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Harriet Wilson, Lecture Notes Bio. Sci. 4 - Microbiology Sierra College
Introduction to Microbiology
Microbiology may be defined as the science or study of microscopic organisms, i.e., organisms too small to be observed with the naked eye (from the Greek terms – micro = small, Bio = life, and logos = discourse or study of). These microorganisms, or microbes as they are sometimes called, include bacteria, archaea, protozoa and microscopic forms of fungi and algae. Since certain multicellular organisms are microscopic (or have microscopic stages during their life cycles) and some play a role in disease transmission, they are also included in microbiology courses. Non- cellular forms such as viruses, viroids and prions are not true organisms, but since they do infect and reproduce within living organisms, they fall within the realm of microbiology. Eukaryotic cell types Prokaryotic cell types Non-cellular types Protozoa Bacteria Viruses Microscopic algae Archaea Viroids Microscopic fungi Prions Microscopic animals When compared to the other natural sciences, Microbiology is relatively young, i.e., has not existed for very long. Can you think of a reason for this? Exactly! Although humans have been interacting with microorganisms for thousands of years, they are not visible without the aid of a microscope, so nobody knew they were there. The first living microorganisms were observed a little more than 300 years ago, but their significance was not appreciated until nearly 200 years later. Today microbiology is recognized as a subject of major importance since microbes play a role in nearly every aspect of our lives. Early Uses for Microorganisms Humans began to interact with microorganisms long before they were able to observe them or recognized that they existed. Though they sometimes caused disease, microbes were also beneficial under various circumstances, and people began to put them to work. What do you suppose humans first used microorganisms for? Records indicate that prior to 6000 BC human societies such as the Sumerians and Babylonians were using microorganisms to ferment grain and make beer. This practice probably began as soon as people had excess grain to store for future use. Grain stored in depressions in the ground was sometimes wetted by rain, would have been fermented by wild yeasts and alcohol was produced. As people found the fermented grain and juice palatable, they undoubtedly took steps to increase production. Thus the first “beer” was made.
Alcoholic beverages made from rice were produced in China at least as early as 2300 BC and reference to wine is common in association with early cultures. Around 4000 BC the Egyptians discovered that bread dough treated in a certain manner, would rise into a light airy loaf. This was due to yeast cells producing carbon dioxide. The practice of saving a small bit of dough as a "starter" probably began long before people recognized yeast as a microbe. Cultured foods such as cheese and yogurt were initially produced as the result of storing milk without refrigeration (often in containers made of animal skin or stomach). When people found that these products would keep without spoiling longer than would fresh milk, they were made intentionally. So the earliest uses for microorganisms were in food processing and preservation. Wine could be stored longer than fresh grape juice, and cheese longer than fresh milk. The fermentation of materials such as milk, grains, grapes, cabbages, cucumbers etc. yielded products that remained palatable and could be stored for long periods of time. Anton Van Leeuwenhoek (1674-1676) The discovery of microbiology is usually credited to a Dutch naturalist by the name of Anton Van Leeuwenhoek. He is sometimes referred to as the "father of Microbiology". Van Leeuwenhoek ground fine glass lenses (which could magnify objects about 266 times) and observed living microorganisms (which he called "animacules") from a variety of environments. His investigations were apparently made around 1674 but he was rather secretive about his work and did not explain exactly how he made his lenses or his observations. Van Leeuwenhoek’s observations may not have been the first, but they were significant because he made numerous drawings and wrote accurate descriptions of what he saw. He documented his findings. For several years, starting about 1684, he sent correspondence to the British Royal Society or Royal Society of London, and thereby aroused considerable interest in microbiology. Spontaneous Generation (Abiogenesis) Van Leeuwenhoek’s discoveries did much to revitalize arguments between scientists, philosophers and theologians about the origin of life. It was, at one time, generally accepted that living organisms arose spontaneously from non-living material. This belief, sometimes called the theory of abiogenesis or spontaneous generation (a=without, bio=life, genesis=origins or beginnings) was taught by Aristotle around 346 BC. He believed that life could and did appear spontaneously from non-living and/or decomposing materials. For example, he wrote that snakes and frogs came from the mud along river banks, that insects came from dew, that flies arose from decaying meat and that rats sprang from refuse heaps. These, like many other beliefs of the Greek scholars, were maintained until relatively recent times. During the 17th century (1600s), a Belgian clergyman by the name of Van Helmont wrote a recipe for the generation of mice. He suggested that if a dirty garment were placed in a
Louis Pasteur (1860s) Louis Pasteur , a young French chemist and physicist, had been hired by French distillers to determine why the contents of their fermentation vats sometimes turned sour (vinegar) instead of brewing as expected (ethanol). Pasteur determined that microorganisms including bacteria and yeast (fungi) were present in the vats. Over a period of time, he was able to prove that fermentation was indeed the result of microbial activity. By taking samples from various vats and transferring them to fresh juice samples, he was able to show that each type of fermentation product was mediated by a specific type of microorganism. Although they were not pure cultures, the collections of organisms in Pasteur's fermentation vats were predominantly of one type or another, and he was able to identify them with a fair degree of accuracy. Pasteur also developed a process that could be used to greatly reduce the number of unwanted microorganisms in juice. It involved heating the juice briefly to a specific temperature, and thereby killing most of the cells present. Can you guess what this process is called? (Pasteurization) Despite mounting evidence to the contrary, the proponents of abiogenesis continued to argue their cause and to publish their evidence in support of spontaneous generation. Pasteur was irritated by the seemingly endless controversy, and set out to settle the question "once-and-for-all". He reported the results of his experiments in 1864, and is usually credited with disproving the abiogenesis of microorganisms. By passing air through gun cotton, Pasteur was able to show that microorganisms were abundant in air (they had been collected and observed on the cotton). When placed into flasks of broth, these microorganisms grew readily. Pasteur also constructed "goose necked" flasks in which he could boil nutrient broths but which, by their shape, prevented the entrance of microorganisms from air. Though these were left open to whatever "vital forces" might be present in air, no organisms grew. Fortunately, Pasteur's broths contained no endospore- forming bacteria, since endospores are resistant to boiling and had they been present, would have grown. Though Pasteur's work was not universally accepted, he had many supporters. One of these was an English physicist by the name of John Tyndall. Tyndall set up an elaborate box containing only clean (filtered) air, and showed that broths exposed to this clean air did not grow microorganisms. Tyndall also discovered that some microorganisms were very resistant to being killed by boiling, i.e., those that produced heat resistant endospores. This helped to explain the varied results obtained by other investigators. Tyndall found that by alternately boiling and cooling his broths over a period of three days he could eliminate the spore-forming organisms. This process is called tyndallization. Though many investigators worked to disprove the theory of abiogenesis at the microscopic level, it is Pasteur who usually receives credit for finally laying the theory to rest. Once this was accomplished, the supernatural, mysterious or magical aspects of microorganisms were explained away, and Microbiology could be recognized as a true science. Something to consider - Why is the theory of abiogenesis not compatible with science?
Germ Theory of Disease Even after microorganisms were observed and found to play an important role in fermentation, it was a number of years before people recognized their involvement in disease processes. Some of the earliest physicians, including Hypocrites, believed that people could transmit disease from one to another, but they did not understand how. Around 1546, Girolamo Fracastoro , an Italian physician, recorded his belief that disease was due to organisms too small to be seen with the naked eye. This was referred to as the contagion theory, but since Fracastoro had no real proof, his writings were largely ignored. Prior to Microbiology, people generally associated disease with natural phenomena such as earthquakes, floods, or exposure to bad weather. Disease was also attributed to mysterious or supernatural causes. Many religious leaders encouraged the belief that disease resulted from disobedience to God. People stricken by illness and death were undoubtedly being punished for their evil deeds. The threat of such punishment was useful for controlling people. Since people were unaware of disease causing microbes and their manner of transmission, practices we take for granted today (to prevent infection and contamination) did not occur to people. Around 1840 there was a turning point in surgery due to the advent of anesthesia. Prior to that time, people undergoing surgery often died of shock unless the surgeon was quick. The most successful surgeons, therefore, were those who were fastest at their work. With the advent of anesthesia, surgeons could work at a slower pace and their patients did not suffer from shock. Unfortunately however, longer exposure to the microbes associated with the surgeon's hands, instruments and the surrounding air resulted in more wound infections. Physicians did not wash their hands or instruments between patients, and most surgery was conducted in open rooms containing large numbers of people. Patients no longer died of shock, but many died of disease. Around 45% of those undergoing surgical procedures died as a result of the associated wound infections. Joseph Lister (1867) During the 1860s Joseph Lister , an English surgeon, reasoned that surgical infection (sepsis) might be caused by microorganisms. ( Sepsis = The condition resulting from the presence of pathogenic microbes or their products in blood or tissues.) Lister devised methods to prevent microbes from entering the wounds of his patients. His procedures came to be known as antiseptic (against sepsis) surgery, and included hand washing, sterilizing instruments, and dressing wounds with carbolic acid (phenol). Lister was well aware of microorganisms and is credited with developing techniques to obtain and maintain the first pure bacterial cultures. Though he did not publish proof that microbes were responsible for disease, he firmly believed they were. About this same time (1840s), a physician by the name of Ignaz Philip Semmelweis began using antiseptic procedures to prevent "childbirth" or puerperal fever (a serious and often fatal disease associated with infection contracted during delivery). Semmelweis also strongly discouraged doctors involved in conducting autopsies and teaching anatomy (in
Richard J. Petri - developed the Petri dish in which microbial cultures could be grown and manipulated. Fanny Hesse - developed the use of agar as a solidifying agent for microbiological media. Hans Christian Gram - developed the Gram stain , a stain technique that could be used to separate two major groups of disease causing bacteria. Immunization – Using microorganisms in disease prevention In science, many important discoveries are made accidentally, and such was the case with Pasteur's discovery of immunization. In 1880 , Louis Pasteur had isolated the bacteria responsible for causing chicken cholera (organisms similar to the Vibrio cholerae causing cholera in humans). He later arranged for a public demonstration of Koch's postulates and inoculated a number of animals with the pure culture he had prepared. Much to his dismay, the animals did not develop disease symptoms, but remained perfectly healthy. Upon reviewing his records, Pasteur found that the experimental animals had been inoculated with a culture several weeks old. Pasteur reasoned that this old culture would be weakened ( attenuated ) and might therefore be unable to cause disease. He arranged to repeat the demonstration, and this time inoculated the subject animals with a fresh culture. Fortunately he also chose to inoculate a new group of animals with the same culture. The original animals again did not develop disease symptoms, but the newly inoculated animals did. As expected, they all developed cholera and died. Pasteur knew that the experimental animals had all been inoculated with the same type of disease causing bacteria. Since they all came from a similar source, he suspected that exposure to the attenuated culture had somehow made the first ones resistant to the disease. He repeated the experiments and eventually concluded that this was indeed the case. Bacteria that were killed or attenuated could be used to prevent disease. Pasteur called his attenuated cultures vaccines , and thus gave credit to an earlier investigator named Edward Jenner. In 1796, Edward Jenner (a British Physician) reported the use of material scraped from the skin of an individual infected with cowpox to immunize a child against smallpox. Jenner had noticed that dairymaids (young women responsible for milking cows) frequently contracted cowpox, a relatively mild disease, but were resistant to smallpox. Since both of these diseases are caused by viruses, there was no way for Jenner to see the disease causing agents, but his method was successful. He called his technique vaccination (vacca = cow). The Magic Bullet By the early 1900s, physicians knew that microorganisms could cause disease, and under certain circumstances could be used to prevent disease, but they did not know how to cure disease. Many strange and sometimes brutal practices had been used in attempts to cure disease, but most were useless and some were dangerous (for example the ingestion of
precious metals - gold and silver). What was needed was a substance that could be taken into the body and would somehow seek out and kill the pathogenic microorganisms without harming the patient, i.e., a "magic bullet". A German physician by the name of Paul Ehrlich searched for a “magic bullet”, and in around 1910 developed the first effective cure for a bacterial disease. The drug he developed was called salvarsan , and was an arsenic compound that was effective against syphilis. A short time later (1928), Alexander Fleming , a Scottish physician, discovered penicillin. He had noticed that a mold growing on one of his culture plates inhibited the growth of bacteria there, and eventually isolated the substance responsible. Penicillin was among the first antibiotics to be used in the treatment of disease. Although Salvarsan was a synthetic compound, and penicillin is produced by mold, many compounds now used to treat disease in humans (and other animals) are made by bacteria. Thus bacteria play a critical role in health and disease (cause, prevention and cure). During the 20th century, microbiology has expanded and increased in importance. Immunology, virology and molecular genetics (recombinant DNA technology) have arisen as branches of microbiology. New discoveries in microbiology may lead to better methods for food and fuel production as well as environmental remediation that will become more and more critical as the human population continues to expand. Or perhaps microbes will eventually force humans to live in balance with the natural world.
