After reading this section, students will be able to...
Give some examples of the diseases microbes cause.
Explain the impact of vaccines, antibiotics and other advances on infectious disease in the developed world.
List some of the new illnesses caused by microorganisms that are emerging and the known pathogens that are becoming resistant to treatment.
Give examples of the mutualistic relationships that microbes form with other organisms.
If you ask the average person how microbes (or germs) impact their lives, they would immediately think of disease. This is not a silly view, as pathogenic microorganisms have greatly affected human populations throughout our existence. Until about 1930, microbes were the major cause of death in humans. In 1900, infant mortality rates in some urban areas were up to 30%. From today's perspective, this is a horrendous statistic, over half of all infants did not make it to adulthood! With the advent of antibiotics, vaccines and better water sanitation, humanity has reduced the impact of pathogenic microbes, but they will always remain an important social concern. The discipline of microbiology emerged from the study of these diseases and most advances in treating various ailments had their roots in this relatively young science. Figure 1.3 shows a number of important pathogens.
Figure 1.3. Some important pathogens. Many microbes cause disease in humans. Depicted here are several pathogens that cause important illnesses. A, Influenza virus; B, West Nile Virus; C, Staphylococcus aureus; D, Streptococcus pneumoniae. (Sources: A, Dr. Erskine/L. Palmer/ Dr. M. L. Martin; B, Cynthia Goldsmith; C, Janice Carr/ Jeff Hageman, M.H.S.; D, Dr. Mike Miller; all individuals are at the CDC.)
While death from infectious disease in the U.S. has been greatly diminished, infection rates in developing nations remain unacceptably high. "Ancient" diseases continue to be a problem where nutrition and sanitation are poor, and emerging diseases such as Acquired Immunodeficiency Syndrome (AIDS) are even more dangerous for such populations. The Centers for Disease Control and Prevention (the U.S. government agency charged with protecting human health and safety) estimate that about 9% of adults between the ages of 18-49 in Sub-Saharan Africa are infected with HIV. Yet as you can see in Table 1, AIDS is only one of a number of new diseases that have emerged. Many of the new diseases are viral in nature, making them notoriously difficult to treat and they have no known cure. In addition, influenza and pneumonia are leading killers of the elderly even in the U.S. and other developed nations. Even the common cold causes illness and misery for almost everyone and drains the productivity of all nations.
Disease due to food-borne pathogens is an increasing problem, largely because of consumption of improperly processed or stored foods. Understanding the sources of contamination and developing ways to limit the growth of pathogens in food is the job of food microbiologists.
|1973||Rotavirus||Virus||Major cause of infantile diarrhea worldwide|
|1975||Parvovirus B19||Virus||Severe anemia|
|1976||Cryptosporidium parvum||Parasite||Acute and chronic diarrhea|
|1977||Ebola||Virus||Ebola hemorrhagic fever/uncontrolled bleeding and kidney failure|
|1977||Legionella pneumophila||Bacteria||Legionnaire's disease|
|1977||Hanta virus||Virus||Hemorrhagic fever|
|1977||Campylobacter jejuni||Bacteria||Short-term diarrhea|
|1980||Human T-lymphotropic virus I (HTLV-I)||Virus||T-cell lymphoma-leukemial cancer of the blood|
|1981||Toxic strains of Staphyloccus aureus||Bacteria||Toxic shock syndrome|
|1982||Escherichia coli O157:H7||Bacteria||Hemorrhagic colitis; hemolytic uremic syndrome|
|1982||HTLV-II||Virus||Hairy cell leukemia|
|1982||Borrelia burgdorferi||Bacteria||Lyme disease|
|1983||Human immunodeficiency virus (HIV)||Virus||Acquired immune deficiency syndrome (AIDS)|
|1983||Helicobacter pylori||Bacteria||Peptic ulcer disease|
|1985||Entercytozoon bieneusi||Parasite||Persistent diarrhea|
|1986||Cyclospora cayetanensis||Parasite||Persistent diarrhea|
|1988||Human herpesvirus-6 (HHV-6)||Virus||Roseola subitum/skin rash|
|1988||Hepatitis E||Virus||Liver infection; epidemic hepatitis|
|1989||Ehrlichia chaffeensis||Bacteria||Human ehrlichiosis/influenza-like infection|
|1989||Hepatitis C||Virus||Chronic liver infection|
|1991||Guanarito virus||Virus||Venezuelan hemorrhagic fever|
|1991||New species of Babesia||Parasite||Atypical babesiosis/infection with fever, chills and fatigue|
|1992||Bartonella henselae||Bacteria||Catch scratch disease/bacillary angionmatosis|
|1993||sin nombre virus||Virus||Adult respiratory distress syndrome|
|1993||Encephalitozoon cuniculi||Virus||Infection with fever, chills and fatigue|
|1994||Sabia virus||Virus||Brazilian hemorrhagic fever|
|1995||HHV-8||Virus||Associated with Kaposi sarcoma in AIDS patients|
Source: WHO, The World Health Report 1996: 112
New infections continually appear. Having an available food source to grow on (humans) inevitably results in a microorganism that will take advantage. Some of these feeders will interfere with our own well being, causing disease.
Surprisingly, many diseases that were previously thought to have only behavioral or genetic components have been found to involve microorganisms. The clearest case is that of ulcers, which was long thought to be caused by stress and poor diet. However the causative agent is actually a bacterium, Helicobacter pylori, and many ulcers can be cured with appropriate antibiotics. Work on other non-infectious diseases such as heart disease, stroke and some autoimmune diseases also suggest a microbial component that triggers the illness.
Finally, some pathogenic microbes that had been "controlled" through the use of antibiotics are beginning to develop drug resistance and therefore reemerge as serious threats in the industrialized world as well as developing nations. Tuberculosis is an illness that was on the decline until the middle 80's. It has recently become more of a problem, partly due to drug resistance and partly due to a higher population of immunosuppressed individuals from the AIDS epidemic. Staphylococcus aureus strains are emerging that are resistant to many of the antibiotics that were previously effective against them. These staph infections are of great concern in hospital settings around the world. Understanding both familiar killers and new pathogens will require an understanding of their biology, and thus an understanding of the field of microbiology.
From the beginning of microbiology, significant resources have been spent to understand and fight disease-causing microorganisms. You may be surprised to learn that only a small fraction of microbes are involved in disease; many other microbes actually enhance our well being. The harmless microbes that live in our intestines and on our skin actually help us fight off disease. They actively antagonize other bacteria and take up space, preventing potential pathogens from gaining a foothold on our bodies. The microbial community in humans not only protects us from disease, but also provides needed vitamins, such as B12. We have entire communities of microorganisms in our digestive systems that contribute to our overall health. In fact, like all other large organisms, humans are actually consortia of different organisms - there are more non-human cells in and on our bodies than there are human cells! Recent experiments, that have examined microorganisms inside our digestive tract by intensive sequencing experiments have revealed many interesting findings. More than 80% of the microbes in our guts have not been cultured. In addition, the microbial flora of a person is unique to that person, and there are differences based upon body type and genetic background. This has profound effects on physical well-being of the individual.
Human health and nutrition also depends on healthy farm animals. Cows, sheep and other ruminant animals utilize their microbial associates to degrade plant material into useful nutrients. Figure 1.4 shows the cow, one example of a ruminant animal. Without these bacteria inside ruminants, growth on plant material would be impossible.
Figure 1.4. The cow as an example of a ruminant animal. In contrast to humans, ruminant animals have a complex system of stomachs that harbor large numbers of microorganisms. These microbes degrade the tough plant material eaten by the animal into usable nutrients. Without the assistance of the microbes, ruminant animals would not be able to digest the food they eat. (Source: Keith Weller, USDA.)
Commercial crops are also central to human prosperity, and much of agriculture depends upon the activities of microbes. For example, an entire group of plants, the legumes, forms a cooperative relationship with certain bacteria. These bacteria convert nitrogen gas to ammonia for the plant, an important nutrient that is often limiting in the environment. Figure 1.5 shows a leguminous plant and the special structures on the roots that house these helpful bacteria. Most plants also form cooperative relationships with fungi, the mycorrhizal fungi, that help the plant sequester and absorb needed nutrients. Microbes also serve as small factories, producing valuable products such as cheese, yogurt, beer, wine, organic acids and many other items. In conclusion, while it is less apparent to us, the positive role of microbes in human health is at least as important as the negative impact of pathogens.
Figure 1.5. A leguminous plants.. Nitrogen-fixing bacteria form special structures, called nodules, on the roots of leguminous plants. In the picture, peas are shown that have either been exposed to bacteria (left) or not (right). The small bumps on the roots on the left are the nodules and contain millions of bacteria actively fixing N2 for the plant's use. Though not obvious from this figure, the plants on the left are more robust because of that nitrogen. (Source: E. B. Fred, et al., 1921. Soil Science 11:479-491.)
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