Vital Activities of Microorganisms
The activities of microorganisms are responsible for the survival of all other organisms, including humans. A few examples prove this point. Nitrogen is an essential part of most of the important molecules in our bodies, such as nucleic acids and proteins. Nitrogen is also the most common gas in the atmosphere. Neither plants nor animals, however, can use nitrogen gas. Without certain bacteria that are able to convert the nitrogen in air into a chemical form that plants can use, life as we know it would not exist on earth.
All animals including humans require oxygen (O2) to breathe. The supply of O2 in the atmosphere, however, would be depleted in about 20 years, were it not replenished. On land, plants are important producers of O2, but when all land and aquatic environments are considered, microorganisms are primarily responsible for continually replenishing the supply of O2.
Microorganisms can also break down a wide variety of materials that no other forms of life can degrade. For example, the bulk of the carbohydrate in terrestrial (land) plants is in the form of cellulose, which humans and most animals cannot digest. Certain microorganisms can, however. As a result, leaves and downed trees do not pile up in the environment. Cellulose is also degraded by billions of microorganisms in the digestive tracts of cattle, sheep, deer, and other ruminants. The digestion products are used by the cattle for energy. Without these bacteria, ruminants would not survive. Microorganisms also play an indispensable role in degrading a wide variety of materials in sewage and wastewater.
In addition to the crucial roles that microorganisms play in maintaining all life on earth, they also have made life more comfortable for humans over the centuries. Biotechnology is the application of biology to solve practical problems and produce useful products economically.
By taking advantage of what microorganisms do naturally, Egyptian bakers as early as 2100 B.C. used yeast to make bread. Today, bakeries use essentially the same technology. ■ breadmak-ing, p. 810
The excavation of early tombs in Egypt revealed that by 1500 B.C., Egyptians employed a highly complex procedure for fermenting cereal grains to produce beer. Today, brewers use the same fundamental techniques to make beer and other fermented drinks. ■ beer, p. 809
Virtually every human culture that has domesticated milk-producing animals such as cows and goats also has developed the technology to ferment milk to produce foods such as yogurt, cheeses, and buttermilk. Today, the bacteria added to some fermented milk products are being touted by nutritionists as protecting against intestinal infections and bowel cancer, the field of probiotics. ■ milk products, p. 804
The use of living organisms to degrade environmental pollutants is termed bioremediation. Bacteria are being used to
1.2 Microbiology: A Human Perspective 5
destroy such dangerous chemical pollutants as polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), and trichloroethylene, a highly toxic solvent used in dry cleaning. All three organic compounds and many more have been detected in soil and water. Bacteria are also being used to degrade oil, assist in the cleanup of oil spills, and treat radioactive wastes. A bacterium was discovered recently that can live on trinitrotoluene (TNT). ■ bioremediation, p. 797
Bacteria can synthesize a wide variety of different products in the course of their metabolism. Many of these products have great commercial value. Although these same products can be synthesized in factories, bacteria often can do it faster and cheaper. For example, different bacteria produce:
■ Cellulose used in stereo headsets
■ Hydroxybutyric acid used in the manufacture of disposable diapers and plastics
■ Ethanol, which is added to gasoline to make it burn cleaner
■ Chemicals poisonous to insects
■ Antibiotics used in the treatment of disease
■ Amino acids, which are used as dietary supplements
It is now possible to introduce genes from one organism into a related or an unrelated organism and confer new properties on that organism. This is the process of genetic engineering. Genetically engineered microorganisms often appear in the popular press because they are being used to solve many problems associated with an industrial society. Genetic engineering has expanded the power of biotechnology enormously. Here are examples of the roles that microorganisms play in this new biotechnology:
■ Microorganisms can be genetically engineered to produce a variety of medically important products. These include interferon, insulin, human growth hormone, blood clotting factors, and enzymes that dissolve blood clots. ■ genetic engineering, p. 220
■ Microorganisms are being modified so that they will produce vaccines against rabies, gonorrhea, herpes, leprosy, malaria, and hepatitis.
■ A bacterium can be used to genetically engineer plants so they become resistant to insect attacks and viral diseases, and produce large amounts of ^-carotene.
■ genetically engineered plants, p. 224
■ A bacterium can be used to transfer antibody-eliciting genes into bananas which then confer resistance to certain diarrheal diseases. ■ vaccines, p. 421
■ Viruses are being studied as a means of delivering genes into humans to correct conditions such as cystic fibrosis, heart disease, and cancer. This is the process of gene therapy. ■ gene therapy, p. 367
These examples represent only a few of the ways that microorganisms and viruses are being used to promote human welfare. In the past, microorganisms were considered only as dangerous organisms because they caused disease. The current and future use of microorganisms to increase the quality of human life, however, will receive increasing attention in scientific laboratories.
The DNA in bacteria carries all of the information that gives the organism its unique characteristics and the ability to carry out the activities that are essential to life on earth. To fully understand the basis for their remarkable diversity and what makes them tick, we need to sequence their DNA and thereby reveal this storehouse of information. This is the science of genomics. The DNA of about 100 different bacteria has now been sequenced, revealing the innermost secrets of these organisms. Scientists are now better able to understand how bacteria can live in widely diverse environments and their relationships to other organisms. This will allow scientists to improve organisms' usefulness in biotechnology.
In addition to the important roles that microorganisms play in our daily lives, some also play a sinister role. For example, more Americans died of influenza in 1918-1919 than were killed in World War I, World War II, the Korean War, and the Vietnam War combined. Modern sanitation, vaccination, and effective antibiotic treatments have reduced the incidence of some of the worst diseases, such as smallpox, bubonic plague, and influenza, to a small fraction of their former numbers. Another disease, acquired immunodeficiency syndrome (AIDS), however, has risen as a modern-day plague.
About the time that spontaneous generation was finally disproved to everyone's satisfaction, the Golden Age of medical microbiology was born. Between the years 1875 and 1918, most disease-causing bacteria were identified, and early work on viruses had begun. Once people realized that some of these invisible agents could cause disease, they tried to prevent their spread from sick to healthy people. The great successes in the area of human health in the last 100 years have resulted from the prevention of infectious diseases with vaccines and treatment of these diseases with antibiotics. The results have been astounding!
The viral disease smallpox was one of greatest killers the world has ever known. Approximately 10 million people have died from this disease over the past 4,000 years. It was brought to the New World by the Spaniards and made it possible for Hernando Cortez, with fewer than 600 soldiers, to conquer the Aztec Empire, whose subjects numbered in the millions. During a crucial battle in Mexico City, an epidemic of smallpox raged, killing only the Aztecs who had never been exposed to the disease before. In recent times, an active worldwide vaccination program has resulted in no cases being reported since 1977. Although the disease will probably never reappear on its own, its potential use as an agent in bioter-rorists attacks is raising great concern.
Bubonic plague has been another great killer. One-third of the entire population of Europe, approximately 25 million people, died of this bacterial disease between 1346 and 1350. Now, generally less than 100 people in the entire world die each year from bubonic plague. In large part, this dramatic decrease is a result of controlling the population of black rats that harbor the bacterium. Further, the discovery of antibiotics in the early twentieth century made the isolated outbreaks treatable and the disease no longer the scourge it once was.
Epidemics are not limited to human populations. In 2001, a catastrophic outbreak of foot-and-mouth disease of animals ran out of control in England. To control this disease, one of the most contagious diseases known, almost 4 million pigs, sheep, and cattle were destroyed.
Although progress has been very impressive against bacterial diseases, a great deal still remains to be done, especially in the treatment of viral diseases and diseases that are prevalent in developing countries. Even in wealthy developed countries with their sophisticated health care systems, however, infectious diseases remain a serious threat. For example, about 750 million cases of infectious diseases of all types occur in the United States each year. Every year these diseases lead to 200,000 deaths and cost tens of billions of health care dollars. Respiratory infections and diarrheal diseases cause most illness and deaths in the world today.
Emerging Diseases In addition to the well-recognized diseases, seemingly "new" emerging diseases continue to arise. In the last several decades, they have included:
■ Acquired immunodeficiency syndrome (AIDS), p. 655
■ Hantavirus pulmonary syndrome, p. 589
■ Hemolytic-uremic syndrome, p. 614
■ Cryptosporidiosis, p. 626
None of these diseases are really new, but an increased occurrence and wider distribution have brought them to the attention of health workers. Using the latest techniques, biomedical scientists have isolated, characterized, and identified these agents of disease. Now, methods need to be developed to prevent them.
A number of factors account for these emerging diseases arising even in industrially advanced countries. One reason is that changing lifestyles bring new opportunities for infectious agents to cause disease. For example, the vaginal tampons used by women provide an environment in which the organism causing toxic shock syndrome can grow and produce a toxin. In another example, the suburbs of cities are expanding into rural areas, bringing people into closer contact with ani mals previously isolated from humans. Consequently, people become exposed to viruses and infectious organisms that had been far removed from their environment. A good example is the hantavirus. This virus infects rodents, usually without causing disease. The infected animals, however, shed virus in urine, feces, and saliva; from there, it can be inhaled by humans as an aerosol. This disease, as well as Lyme disease, are only two of many emerging human diseases associated with small-animal reservoirs.
Some emerging diseases arise because the infectious agents change abruptly and gain the ability to infect new hosts. It is possible that HIV (human immunodeficiency virus), the cause of AIDS, arose from a virus that once could infect only monkeys. Some bacterial pathogens, organisms capable of causing disease, differ from their non-pathogenic relatives in that the pathogens contain large pieces of DNA that confer on the organism the ability to cause disease. These pieces of DNA may have originated in unrelated organisms.
Figure 1.3 shows the countries in the world where, since 1976, new infectious diseases of humans and animals have first appeared. Are there other agents out there that may cause "new" diseases in the future? The answer is undoubtedly yes!
Resurgence of Old Diseases Not only are "new" diseases emerging, but many infectious diseases once on the wane in the United States have begun to increase again. Further, many of these diseases are more serious today because the causative agents resist the antibiotics once used to treat them. One reason for this resurgence is that thousands of foreign visitors and U.S. citizens returning from travel abroad enter this country daily.
1.2 Microbiology: A Human Perspective 7
About one in five comes from a country where such diseases as malaria, cholera, plague, and yellow fever still exist. In developed countries these diseases have been eliminated largely through sanitation, vaccination, and quarantine. An international traveler incubating a disease in his or her body, however, could theoretically circle the globe, touch down in several countries, and expose many people before he or she became ill. As a result these diseases are recurring in countries where they had been virtually eliminated.
A second reason that certain diseases are on the rise is that in both developed and developing countries many childhood diseases have been so effectively controlled by childhood vaccinations that some parents have become lax about having their children vaccinated. The unvaccinated children are highly susceptible, and the number of those infected has increased dramatically. These diseases include measles, polio, mumps, whooping cough, and diphtheria.
A third reason for the rise in infectious diseases is that the population contains an increasing proportion of elderly people, who have weakened immune systems and are susceptible to diseases that younger people readily resist. In addition, individuals infected with HIV are especially susceptible to a wide variety of diseases, such as tuberculosis and Kaposi's sarcoma.
Chronic Diseases Caused by Bacteria In addition to the diseases long recognized as being caused by microorganisms or viruses, some illnesses once attributed to other causes may in fact be caused by bacteria. The best-known example is peptic ulcers. This common affliction has recently been shown to be caused
8 Chapter 1 Humans and the Microbial World by a bacterium, Helicobacter pylori, and is treatable with antibiotics. Chronic indigestion, which affects 25% to 40% of the people in the Western world, may also be caused by the same bacterium. Some scientists have also suggested that a bacterium is involved in cardiovascular disease.
In 2002, it was shown that the worm responsible for the tropical disease river blindness must contain a specific bacterium which apparently causes the disease. Infectious agents likely play roles in other diseases of unknown origin.
Only a very small minority of bacteria cause disease. All surfaces of the human body are populated with bacteria, most of which protect against disease. They successfully compete with the occasional disease-causing bacteria and keep them from breaching host defenses. The body is an ecological habitat for a vast number of different bacteria interacting with one another and with the surfaces of the body. Some pathogenic bacteria have the ability to enter cells in the body where they can escape competition from other bacteria, and the defenses of the host, and find a source of nutrients. These host-pathogen interactions are very complex, involving combat between host defenses and the disease-causing ability of the invading organisms.
Microorganisms are wonderful model organisms to study because they display the same fundamental metabolic and genetic properties found in higher forms of life. For example, all cells synthesize protein from the same amino acids by the same mechanism. They all duplicate their DNA by similar processes, and they degrade food materials to generate energy via the same metabolic pathways. To paraphrase a Nobel Prize-winning microbiologist, Dr. Jacques Monod, what is true of an elephant is also true of bacteria. Bacteria are easy to study and results can be obtained very quickly because they grow rapidly and form billions of cells per milliliter on simple inexpensive media. Thus, most of the major advances that have been made in the last century toward understanding life have come through the study of microorganisms. The number of Nobel Prizes that have been awarded to microbiologists, and especially the ones awarded in 2001, proves this point (see inside cover). Such studies constitute basic research, and they continue today.
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