experiment demonstrated that sterile infusions would remain sterile in specially constructed flasks even when they were left open to the air. Organisms from the air settled in the bends and sides of these swan-necked flasks, never reaching the fluid in the bottom of the flask (figure 1.2). Only when the flasks were tipped would bacteria be able to enter the broth and grow. These simple and elegant experiments ended the arguments that unheated air or the infusions themselves contained a "vital force" necessary for spontaneous generation.
Trapped air escapes from open end of flask
Bacteria and dust from air settle in bend
Broth allowed to cool slowly
Broth stays sterile indefinitely
Flask tilted so that the sterile broth comes in contact with bacteria and dust from air
Figure 1.2 Pasteur's Experiment with the Swan-Necked Flask If the flask remains upright, no microbial growth occurs. (1-3) If the flask is tipped, microorganisms trapped in the neck reach the sterile liquid and grow. (4, 5) Why did bacteria grow in the flask only after the flask was tipped?
Experiments of Tyndall
Although most scientists were convinced by Pasteur's experiments, others refUsed to give up the concept of spontaneous generation. Some scientists still could not verify Pasteur's results. One of these was an English physicist, John Tyndall. It was Tyndall who finally explained differences in experimental results obtained in different laboratories and proved Pasteur correct. Tyndall concluded that different infusions required different boiling times to be sterilized. Thus, boiling for 5 minutes would sterilize some materials, whereas others, most notably hay infusions, could be boiled for 5 hours and they still contained living organisms! Furthermore, if hay was present in the laboratory, it became almost impossible to sterilize the infusions that had previously been sterilized by boiling for 5 minutes. What did hay contain that caused this effect? Tyndall finally realized that heat-resistant forms of life were being brought into his laboratory on the hay. These heat-resistant life forms must then have been transferred to all other infusions in his laboratory on dust particles, thereby making everything difficult to sterilize. Tyndall concluded that some microorganisms could exist in two forms: a cell that is readily killed by boiling, and one that is heat resistant. In the same year (1876), a German botanist, Ferdinand Cohn, also discovered the heat-resistant forms of bacteria, now termed endospores. The following year (1877), Robert Koch demonstrated that anthrax was caused by Bacillus anthracis and that the usual means of transmission in animals was by means of resistant spores. In 2001, the deliberate transmission of anthrax to humans by means of spores was instigated by bioterrorists in the United States. ■ endospores, p. 67
The extreme heat resistance of endospores explains the differences between Pasteur's results and those of other investigators. Organisms that produce endospores are commonly found in the soil and most likely were present in hay infusions. Because Pasteur used only infusions prepared from sugar or yeast extract, his broth most likely did not contain endospores. At the time these experiments on spontaneous generation were performed, the importance of the source of the infusion was not appreciated. In hindsight, the infusion source was critical to the results observed and conclusions drawn.
These experiments on spontaneous generation point out an important lesson for all scientists. In repeating an experiment and comparing results with previous experiments, it is absolutely essential to reproduce all conditions of an experiment as closely as possible. It may seem surprising that the concept of spontaneous generation was disproved less than a century and a half ago. Table 1.1 lists some of the other important advances in microbiology that have been made in the course of history. Rather than cover the entire history of microbiology here, we will return to many of these major milestones in more detail as our study of microbiology continues. How far the science of microbiology and all biological sciences have advanced over the last 140 years!
The First Microorganisms
Where did microorganisms really come from? No one knows, but many theories abound. The progenitors of microorgan-