MDC Micro Unit 1 Notes PDF

Discovery Era: The discovery of Microbial World and Microscope: (Contributions of Anton von Leeuwenhoek and Robert Hooke) Active in the 17th century, Robert Hooke is one of the most important scientists of his generation and contributed in an amazing variety of scientific fields. He was the first...

Discovery Era: The discovery of Microbial World and Microscope: (Contributions of Anton von Leeuwenhoek and Robert Hooke) Active in the 17th century, Robert Hooke is one of the most important scientists of his generation and contributed in an amazing variety of scientific fields. He was the first to discover the cell. Robert Hook refined the design of the compound microscope and used it to study the bark of a cork tree. He took a good clear piece of cork, and with a sharpened knife he cut a piece of it and examined it with a microscope. To him the cork appeared a little porous much like a honeycomb. Hooke called the pores as “cells” - the building block of life, because they reminded him of the cells inhabited by monks living in a monastery. In reality, Hooke had observed the empty cell walls of dead plant tissue. (a) Robert Hooke’s microscope (b) Dead plant cells of cork tissue. Microbiology has been defined as the study of organisms and agents too small to be seen clearly by the unaided eye—that is, the study of microorganisms. Because objects less than about one millimeter in diameter cannot be seen clearly and must be examined with a microscope, microbiology is concerned primarily with organisms and agents this small and smaller. THE DISCOVERY OF MICROORGANISMS The first person to publish extensive, accurate observations of microorganisms was the amateur microscopist Antony van Leeuwenhoek (1632–1723) of Delft, The Netherlands. Leeuwenhoek earned his living as a draper and haberdasher (a dealer in men’s clothing and accessories), but spent much of his spare time constructing simple microscopes composed of double convex glass lenses held between two silver plates. His microscopes could magnify around 50 to 300 times. Beginning in 1673, Leeuwenhoek sent detailed letters describing his discoveries to the Royal Society of London. It is clear from his descriptions that he saw both bacteria and protozoa. Contributions of Antony Van Leeuwenhoek (Father of Microbiology) He constructed simple microscopes composed of double convex glass lenses held between two silver plates. His microscopes could magnify around 50 to 300 times. With his microscopes, Leeuwenhoek observed a variety of things like rain water, pond water and scrapings from his own teeth. He saw minute moving objects and called them as “animalcules”. It is clear from his descriptions that he saw both bacteria and protozoa. He was first to record microscopic observations of muscle fibers, blood cells, algae, etc. The development of microbiology suffered for the next 200 years. Little progress was made primarily because microscopic observations of microorganisms do not provide sufficient information to understand their biology. For the discipline to develop, techniques for isolating and culturing microbes in the laboratory were needed. Many of these techniques began to be developed as scientists dealt with the conflict over the Theory of Spontaneous Generation. TRANSITION ERA: THE CONFLICT OVER SPONTANEOUS GENERATION From earliest times, people had believed in spontaneous generation—that living organisms could develop from non-living matter. Even Aristotle (384–322 B.C.) thought some of the simpler invertebrates could arise by spontaneous generation. This view finally was challenged by the Italian physician Francesco Redi (1626–1697), who carried out a series of experiments on decaying meat and its ability to produce maggots spontaneously. Redi placed meat in three containers. One was uncovered, a second was covered with paper, and the third was covered with a cheesecloth that would exclude flies. Flies laid their eggs on the uncovered meat and maggots developed. The other two pieces of meat did not produce maggots spontaneously. However, flies were attracted to the cloth-covered container and laid their eggs on the cloth; these eggs produced maggots. Thus, the generation of maggots by decaying meat resulted from the presence of fly eggs, and meat did not spontaneously generate maggots as previously believed. Leeuwenhoek’s discovery of microorganisms renewed the controversy. Some proposed that microorganisms arose by spontaneous generation even though larger organisms did not. They pointed out that boiled extracts of hay or meat would give rise to microorganisms after a while. In 1748, the English priest John Needham (1713–1781) reported the results of his experiments on spontaneous generation. Needham boiled mutton broth and then tightly closed the flasks. Eventually many of the flasks became cloudy and contained microorganisms. He thought organic matter contained a vital force that could confer the properties of life on non-living matter. A few years later, the Italian priest and naturalist Lazzaro Spallanzani (1729–1799) improved on Needham’s experimental design by first sealing glass flasks containing broth. If the sealed flasks were placed in boiling water for 45 minutes, no growth took place as long as the flasks remained sealed. He proposed that air carried germs to the culture medium, but also commented that the external air might be required for spontaneous generation the medium. The supporters of spontaneous generation believed that heating the air in sealed flasks destroyed its ability to support life. Theodore Schwann (1810–1882) allowed air to enter a flask containing a sterile nutrient solution after the air had passed through a red-hot tube. The flask remained sterile. Subsequently Georg Friedrich Schroder and Theodor von Dusch allowed air to enter a flask of heat-sterilized medium after it had passed through sterile cotton wool. No growth occurred in the medium even though the air had not been heated. Louis Pasteur (1822–1895) came forward to settle the matter once and for all. He placed nutrient solutions in flasks, heated their necks in a flame and drew them out into a variety of curves resembling the neck of a swan while keeping the ends of the necks open to the atmosphere. Pasteur then boiled the solutions for a few minutes and allowed them to cool. No growth took place even though the contents of the flasks were exposed to the air. Pasteur pointed out that no growth occurred because dust and germs had been trapped on the walls of the curved necks. If the necks were broken, growth was seen immediately. Pasteur had not only resolved the controversy by 1861 but also had shown how to keep solutions sterile. THE GOLDEN AGE OF MICROBIOLOGY Pasteur’s work with swan neck flasks ushered in the Golden Age of Microbiology. Within 60 years (1857– 1914), a number of disease-causing microbes were discovered, great strides in understanding microbial metabolism were made, and techniques for isolating and characterizing microbes were improved. Scientists also identified the role of immunity in preventing disease and controlling microbes, developed vaccines, and introduced techniques used to prevent infection during surgery. Golden era of microbiology started with the work of Louis Pasteur (France) and Robert Koch (Germany) which includes rapid advances in this field which led to the establishment of Microbiology as a science. 1) Contributions of Louis Pasteur (Father of Modern Microbiology) After proving the theory of biogenesis, he became interested in the process of fermentation. Fermentation is the conversion of sugars to acid/alcohol and gas under anaerobic conditions (in the absence of oxygen). Some scientists proposed that microbes were involved in fermentation but leading chemists believed that it was purely a chemical reaction. Pasteur (1857 - chemist) became interested in fermentation. He discovered that yeast (unicellular fungus) was responsible for alcohol fermentation. Hence, he concluded that fermentation is caused by microorganisms and this is popularly known as “Germ theory of fermentation”. At the same time French industrialists asked Pasteur to find out the reason of wine and beer souring. To solve this problem, he found out experimentally that it is sufficient to heat wine to only about 50–60 °C for a short time to kill the microbes, and that the wine could subsequently be aged without sacrificing the final quality. In honour of Pasteur, this process is known as "pasteurization". Pasteur referred to the sour wines as diseased and concluded that particular wine disease was caused by a particular microbe contaminating the wine. This finding led Pasteur to state that it could also be caused by microorganisms. His findings led to the concept he proposed, that microorganisms are responsible for diseases - “Germ theory of disease”. 1) Contributions of Robert Koch (Father of Bacteriology) Germ theory of disease The first direct demonstration of the role of bacteria in causing disease came from the study of anthrax by the German physician Robert Koch (1843–1910). Koch used the criteria proposed by his former teacher, Jacob Henle (1809–1885), to establish the relationship between a causative agent and its disease. His criteria for proving the relationship between a microorganism and a specific disease are known as Koch’s postulates. In 1884, he reported that this disease was caused by a rod-shaped bacterium, Mycobacterium tuberculosis; he was awarded the Nobel Prize in Physiology or Medicine in 1905 for his work. Koch’s postulates quickly became the cornerstone of connecting many diseases to their causative agent. He also discovered that cholera is caused by Vibrio cholerae. Koch’s Postulates 1. The microorganism must be present in every case of the disease but absent from healthy organisms. 2. The suspected microorganisms must be isolated and grown in a pure culture. 3. The same disease must result when the isolated microorganism is inoculated into a healthy host. 4. The same microorganism must be isolated again from the diseased host. During Koch’s studies on bacterial diseases, it became necessary to isolate suspected bacterial pathogens in pure culture - a culture containing only one type of microorganism. Koch started by using natural surfaces such as a potato slice to obtain pure cultures, but quickly developed more reliable and reproducible growth media employing liquid nutrient solutions (using meat extracts and protein digests because of their similarity to body fluids) solidified with gelatin, and later with agar. Along with his associate Walther Hesse, Koch observed that when a solid surface was incubated in air, masses of bacterial cells called colonies developed, each having a characteristic shape and color. Koch reasoned that each colony harbored a population of identical cells, that is, a pure culture, and Koch quickly realized that solid media provided an easy way to obtain pure cultures. Some of these developments directly stimulated progress in all areas of bacteriology. Working independently, the German botanist Ferdinand Cohn (1828–1898) discovered the existence of heat-resistant bacterial endospores. He experimented with sterile infusions from dried hay and obtained conflicting results. He could observe growth in tubes. This let him to come to a conclusion that microorganisms might exist in thermostable forms (spores) which become active when proper nutrients are available. Contributions of Joseph Lister (Father of Aseptic surgery) He was deeply interested in the prevention of postoperative sepsis as his patients were dying due to infections after being operated. He was attracted by Pasteur’s Germ theory of disease and concluded that sepsis or wound infection may be due to microbes from the atmosphere. He developed a method to destroy microorganisms in the operating theater by spraying a mist of carbolic acid (phenol) into the air. He applied dressings soaked in carbolic acid on wounds. As a result, there was a marked reduction of postoperative sepsis. He successfully prevented postoperative sepsis by introducing antiseptic techniques. This dramatically reduced the number of people dying due to surgical sepsis. Contributions of Alexander Flemming In 1928, Fleming began a series of experiments involving the common bacteria, Staphylococcus aureus. He had returned from a long vacation with his family to find his laboratory quite messy. An uncovered Petri dish sitting next to an open window became contaminated with fungus. Fleming observed that the bacteria in proximity to the fungus were dying. He realised that it was not the fungus itself but some ‘juice’ it had produced that had killed the bacteria. He named the juice penicillin. He accidentally discovered the 1st antibiotic, Penicillin produced by a fungus Penicillium notatum that destroy several pathogenic bacteria. Contributions of Paul Ehrlich (Father of Chemotherapy) He was a German bacteriologist, who pioneered the technique of chemotherapy in medicine. In order to create drugs capable of specifically killing certain microbes, Ehrlich synthesized a series of specific antimicrobial drugs, the most famous example being arsphenamine (Salvarsan), the first synthetic agent against syphilis. Due to the huge success of this drug, Ehrlich was able to popularize his new concept of a ‘magic bullet’, a drug specifically targeting a particular pathogen without affecting normal host cells. Contributions of Selman Waksman (Father of Antibiotics) Waksman had been studying the Streptomyces spp. since his college student days, specifically studying the organism Streptomyces griseus. Streptomycin was isolated from S. griseus and found effective against tuberculosis by one of Waksman's graduate students, Albert Schatz. These results were later confirmed by Elizabeth Bugie Gregory, whose name was also published with Schatz and Waksman. He won the Nobel Prize in physiology or medicine in 1952 “for his discovery of streptomycin, the first antibiotic effective against tuberculosis.” This distinction earned him the title of “Father of Antibiotics”. Later, he discovered several antibiotics (and introduced the modern sense of that word to name them), and he introduced procedures that have led to the development of many others.