ADVERTISEMENTS:
Here is a term paper on the ‘Prokaryotes’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short term papers on the ‘Prokaryotes’ especially written for school and college students.
Term Paper on the Prokaryotes
Term Paper Contents:
- Term Paper on the Introduction to Prokaryotes
- Term Paper on the Prokaryotic Cell
- Term Paper on the Diversity of Form
- Term Paper on the Types of Motility in Prokaryotes
- Term Paper on the Reproduction and Resting Forms in Prokaryotes
- Term Paper on the Prokaryotic Nutrition
- Term Paper on the Blue-Green “Algae”
- Term Paper on Viruses
- Term Paper on Microorganism and Human Ecology
ADVERTISEMENTS:
Term Paper # 1. Introduction to the Prokaryotes:
ADVERTISEMENTS:
The prokaryotes are the oldest and most abundant group of organisms in the world. About 1,600 species have been described. They are the smallest cellular organisms; a single gram (about 1/28 of an ounce) of fertile soil can contain as many as 2½ billion individuals.
The success of the prokaryotes, biologically speaking, is undoubtedly due to their rapid rate of cell division and their great metabolic versatility. Growing under optimum conditions, a population can double in size every 20 or 30 minutes. Prokaryotes can survive in many environments that support no other form of life.
They have been found in the icy wastes of Antarctica, the near- boiling waters of natural hot springs, and even in the dark depths of the ocean. Some bacteria are among the very few modern organisms that can survive without free oxygen, obtaining their energy by anaerobic glycolysis. Oxygen is lethal to some types (obligate anaerobes), whereas others can exist with or without oxygen (facultative anaerobes).
A few prokaryotes can form thick-walled, dry spores. These spores are inactive, resistant forms that enable the cells to survive for long periods of time without water or nutrients or in conditions of extreme heat or cold. They may stay dormant for years, and some remain viable even when boiled in water for as long as 2 hours.
From the ecological point of view, prokaryotes are important as decomposers, breaking down organic material to a form in which it can be used by plants. They also play a major role in the process known as nitrogen fixation, by which nitrogen gas (N2) is reduced to ammonia (NH3) or ammonium (NH4+).
Although nitrogen is abundant in the atmosphere, no eukaryotes are able to use atmospheric nitrogen, and so the crucial first step in the incorporation of nitrogen into organic compounds depends largely on a few species of prokaryotes, some free-living and some in symbiotic association with plants. Some forms are photosynthetic, and some few are both photosynthetic and nitrogen-fixing.
Term Paper # 2. The Prokaryotic Cell:
The essential features of a prokaryotic cell are reviewed in Figure 7.2. The cell shown here are the familiar Escherichia coli, one of the eubacteria. The eubacteria, in general, and E. coli, in particular, are the best studied of the prokaryotes.
The Cytoplasm:
The cytoplasm of prokaryotes is relatively unstructured. Except for the blue-green “algae,” the cytoplasm is not divided or compartmentalised by membranes and does not contain any membrane-bound organelles. The most prominent feature within the cytoplasm is the chromosome.
All prokaryotic chromosomes analyzed so far have proved to be one single, continuous (“circular”) molecule of DNA. In addition, a prokaryotic cell may contain one or more plasmids.
The cytoplasm often has a fine granular appearance due to its many ribosomes. These are somewhat smaller than eukaryotic ribosomes but have the same general shape.
ADVERTISEMENTS:
The Cell Membrane:
The membrane surrounding a prokaryotic cell is similar in chemical composition to that of a eukaryotic cell but, with the exception of the mycoplasma (the smallest free-living cells known), it lacks cholesterol or other steroids.
In the aerobic prokaryotes, the cell membrane incorporates the electron transport system found in the mitochondrial membrane of eukaryotic cells, and in one group of photosynthetic prokaryotes (the purple bacteria); the sites of photosynthesis are found in the outer membrane.
In the purple bacteria and in aerobes with large energy requirements, the membrane is often extensively convoluted, with folds extending into the interior. These, of course, greatly increase the working surface of the membrane. The membrane appears to contain specific attachment sites (called mesosomes) for the DNA molecules; these sites are believed to play a role in ensuring the separation of the duplicate chromosomes at cell division.
ADVERTISEMENTS:
The Cell Wall:
Almost all prokaryotes are surrounded by a cell wall, which ranges from 5 to 80 nanometers in diameter and gives the different types their characteristic shapes. Many prokaryotes have rigid walls some have flexible walls and only the mycoplasma have no cell walls at all.
Because most bacterial cells are hypertonic in relation to their environment, they would burst without their walls. (The mycoplasma lives as intracellular parasites in an isotonic environment.)
Chemical Structure of the Cell Wall:
ADVERTISEMENTS:
The cell walls of prokaryotes are complex and contain many kinds of molecules not present in eukaryotes. The walls of prokaryotic cells contain complex polymers known as peptidoglycans, which are primarily responsible for the mechanical strength of the wall.
In addition, in some prokaryotes, large molecules of lipopolysaccharide are deposited over the peptidoglycan layer. Bacterial cell walls that lack the lipopolysaccharide layer combine firmly with such dyes as gentian violet, and those in which it is present do not.
Those that combine with the dyes are known as gram-positive, whereas the others are gram-negative, after Hans Christian Gram, the Danish microbiologist who discovered this distinction. Gram staining is widely used as a basis for classifying bacteria, since it reflects a fundamental difference in the architecture of the cell wall.
This architecture in turn affects various other characteristics of the bacteria, such as their patterns of susceptibility to antibiotics. Gram-positive bacteria are more susceptible to most antibiotics than are gram-negative bacteria.
ADVERTISEMENTS:
They are also more susceptible to lysozyme, an enzyme found in nasal secretions, saliva, and other body fluids, which digests the cell walls of bacteria. In certain bacteria, a gluey polysaccharide capsule, which is secreted by the bacterium, is present outside the cell wall. The function of the capsule is not entirely clear, but its presence is associated with pathogenic activity in certain organisms.
For example, the encapsulated form of Diplococcus pneumoniae is virulent, whereas the non-encapsulated form is generally non-virulent. It appears that the capsule interferes with phagocytosis by host white blood cells.
Flagella and Pili:
Some types of bacteria have long, slender extensions, known as flagella and pili. Bacterial flagella, are made up of monomers of protein known as flagellin, which are assembled in chains and wound around a hollow central core.
The flagella of different species differ slightly in diameter (12 to 18 nanometers), probably due to slight differences in the composition of their flagellin.
Electron microscopy has recently revealed that bacterial flagella are anchored into the cell wall and membranes by a fascinatingly complicated assembly. The filament (the helix of flagellin monomers with its hollow core) terminates in a hook made up of a different protein. The hook is inserted into a basal body that consists of a rod and, in gram-negative bacteria, two pairs of rings, one pair of which is embedded in the cell wall and one pair in the cell membrane. In gram-positive bacteria, only the inner pair of rings is present.
Pili (singular, pilus) are assembled from protein monomers in much the same way that the filaments of flagella are. (You will not be surprised to learn that the protein is called pilin.) They are rigid, cylindrical rods that extend out from the cell, sometimes to a considerable distance. They are shorter and thinner than flagella and are often present in large numbers (hundreds on a single cell).
They serve to attach bacteria to a food source, to the surface of a liquid (where oxygen is present), or, in the case of conjugating bacteria, to one another.
Term Paper # 3. Diversity of Form:
The oldest method of identifying microorganisms is by their physical appearance. The shape of individual prokaryotes is a result, of their cell wall. In addition, different types of bacteria have characteristic patterns of growth, producing colonies that also have a distinctive shape, and many forms have sheaths or other coatings outside their cell walls.
The eubacteria, or true bacteria, for example, may have any one of four forms. Straight, rod-shaped forms like E. coli are known as bacilli; spherical ones are called cocci; long, spiral rods are called spirilla; and short, curved rods, probably incomplete spirals, are called vibrios.
Cocci may stick together in pairs after division (diplococci), they may occur in clusters (staphylococci), or they may form chains (streptococci). One bacterium that causes pneumonia is a diplococcus, while the staphylococci are responsible for many serious infections characterised by boils or abscesses.
ADVERTISEMENTS:
Rod-shaped bacilli usually separate after cell division. When they do remain together, they spread out end to end in filaments, since they always divide in the same plane (transversely). Because these filaments are fungus like in appearance, the combining form myco-(from the Greek word for “fungus”) is often a part of the name of these organisms. Mycobacterium tuberculosis, for example, the cause of tuberculosis, is a rod-shaped bacillus that forms a filamentous, fungus like growth.
Spirochetes are among the easiest microorganisms to identify. They are very long (5 to 500 micrometers) and slender (about 0.5 micrometer in diameter) and have an unusual structure, known as an axial filament, attached at each end of the cell.
Axial filaments have now been shown to contain two fibrils, identical to flagella, in their structure, and so they are recognised as modified flagella. They are wrapped around the cell between the cell membrane and the delicate wall, with the fibrils of each filament overlapping at the middle of the cell.
Rickettsia, another type of prokaryote distinguishable by its form.
Term Paper # 4. Types of Motility in the Prokaryotes:
Prokaryotes move in a variety of ways, all of which are more or less mysterious. Flagella beat with a rotary movement. The flagella are so fine and the beat so fast, the motion of the flagellum itself cannot be seen. (The flagella of Spirillum serpens, for instance, are reported to have been clocked at 2,400 rpm.) Ingenious methods have been devised by which the cells can be tethered in place by the flagella.
As a result, the cells rotate instead of the flagella, and the rotational movement can be observed. The basal end of the flagella, with its complex structure, apparently acts as a motor, perhaps running on chemiosmotic power.
Another characteristic form of movement for some bacteria is gliding. Gliding requires attachment to a solid surface and the secretion of some sort of slime or mucus along which the cells slide. It has been suggested that cytoplasmic streaming may play a role in imparting motility to these cells, but so far the mechanism remains unknown.
Spirochetes move by a corkscrew motion caused, apparently, by contractions of the axial filament.
Term Paper # 5. Reproduction and Resting Forms in the Prokaryotes:
Most prokaryotes reproduce by simple cell division, also called binary fission. In some forms, reproduction is by budding or by the breaking off of fragments of the cell. As they multiply, these prokaryotes, barring mutations, produce clones of genetically identical cells.
Genetic recombination’s take place as a result of conjugation, transformation, transduction, and exchanges of plasmids. It is not known how common such genetic recombination’s are in nature or whether or not they occur in all types of prokaryotes.
Many prokaryotes have the capacity to form spores, which are dormant, resting cells. Again, the process has been studied most extensively in the rod-shaped eubacteria. It occurs characteristically when a population of cells, growing very rapidly, has begun to use up its food supply.
Each cell, at the beginning of sporulation (spore formation), contains two duplicate chromosomes. A cell membrane grows around one of the chromosomes, separating it from the rest of the cell, which then engulfs the newly formed cell (as in phagocytosis). Thus the spore-to-be is now surrounded by two membranes, its own and that of the larger cell.
A spore coat containing a unique peptidoglycan forms around this smaller cell. Covered by an outer layer of proteins composed largely of hydrophobic amino acids, this peptidoglycan is completely different from that present in the bacterial cell wall. The spore remains in this protected state until appropriate events trigger its germination.
Germination takes place rapidly, with the uptake of water, the dissolution of the spore coat, and the formation of a new cell wall. Genetic studies of the spore-making Bacillus subtilis indicate that some 50 genes, clustered in about five segments of the chromosome, are involved in spore production.
The mycobacteria, a type of gliding bacteria, form fruiting bodies, which are brightly coloured collections of spores and slime large enough to be seen by the unaided eye.
Spore formation greatly increases the capacity of prokaryotic cells to survive. The spores of Clostridium botulinum, for instance, the bacterium that causes botulism, are not destroyed by boiling for several hours.
Term Paper # 6. Prokaryotic Nutrition:
Heterotrophs:
Most prokaryotes are heterotrophs; most types feed on dead organic matter. Bacteria and other microorganisms are responsible for the decay and recycling of organic material in the soil. Typically, different groups of bacteria play different, specific roles-such as the digestion of cellulose, starches, or other polysaccharides, or the hydrolysis of specific peptide bonds, or the breakdown of amino acids.
Because of their high degree of nutritional specialisation, prokaryotes are able to live in large numbers in the same small area with reduced competition and, indeed, with mutual assistance, as the activities of one group make food molecules available to another group.
These combined activities release the nutrients and make them available to plants and, through plants, to animals. Thus, they are an essential part of ecological systems.
Some heterotrophic bacteria are symbionts. Some of these are parasites that break down organic material in the bodies of living organisms. The disease-causing (pathogenic) bacteria belong to this group, as do a number of other nonpathogenic forms.
Some of the symbiotic bacteria have little effect on their hosts, and some are actually beneficial. Cows and other ruminants can digest cellulose only because their stomachs contain bacteria and certain symbiotic protozoans.
Our own intestines contain a number of types of generally harmless bacteria (including E. coli). Some supply vitamin K, which is necessary for blood clotting. Others prevent us from developing serious infections.
When the normal bacterial inhabitants of the human intestinal tract are destroyed-as can happen, for example, following prolonged antibiotic therapy-our tissues are much more vulnerable to disease-causing microorganisms.
One group of very small bacteria, members of the genus Bdellovibrio (from bdello, the Greek word for leech) are parasites on other bacteria. They make a hole in the host cell wall and multiply between the wall and the membrane, digesting the host cell as they multiply.
Chemoautotrophs:
Chemoautotrophic prokaryotes obtain their energy from the oxidation of inorganic compounds. Only prokaryotes are able to use inorganic compounds as an energy source.
Certain chemosynthetic bacteria are essential components of the nitrogen cycle, the process by which nitrogen compounds are cycled and recycled through ecosystems. One group oxidises ammonia or ammonium (derived from the breakdown of organic materials, the activities of nitrogen-fixing prokaryotes, or, to a minor extent, from lightning or volcanic activities).
The products of this reaction are nitrite (NO2) and energy. Another group oxidises nitrites, producing nitrate (NO3) and energy. Nitrate is the form in which nitrogen moves from the soil into the roots of plants.
Sulfur is also required by plants for amino acid synthesis. Like nitrogen, it is converted to the form in which it is taken up by plant roots by the activities of chemoautotrophic bacteria that oxidise elemental sulfur to sulfate:
2S + 2H2O + 3O2 → 2H2SO4
Other sulfur bacteria, such as Thiothrix and Beggiatoa, obtain energy by oxidising hydrogen sulfide.
Photosynthetic Prokaryotes:
Among the eubacteria are three photosynthetic forms: the green sulfur bacteria, the purple sulfur bacteria, and the purple nonsulfur bacteria. (The colours of the third group may actually range from purple to red or brown.)
The chlorophyll found in the green sulfur bacteria, chiorobium chlorophyll, is chemically similar to chlorophyll a. The chlorophyll found in the two groups of purple bacteria is bacteriochlorophyll, which differs chemically in several details from chlorophyll a and is a pale blue-gray. The colours of the purple bacteria are due to the presence of several different yellow and red carotenoids, which function as accessory pigments.
In the photosynthetic sulfur bacteria, the sulfur compounds are the electron doctors, playing the same role in bacterial photosynthesis that water does in photosynthesis in eukaryotes.
Photosynthesis by eubacteria is carried out anaerobically, and it never results in the production of molecular oxygen (O2). However, in all species except the methanogens, carbon is fixed by means of the Calvin cycle.
In the non-sulfur photosynthetic bacteria, other compounds, including alcohols, fatty acids, and a variety of other organic substances, serve as electron donors for the photosynthetic reaction.
Term Paper # 7. The Blue-Green “Algae”:
The blue-green “algae” are essentially bacteria, members of the gliding group. Unlike other prokaryotes, they contain chlorophyll a, which is also found in all photosynthetic eukaryotes. They have several kinds of accessory pigments, including xanthophyll, which is a yellow carotenoid, and several other carotenoids. (Carotenoids can also be found in photosynthetic eukaryotes and in some other bacteria.)
The cells of blue-green algae may also contain one or two pigments known as phycobilins-phycocyanin, a blue pigment, which is always present, and phycoerythrin, a red one, which is often present. Chlorophyll and the accessory pigments are not enclosed in chloroplasts, as they are in plant cells, but are part of a membrane system distributed in the peripheral portion of the cell.
Cells of the blue-greens, as with many other prokaryotes, have an outer polysaccharide sheath, or coating. The outer sheath is often deeply pigmented, particularly in species that spread up onto the land; the colours include a light golden yellow, brown, red, emerald green, blue, violet, and blue-black.
In addition, the carotenoids and phycobilins modify the colour of the cells in which they occur. Thus, not only are they not algae, but only about half of the known species are actually blue-green. Indeed, the Red Sea was so named because of the dense concentrations, or “blooms,” of red-pigmented blue-green algae that float on its surface.
Some species of blue-green algae are capable of nitrogen fixation.
The photosynthetic, nitrogen-fixing blue-green algae have gone the farthest along the pathway to independence hypothesized for prokaryote evolution.
They have the simplest nutritional requirements of any living thing, needing only N2 and CO2 which are always present in the atmosphere, a few minerals, and water.
The ecological importance of the blue-green algae appears to be less than that of the nitrogen-fixing bacteria, at least for agriculture. However, in Southeast Asia, rice can be grown on the same land for years without the addition of fertilisers because of the rich growth of nitrogen-fixing blue-green algae in the rice paddies.
Because of their nutritional independence, the blue-green algae are able to colonise bare areas of rock and soil. A dramatic example of such colonization was seen on the island of Krakatoa in Indonesia, which was denuded of all visible plant life by its cataclysmic volcanic explosion of 1883.
Filamentous blue-green algae were the first living things to appear on the pumice and volcanic ash; within a few years they had formed a dark-green gelatinous growth. The layer of blue-green algae eventually became thick enough to provide a substrate for the growth of higher plants. It is very probable that the blue-green algae were similarly the first colonisers of land in the course of biological evolution.
Term Paper # 8. Viruses:
Viruses do not fit easily into any of the traditional kingdoms of living organisms, and the problem of categorising them is made even more difficult by the fact that there is doubt about whether or not they should be considered living.
Structure of Viruses:
In size, viruses range from about 17 nanometers (a hemoglobin molecule is 6.4 nanometers in diameter) to about 300 nanometers, larger than small bacteria. The larger ones are at the limits of resolution of the light microscope.
Viruses are made up of nucleic acid, either DNA or RNA, enclosed in a protein coat. The protein coat determines the specificity of the virus; a cell can be infected by a virus only if that type of cell has a receptor site for the virus protein.
Thus cold viruses infect cells in the mucous membranes of the respiratory tract; measles and chicken pox infect skin cells; and polio infects the upper respiratory tract, the intestinal lining, and sometimes nerve cells. Even bacteria, have their own set of specific viruses, the bacteriophages.
In some virus infections, the protein coat is left outside the cell; in others, the intact virus enters the cell, but once inside, the protein is destroyed by enzymes. The DNA of a DNA-containing virus serves as a template for more viral DNA and also for messenger RNA, which codes for viral enzymes, viral coat protein, and probably, at least in some cases, repressors and other regulatory chemicals.
The virus uses the equipment of the host cell; including ribosomes, transfer RNA molecules, amino acids, and nucleotides. Many viruses use host enzymes as well as those coded for by their own nucleic acids, and some break up host DNA and recycle the nucleotides as viral DNA.
In the case of the RNA viruses, the viral RNA not only serves as a template for more RNA but also acts directly as messenger RNA. Viral RNA can also serve as a template for viral DNA. This phenomenon of reverse transcription is observed almost exclusively with cancer-causing viruses.
Virus particles are assembled within the host cell. They then leave the cell, often lysing the membrane. Some viruses, such as influenza virus, bud off from the host cell membrane and, in so doing, become wrapped in fragments of it. Each new viral particle is capable of setting up a new infective cycle in an uninfected cell.
Origin of Viruses:
Are viruses very simple forms of life? Can they give us clues to the nature of a pre-cellular living system of which no fossil is likely to remain? As information increases on the nature of viruses and, in particular, of their relationships with host cells, this interpretation of viral origins seems less and less likely. Viruses are able to reproduce and to make their protein coats only because they are capable of commandeering the enzymes and other metabolic machinery of the host cell.
Without this machinery, they are as inert as any other macromolecule, lifeless by many criteria. It seems more likely that viruses are cellular fragments that have set up a partially independent existence. S. E. Luria has called viruses “bits of heredity looking for a chromosome.”
Term Paper # 9. Microorganism and Human Ecology:
Symbiosis:
Symbiosis (“living together”) is a close and permanent association between organisms of different species. There are three types of symbiotic relationships. If the relationship is beneficial to both species, it is called mutualism. If one species benefits from the association while one is neither harmed nor benefited, it is called commensalism.
If one species benefits and the other is harmed, the relationship is known as parasitism. Microorganisms and human beings are often symbionts. When the microorganisms are parasites, they are said to be pathogenic-disease causing. The lines of demarcation between the different categories of symbiosis are not clear-cut.
A bacterium such as E. coli, which lives in the lower intestine, may be commensal, depending on its host for food and shelter and neither helping nor harming. If it produces a needed vitamin or digestive enzyme, the relationship is then mutualistic. If it gets in the bloodstream and causes septicemia (blood poisoning), it is a parasite and a pathogen.
Evolutionary success is measured, in terms of surviving progeny. A parasite that destroys its host is less likely to be successful by the evolutionary criterion than one that enjoys a long and comfortable relationship with its protector. Disease is likely to be the result of a sudden change in the parasite, in the host, or in their relationship.
For instant many persons harbor small numbers of Mycobacterium tuberculosis without any symptoms of disease; however, factors such as malnutrition, fatigue, or other diseases may weaken host defenses so that the signs of tuberculosis appear. Similarly, herpes simplex, the virus that causes “cold sores,” may remain latent for months or years at a time, with the lesion appearing only in response to some change in the condition of the host.
How Microbes Cause Disease:
The pathogenic effects of microbes are produced in a variety of ways. The viruses, enter particular types of cells and often destroy them. Bacteria may produce cell destruction also.
Frequently, however, the effects we recognise as disease are caused not by the direct action of the pathogens but by toxins, or poisons, produced by them. For instance, diphtheria is caused by a bacillus, Corynebacterium diphtheriae. The organisms are inhaled and establish infection in the upper respiratory tract.
ADVERTISEMENTS:
They do not invade the bloodstream, but the powerful toxin they produce does. (This toxin is made only when the bacterium is harboring a particular prophage.) The toxin is absorbed by the body cells. Under the influence of the toxin, an enzyme involved in the assembly of activated amino acids into polypeptides becomes irreversibly bound to a fragment of NAD+. The cells can no longer make proteins and so can no longer function.
Some diseases are the result of the body’s reaction to the pathogen. In pneumonia caused by Streptococcus pneumoniae, the infection causes a tremendous outpouring of fluid and cells into the air sacs of the lungs, thus interfering with respiration. The symptoms caused by fungus infections of the skin similarly result from inflammatory responses.
A single disease agent can cause a variety of diseases. Skin infections of Streptococcus pyogenes cause the disease known as impetigo. Throat infections by the same bacteria are the familiar strep throat.
Throat infections with strains of the bacteria that produce toxins (again as a result of a bacteriophage) are known as scarlet fever. Among the persons with untreated strep throat or scarlet fever, about 0.5 percent develops rheumatic fever, which is characterised by inflammatory changes in the joints, heart, and other tissues, apparently as a result of reactions involving the body’s own immune system. Conversely, many agents may cause the same symptoms; the “common cold” can result from infection with any one of a large number of viruses.
Prevention and Control of Infectious Disease:
Although microorganisms were first seen and depicted with remarkable accuracy by Antony van Leeuwenhoek in the late seventeenth century, they were not generally recognised as a cause of disease until 100 years ago.
The relationship was made clear by the work of Louis Pasteur on disease in silkworms and by Robert Koch, who studied anthrax in sheep and tuberculosis in humans. Recognition of microbes as disease agents opened the way to control measures, among the most important of which were, the introduction of sterile procedures in hospitals.
Among the agents spreading bacterial infection were physicians, surgeons, nurses, and hospitals. Childbed fever, for example, once a leading cause of sickness and death among young women, was transmitted almost exclusively by physicians who carried streptococci from patient to patient on their hands or unsterilised instruments.
The battlefields of World War II were the proving grounds for new antimicrobial drugs, such as sulfa and penicillin. These drugs made possible not only treatment of wounds but also the widespread and often life-saving use of major surgery as a treatment for cancer and other diseases. Ironically, with the advent of strains of bacteria resistant to these drugs and endemic in hospitals, the latter are once more becoming reservoirs of serious bacterial diseases.
Even more important than improvement in medical practices was the institution of public health measures, for example, the eradication of fleas, lice, mosquitoes, and other agents that carry disease; disposal of sewage and other wastes; protection of public water supplies; pasteurization of milk; quarantine; and other procedures.
Not long ago, for instance, infant mortality during the first years of life was often as high as 50 percent in some localities owing to infant diarrhea caused by contaminated milk and water.
Many infectious diseases can be prevented by immunization, a practice that dates back to 1796. Observing that persons who had cowpox, a mild disease, did not develop smallpox, Edward Jenner boldly undertook to inoculate healthy people with cowpox. (Cowpox is called vaccinia and hence immunization came to be called vaccination.) We know now that vaccination is effective because the protein coat of the vaccinia virus is similar enough to that of the smallpox virus to stimulate the production of antibodies effective against both viruses.
Immunization procedures have now been developed against a large number of diseases including mumps, measles, German measles, and polio (all virus-caused) and diphtheria, whooping cough, and tetanus (all bacteria-caused). They involve stimulation of the immune system either with a related microorganism (as in smallpox or tuberculosis), with a killed strain (as in the Salk vaccine or typhus vaccine), or with a strain attenuated (weakened) by adaptation to another host (oral polio vaccine, measles, German measles).
In 1935, sulfanilamide, the first of the new “wonder drugs” for the control of bacterial infection, was discovered in Germany, and in 1940 the effects of penicillin were reported from England. Penicillin was the first-discovered antibiotic by definition, a chemical that is produced by a living organism and is capable of inhibiting the growth of microorganisms.
Many antibiotics are produced by bacteria, especially the actinomycetes; some are formed by fungi. Many, including penicillin, can now be synthesized in the laboratory. The tremendous decrease in deaths from infectious disease over the last decades is a chief cause of the present population explosion.
