FUNDAMENTALS OF MICROBIOLOGY

Lesson 1. HISTORICAL PERSPECTIVE OF MICROBIOLOGY

Module 1. History and scope of microbiology

Lesson 1

HISTORICAL PERSPECTIVE OF MICROBIOLOGY

1.1 Introduction

The history of microbiology can be traced to ‘First Epidemics’ on earth, which affected Caveman and were probably waterborne. People had no real understanding of why disease occurred. As the civilization progressed, people started clustering into cities. They increasingly shared communal water, handled unwashed food, and stepped in excrement from casual discharge. The crowding increased and spread water-borne, insect-borne and skin-to-skin infectious diseases. Yet there was no general understanding of why disease occurred. By the 13th century fear of the diseased took a drastic turn in the formation of small leper colonies intended to isolate people carrying the devastating disease caused by Mycobacterium leprae. In 1348, a mass epidemic caused by a single organism, Yersinia pestis, wiped out nearly one third of Europe's population. The Plague spread rapidly in the unsanitary conditions of the Middle Ages, leaving Medieval Europeans defenseless against its devastation. Entire towns succumbed to the disease, leaving the living to dispose of thousands of contaminated corpses. Perhaps the deadliest disease in history, the plague or ‘Black Death’, claimed over 20 million lives and contributed to the fall of empires.

1.1.1 Bubonic plague

The social and political dislocations as a result of Bubonic plague were immense, and the resulting terror was particularly intense because the cause of the disaster was unknown. Not until 500 years later, in 1890, did microbiologists identify the causative organism, Yersinia pestis, carried by infected fleas. Infection spreads among the rats as fleas bite them. But rats have little resistance to plague and usually die. When rats become scarce, the fleas move to humans and a plague epidemic is under way. Although rare today, Bubonic plague still occurs in parts of the world.

1.1.2 Potato blight

The Irish famine Potato blight, a disease of plants, rather than humans, caused by a fungus rather than a bacterium, had even greater impact than the plague. Potato blight was responsible for the great Irish famine of the 1800s. Potatoes were the staple of the Irish diet, so when the fungus Phytophthora infestans infected potatoes, causing them to rot in the fields, the result was devastating. By 1846, the potato harvest was so meager that starvation and hunger-based diseases were widespread. An estimated 1,240,000 people had died, and 1,200,000 more had migrated to other countries. Potato blight remains an economic threat to potato farmers even today.

1.2 Disease and Warfare

Warfare and infectious diseases have always been intimately connected. The poor sanitation, movement of peoples, and malnutrition that a war brings, all foster outbreaks of disease. For example, in 1812 when Napoleon invaded Russia he lost more of his troops to typhus (a bacterial disease) than to all other causes combined. And more of his soldiers died from wound-related bacterial infections (such as tetanus and gas gangrene) than from the wound itself.

1.3 Optical Visual Aids

The first vision aid was invented (inventor unknown- possibly a monk) called a reading stone around 1000 AD. It was a glass sphere that magnified when laid on top of reading materials. In the year 1590, two Dutch eye-glass makers, Zaccharias Janssen and son Hans Janssen experimented with multiple lenses placed in a tube. The Janssens observed that viewed objects in front of the tube appeared greatly enlarged, creating both the forerunner of the compound microscope and the telescope.

1.3.1 Microscopy

Robert Hooke (1635-1703): Robert Hooke, a young English scientist, became the first person to view and describe fungi using a simple compound microscope. In 1665, Hooke published Micrographia, which detailed his observations of tiny cork-like cells resembling ‘little boxes’. Hooke is known principally for his law of elasticity (Hooke's Law) and for his work as ‘the father of microscopy’ - it was Hooke who coined the term ‘cell’ to describe the basic unit of life.

Antony van Leeuwenhoek (1632-1723): Antony van Leeuwenhoek, a Dutch merchant, made small hand-held microscopes as a hobby. Squinting through the lens at specimens held on a pin, he discovered a world of invisible creatures he called animalcules (small animals). He found them almost everywhere he looked: in water droplets, particles of soil and his teeth scrapings. In 1674, Leeuwenhoek communicated his discoveries to the Royal Society of London, sending detailed drawings. His primary observations shook the scientific community and led to expanded use of microscopy as a standard scientific tool (Fig. 1.1). All his drawings and nine of the estimated 500 microscopes that Leeuwenhoek made still exist. The most powerful of these has a magnification of 266, powerful enough to magnify an average-sized bacterial cell to the size of the period at the end of this sentence.

Fig. 1.1 Leeuwenhoek’s microscope

1.4 Spontaneous Generation

The idea that life routinely arises from non-life was supported by Aristotle (Circa 350 BC). According to him, it was: “readily observable that aphids arise from the dew which falls on plants, fleas from putrid matter, mice from dirty hay”. This belief remained unchallenged for more than 2000 years. From ancient times, spontaneous generation was thought to be the origin of many organisms (such as rats and flies) that routinely appeared in certain materials. Microorganisms always appeared suddenly in certain materials (meat juices or plant extracts, for example) that had previously been free of them. It seemed logical that microbes were products of spontaneous generation (the formation of living things from inanimate matter).

Needham versus Spallanzani: For 80 years the above debate continued. Then the proponents of spontaneous generation seemed to gain ground when in 1745 an English clergyman named John Needham did a well-publicized experiment. Everyone knew boiling killed microorganisms. Therefore, he boiled chicken broth, put it in a flask, and sealed it. Microorganisms could develop in it only by spontaneous generation. Experiments with gravy seemed to show that life could be generated from non-living materials.

But an Italian priest and professor named Lazzaro Spallanzani was not convinced. According to him, perhaps microorganisms entered the broth after boiling but before sealing. Therefore, Spallanzani put broth in a flask, sealed it, and then boiled it. No microorganisms appeared in the cooled broth. Still the critics were not persuaded. Spallanzani didn’t disprove spontaneous generation, they said, he just proved that spontaneous generation required air.

Gradually, spontaneous generation was rejected as the origin of visible organisms. Francesco Redi, a physician in Italy, played a major role. In 1665 he did experiments with covered and uncovered jars of meat. He showed that maggots (fly larvae) developed only in meat that flies could reach to lay eggs on. Apparently, spontaneous generation did not occur, at least in the case of flies. Instead, flies and by extension all living things come only from preexisting living things. Still, many people believed that microorganisms were an exception to this rule. They are very simple and they always appear, in large numbers, soon after a plant or animal dies. Could decomposition form microorganisms instead of microorganisms causing decomposition? Controversy of spontaneous generation gained momentum during the late 18th and 19th centuries, when further advances in microscopy allowed people to view bacteria and other microorganisms.

1.5 Pasteur’s Experiments

Remarkably, the controversy continued another 100 years and became a significant barrier to the development of microbiology as a science. Finally, in 1859 the French Academy of Science sponsored a competition to prove or disprove the theory of spontaneous generation of microbes. A young French chemist named Louis Pasteur (1822-1895) entered to counter the argument that air was necessary for spontaneous generation. Pasteur used barriers that allowed free passage of air but not microorganisms. In his most famous experiment, Pasteur boiled meat broth in a flask and then drew out and curved the neck of the flask in a flame in the shape of a swan’s neck (Fig. 1.2). No microorganisms grew in the flask. But when he tilted the flask so some broth flowed into the curved neck and then tilted it back so the broth was returned to the base of the flask, the broth quickly became cloudy with the growth of microbial cells. Gravity had caused the microbial cells that had entered the flask to settle at the low point of the neck. They never reached the broth in the base until they were washed into it. Thus, Pasteur convinced the scientific world that spontaneous generation of microorganisms does not occur even in the presence of air. Pasteur’s simple but elegant experiments grounded microbiology in scientific reality. Microorganisms could now be studied by rational scientific means. Probably the most famous contribution to microbiology by Pasteur is the heating process he developed to kill spoilage microbes while still preserving flavor.

Fig. 1.2 Pasteur’s swan-neck flask

1.6 Fermentation

Another important contribution of Pasteur was defining fermentation. In 1856, the father of one of Pasteur's chemistry students asked him to help him solve some problems he was encountering in his attempt to make alcohol by fermenting beetroot. Often, instead of alcohol, the fermentations yielded lactic acid. At the time, fermentation was believed to be a pure chemical process in which sugar was transformed into alcohol. But in 1857, Pasteur proved that a microscopic plant caused the souring of milk (lactic acid fermentation). Pasteur was able to prove that living cells, the yeast, were responsible for forming alcohol from sugar, and that contaminating microorganisms found in ordinary air could turn the ferments sour. Next, he identified the microorganisms responsible for both normal and abnormal fermentations, and found that through heating wine, beer, milk, or vinegar briefly, certain living organisms could be killed, thereby sterilizing or 'pasteurizing' the substances. He reported his findings in "Mémoire sur la fermentation appelée lactique" (Memoir on the fermentation of lactic acid) in 1857, and "Mémoire sur la fermentation alcoholique" (Memoir on the fermentation of alcohol) in 1860. Pasteur also made notable contributions in the field of vaccination and immunity. Studying cholera, Pasteur found that attenuated organisms, inoculated into poultry, offered protection against virulent strains. Based on this research, he developed the first rabies and anthrax vaccines.

1.7 Immunology

Edward Anthony Jenner (1749-1823) was an English scientist who studied his natural surroundings in Berkeley, Gloucestershire. Jenner is widely credited as the pioneer of smallpox vaccine, and is sometimes referred to as the 'Father of Immunology'. Though Pasteur's achievements in microbial immunity were revolutionary, Jenner is credited with inventing the first vaccine against smallpox in late 1700s. In 1796, Jenner developed a controversial experiment to determine the validity of rumours that were circulating in rural communities. Milkmaids and villagers often recanted, "if you want to marry a woman who will never be scarred by the pox, marry a milkmaid." Jenner speculated that becoming infected with cowpox could offer protection against the more virulent smallpox. To test his hypothesis, he created an inoculation with scrapings of cowpox lesions from the fingers of Sarah Nelmes, a young milkmaid and injected it into an 8-year old boy named James Phipps. As expected, James developed the mild fever and cowpox lesions typical of the disease. After a few weeks of recovery, Jenner injected James with the live smallpox virus and found that the boy was indeed protected from the disease. In 1798, Jenner published his findings and presented them to the Royal Society.

1.8 Germ Theory of Disease

Once spontaneous generation of microbes was disproved, microbiology exploded. It changed from an observational science to an experimental science. The way was opened to study the cause of infectious diseases. Building on Pasteur’s work, a German physician, Robert Koch, proved that microorganisms (germs, as they were and are still sometimes called) cause disease. He showed further that specific microorganisms cause specific diseases. Koch also introduced higher scientific standards of rigor to microbiology, as exemplified by those called Koch’s postulates.

1.8.1 Koch’s postulates

In 1876 Robert Koch while studying anthrax, a disease of cattle and sheep that also affects humans established “scientific rules” to show a cause and effect relationship between a microbe and a disease known as Koch’r postulates as follows:

• The same organisms must be found in all cases of a given disease.
• The organism must be isolated and grown in pure culture.
• The isolated organism must reproduce the same disease when inoculated into a healthy susceptible animal.
• The original organism must again be isolated from the experimentally infected animal.

During his work on anthrax, Koch made another critically important contribution to microbiology. He developed a technique to obtain a pure culture of a bacterium (one that contains only a single kind of bacteria) and propagate it. In nature many kinds of bacteria are found growing together (mixed cultures). It is difficult to do valid experiments using mixed cultures because they are so complicated, although such studies are now becoming more common.

1.9 Antiseptic Surgery

Joseph Lister (1827-1912), a British surgeon was the pioneer of antiseptic surgery, who promoted the idea of sterile surgery while working at the Glasgow Royal Infirmary. Lister successfully introduced carbolic acid to sterilise surgical instruments and to clean wounds, which led to reduced post-operative infections and made surgery safer for patients.

1.10 Sterilization

In 1877, John Tyndall published his method for fractional sterilization and clarified the role of heat resistant factors (spores) in putrefaction. Tyndall's conclusion added a final footnote to the work of Pasteur and others in proving that spontaneous generation is impossible. For some of his experiments on light and gases he needed to cleanse the air of particles and so developed a completely novel way to assay the purity of air. ‘The eye being kept sensitive by darkness’, Tyndall reported, ‘a concentrated beam of light was found to be a most searching test for suspended matter – a test indeed indefinitely more searching than that furnished by the most powerful microscope’. Using his new piece of equipment, which relied on observing light scattered by dust particles, Tyndall quickly made a series of important observations on the properties of the ‘floating matter’ of air.

1.11 Milk Fermentation

In 1878, Joseph Lister published his study of lactic fermentation of milk, demonstrating the specific cause of milk souring. His research was conducted using the first method developed for isolating a pure culture of a bacterium, which he named Bacterium lactis.

1.12 Virology

Virology, the study of viruses, began in 1892, when the Russian microbiologist Dmitri Iwanowski discovered the tobacco mosaic virus. Iwanowski was studying a disease of tobacco plants called tobacco mosaic disease. To identify its cause, he forced juice from diseased plants through filters that retained the smallest bacteria. He found the filtered juice still caused disease. Because bacteria were believed to be the smallest microorganisms, Iwanowski first thought his methodology might be flawed. But repeated experimentation convinced him that minute disease-causing agents were passing through the filter. He called these tiny agents ‘filterable viruses’. They could not be seen, even under the most powerful microscopes of that time. Until the electron microscope was developed in the 1930s, we knew viruses existed.

1.12.1 Bacteriophage

In 1915, the first discovery of bacteriophage was done by Frederick Twort. Twort's discovery was something of an accident. He had spent several years growing viruses and noticed that the bacteria infecting his plates became transparent. Later on in 1917 Felix d'Herrelle independently described bacterial viruses and coined the name ‘bacteriophage’. In 1926, Thomas Rivers distinguished between bacteria and viruses, establishing virology as a separate area of study.