Viruses. and Bacteria

Transcription

1 CHAPTER 20 Viruses and Bacteria Quick Review Answer the following without referring to earlier sections of your book. 1. List the properties of life. (Chapter 1, Section 1) 4B 2. Define prokaryote. (Chapter 3, Section 2) 4A 3. Describe a bacteriophage. (Chapter 9, Section 1) 4C 4. Differentiate DNA from RNA. (Chapter 10, Section 1) 6A Did you have difficulty? For help, review the sections indicated. Reading Activity Take a break after reading each section of this chapter, and closely study the figures in the section. Reread the figure captions, and, for each one, write out a question that can be answered by referring to the figure and its caption. Refer to your list of figures and questions as you review the concepts addressed in the chapter before you complete the Performance Zone chapter review. Looking Ahead Section 1 Viruses Is a Virus Alive? Viral Structure Viral Reproduction How HIV Infects Cells Viral Diseases Section 2 Bacteria Bacterial Structure Eubacteria and Archaebacteria Pathogenic Bacteria Antibiotics Importance of Bacteria Streptococcus bacteria include a number of strains that can produce a wide range of illnesses. Some, like strep throat, are easily treated. Others are rare and require immediate medical attention. National Science Teachers Association scilinks Internet resources are located throughout this chapter. CHAPTER 20 Viruses and Bacteria 433

2 Section 1 Viruses Objectives Describe why a virus is not considered a living organism. 4B 4C Summarize the discovery of the tobacco mosaic virus. Describe the basic structure of a virus. 4C Summarize the steps of viral replication. 4C Explain how HIV infects immune system cells. Key Terms virus pathogen capsid envelope glycoprotein bacteriophage lytic provirus lysogenic prion viroid 4C Figure 1 Ebola virus. This virus causes an often-fatal disease and has been recognized only since F Is a Virus Alive? Throughout the book, you have learned about the properties of life. All living things are made of cells, are able to grow and reproduce, and are guided by information stored in their DNA. The smallest organisms that have these properties are prokaryotes. Viruses are segments of nucleic acids contained in a protein coat. Viruses are not cells. Viruses are smaller than prokaryotes and range in size from about 20 nm to 250 nm ( µm) in diameter. (One nanometer is equal to 0.001µm or in.) Most viruses, such as the Ebola virus shown in Figure 1, can be seen only with an electron microscope. Viruses are pathogens agents that cause disease. Viruses replicate by infecting cells and using the cell to make more viruses. Because viruses do not have all the properties of life, biologists do not consider them to be living. Viruses do not grow, do not have homeostasis, and do not metabolize. Because they cause diseases in many organisms, viruses have a major impact on the living world. Discovery of Viruses Near the end of the nineteenth century, scientists were trying to find the cause of tobacco mosaic disease, which stunts the growth of tobacco plants. Scientists filtered bacteria from the sap of infected plants. They were surprised to find that the filtered sap could still cause uninfected plants to become infected. The scientists concluded that the pathogen is smaller than a bacterium. The pathogen was called a virus, a Latin word meaning poison. For many years after this discovery, viruses were thought to be tiny cells. In 1935, biologist Wendell Stanley of the Rockefeller Institute purified tobacco mosaic virus (TMV). He determined that the purified virus is a crystal. Stanley concluded that TMV is a chemical rather than an organism. Each particle of TMV is composed of RNA and protein. Scientists were able to separate the RNA from the protein and reassemble the virus so that it could infect plants. 434 CHAPTER 20 Viruses and Bacteria

3 Viral Structure The virus protein coat, or capsid, may contain either RNA or DNA, but not both. RNA viruses include the human immunodeficiency virus (HIV), which causes AIDS, influenza viruses, and rabies virus. DNA viruses include those viruses that cause warts, chickenpox, and mononucleosis. Many viruses, such as the influenza virus shown in Figure 2, have a membrane, or envelope, surrounding the capsid. The envelope helps the virus enter cells. It consists of proteins, lipids, and glycoproteins (glie koh PROH teenz), which are proteins with attached carbohydrate molecules that are derived from the host cell. Some viruses also contain specific enzymes. Viruses exist in a variety of shapes. Some viruses, such as the Ebola virus, shown in Figure 1, are long rods that form filaments. Spherical viruses, such as the influenza virus shown in Figure 2, typically are studded with receptors. These receptors help the virus enter cells. A helical virus, like the tobacco mosaic virus shown in Figure 2, is rodlike in appearance, with capsid proteins winding around the core in a spiral. Polyhedral viruses have many sides and are roughly spherical. The capsid of most polyhedral viruses has 20 triangular faces and 12 corners. This odd shape is an efficient one for containing a viral genome. Figure 2 shows the polyhedral shape of a adenovirus, which can cause several different kinds of infections in humans. Viruses that infect bacteria, called bacteriophages, have a complicated structure. A T4 bacteriophage, for example, has a polyhedron capsid attached to a helical tail. A long DNA molecule is coiled within the polyhedron. Compare and Contrast To compare and contrast the properties of life as defined in Chapter 1 and the properties of viruses, make a two-column list. In one column, write the properties of life. In the other column, write the properties of life that viruses have. Figure 2 Viral structures Viruses can have characteristic shapes. Magnification: 202,500 Magnification: 1,250,000 Magnification: 135,000 Influenza (enveloped) Tobacco mosaic virus (helical) Adenovirus (polyhedral) SECTION 1 Viruses 435

4 Magnification: 32,512 Figure 3 Bacteriophage infecting a bacterium. Bacteriophages (pink) first attach to a bacterial cell (blue) and then push their DNA into it. The cell then produces more viruses. Figure 4 Viral replication in bacteria. Bacterial viruses provide a model by which viruses replicate through the lytic cycle or lysogenic cycle. Viral Reproduction Viruses lack the enzymes necessary for metabolism and have no structures to make protein. Therefore, viruses must rely on living cells (host cells) for replication, as shown in Figure 3. Before a virus can replicate, it must first infect a living cell. A plant virus, like TMV, enters a plant cell through tiny tears in the cell wall at points of injury. An animal virus enters its host cell by endocytosis. A bacterial virus, or bacteriophage, punches a hole in the bacterial cell wall and injects its DNA into the cell. Lytic Cycle The reproduction of bacterial viruses has been well studied. Once inside a cell, the virus will set out on one of two different paths: the lytic cycle or the lysogenic cycle. In bacterial viruses, the cycle of viral infection, replication, and cell destruction is called the lytic cycle. After the viral genes have entered the cell, they use the host cell to replicate viral genes and to make viral proteins, such as capsids. The proteins are then assembled with the replicated viral genes to form complete viruses. The host cell is broken open and releases newly made viruses. Though reproduction in bacterial viruses is illustrated here, these stages are common to infections by other viruses as well. The lytic cycle is shown in Figure 4. Lysogenic Cycle During an infection, some viruses stay inside the cells but do not make new viruses. Instead of producing virus particles, the viral gene is inserted into the host chromosome and is called a provirus. Lytic cycle Lysogenic cycle 1 The virus attaches to a cell and injects DNA. Bacterial chromosome 5 The provirus may enter the lytic cycle. Many cell divisions 4 The cell breaks open and releases viruses. 3 New viruses are made. 2 Viral DNA enters the lytic cycle or lysogenic cycle. 3 4 The host cell divides normally. Viral DNA integrates with host DNA. 436 CHAPTER 20 Viruses and Bacteria

5 Whenever the cell divides, the provirus also divides, resulting in two infected host cells. In this cycle, called the lysogenic (lie soh JEHN ihk) cycle, the viral genome replicates without destroying the host cell. This cycle is shown in Figure 4. In some lysogenic viruses, a change in the environment can cause the provirus to begin the lytic cycle. This results in the destruction of the host cell. In animal cells, viruses can replicate slowly so that the host cell is not destroyed by the virus. For example, the virus that causes cold sores in humans hides deep in the nerves of the face. When the conditions in the body become favorable for the virus, such as when a person is under stress, the virus then begins to cause tissue damage that is seen as a cold sore or fever blister. Host Cell Specificity Viruses are often restricted to certain kinds of cells. For example, TMV infects tobacco and related plants, but does not infect animals. Scientists hypothesize that this specificity may be due to the viruses origin. Viruses may have originated when fragments of host genes escaped or were expelled from cells. The hypothesis that viruses originated from a variety of host cells may explain why there are so many different kinds of viruses. Biologists think there are at least as many kinds of viruses as there are kinds of organisms. Structure of HIV an Enveloped Virus Many viruses that infect only animals, such as the influenza virus shown in Figure 2, have an exterior viral envelope. Figure 5 shows human immunodeficiency virus (HIV), the virus that causes acquired immune deficiency syndrome (AIDS). Figure 5 illustrates the envelope and other features common to several animal viruses. In many cases, the viral envelope is composed of a lipid bilayer derived from the membrane of the host cell. On the surface of the virus, glycoproteins are embedded within the envelope. Within the envelope lies the capsid, which in turn encloses the virus s genetic material. In the case of HIV, the genetic material is composed of two molecules of single-stranded RNA. The approximately 9,000 nucleotides of HIV make up nine genes. Three of these genes are common to many different viruses. RNA Glycoprotein Figure 5 HIV. HIV infects human white blood cells. Envelope Capsid SECTION 1 Viruses 437

6 How HIV Infects Cells HIV, shown in Figure 6, provides a good example of how animal viruses enter cells. HIV entry is a two-step process. First, the virus attaches to the cell at specific sites called receptors. Second, this attachment triggers endocytosis. Recall that during endocytosis, the cell membrane pushes inward, carrying a particle in this case HIV with it into the cell. Attachment Studding the surface of each HIV are spikes composed of a glycoprotein. This particular glycoprotein precisely fits a human cell surface receptor called CD4. Thus the HIV glycoprotein can bind to any cell that possesses CD4 receptors. In humans, immune system cells called lymphocytes and macrophages, as well as certain cells in the brain, possess CD4 receptors. Entry into Macrophages HIV cannot enter a cell merely by docking onto a CD4 receptor. Rather, the glycoprotein must also activate a second co-receptor, called CCR5. It is this event at CCR5 that starts endocytosis, illustrated in Figure 7. Because human macrophages possess both CD4 and CCR5 receptors, HIV can enter macrophages. Lymphocytes, which are critical to immune system function, do not have CCR5 receptors. HIV therefore does not enter lymphocytes. Topic: AIDS Research in Texas Keyword: HXX4001 Replication Once inside a cell, the HIV particle sheds its capsid. The particle then releases an enzyme called reverse transcriptase. Reverse transcriptase copies the naked viral RNA into a complementary DNA version. This process is mistake-prone, so it creates many new mutations. Translation of the viral DNA by the host cell s machinery directs the production of many copies of the virus. HIV doesn t rupture and kill the cell; instead, the new viruses are released from the cell by budding. The new virus particle is thus covered with an envelope derived from the cell membrane. Figure 6 HIV. The spherical structure of HIV is visible in this transmission electron micrograph of individual virus particles. 438 CHAPTER 20

7 HIV CCR5 receptor CH4 coreceptor HIV RNA Reverse transcriptase New DNA Immune system cell The glycoprotein on HIV s surface docks at a CD4 receptor. A second receptor, CCR5, helps pull HIV into the cell. The viral envelope is left outside the cell. The capsid fragments are located inside the HIV cell. A DNA copy of the HIV RNA is made by the cell. HIV RNA HIV HIV proteins Budding The new viral DNA directs synthesis of new HIV proteins and HIV RNA. New HIV particles leave macrophages by budding. HIV particles leave T cells by budding or bursting through the membrane. AIDS For years after the initial infection, HIV continues to replicate (and mutate). Eventually and by chance, HIV s surface glycoproteins change to the point that they now recognize a new cell surface receptor. This receptor is found on the subset of lymphocytes called T cells. Unlike its activity in macrophages, HIV reproduces in T cells and then destroys them. This increases the number of virus particles in the blood, which then infect other T cells, widening the circle of cell death. It is this destruction of the body s T cells that blocks the body s immune response and signals the onset of AIDS. AIDS is a disease in which an individual is unable to defend his or her own body against infections that do not normally occur in healthy individuals. Usually, HIV-infected people do not develop AIDS symptoms until years after infection. As a result, an HIV-infected individual can feel healthy and still spread the virus to others. HIV is not passed from an infected person to a healthy one through casual contact. It is transmitted in body fluids (such as semen or vaginal fluid) through sexual contact and in blood through the sharing of nonsterile needles. It is also transmitted to infants during pregnancy or through breast milk. Figure 7 HIV infection. HIV docks at specific receptors on cell membranes. The virus is reproduced by the infected cell. SECTION 1 Viruses 439

8 Viral Diseases Diseases caused by viruses have been known and feared for thousands of years. Perhaps the most lethal virus in human history is the influenza virus. Commonly known as the flu, influenza is characterized by chills, fever, and muscular aches. The virus infects cells of the upper respiratory tract. There the viruses replicate and spread to new cells. About 22 million Americans and Europeans died of flu during an 18-month period in Table 1 lists some familiar viral diseases. Certain viruses can also cause some types of cancer. Recall that cancer is a condition in which cells reproduce uncontrollably as a result of the failure of mechanisms that control cell growth and division. Viruses associated with human cancers include hepatitis B (liver cancer), Epstein-Barr virus (Burkitt s lymphoma), and human papilloma virus (cervical cancer). Table 1 Important Viral Diseases Disease Description of illness How the disease is transmitted AIDS Immune system failure Sexual contact, contaminated blood, or contaminated needles Common cold Sinus congestion, muscle aches, cough, fever Inhalation, direct contact Ebola High fever, uncontrollable bleeding Body fluids Hepatitis A Hepatitis B Influenza (flu) Flulike symptoms, swollen liver, yellow skin, painful joints Flulike symptoms, swollen liver, yellow skin, painful joints; can cause liver cancer Fever, chills, fatigue, cough, sore throat, muscle aches, weakness, headache Contaminated blood, food, or water Sexual contact, contaminated blood, or contaminated needles Inhalation Mumps Painful swelling in salivary glands Inhalation Polio Fever, headache, stiff neck, possible paralysis Contaminated food or water Mental depression, fever, restlessness, Rabies difficulty swallowing, paralysis, Bite of infected animal convulsions; fatal Smallpox Blisters, lesions, fever, malaise, blindness, disfiguring scars; often fatal Inhalation Yellow fever Rash, swollen glands, fever; fatal to developing infant in pregnant woman Bite of infected mosquito 440 CHAPTER 20 Viruses and Bacteria

9 Emerging Viruses Viruses that evolve in geographically isolated areas and are pathogenic to humans are called emerging viruses. These new pathogens are dangerous to public health. People become infected when they have contact with the normal hosts of these viruses. In 1999, a mosquito-borne virus called West Nile virus began to spread across the United States. West Nile virus probably was brought from overseas to America by an infected bird. While it is an emerging virus in North America, West Nile virus is common in Africa, eastern Europe, and western Asia. People who are infected with the virus from mosquito bites typically experience mild flulike symptoms. In some people, particularly the elderly, inflammation of the brain may occur, which can be fatal. First detected in the southwestern United States, hantavirus is spread in rodent droppings and can cause a lethal illness in humans. At least 38 percent of its human victims die. Prions and Viroids In 1982, the American scientist Stanley Pruisner, of Stanford University, described a new class of pathogens that he called prions (PREE awnz). Prions are composed of proteins but have no nucleic acid. A disease-causing prion is folded into a shape that does not allow the prion to function. Contact with a misfolded prion will cause a normal prion to misfold, too. In this way the misfolding spreads like a chain reaction. Prions were first linked to a brain disease in sheep called scrapie. Later, brain diseases such as mad cow disease, displayed by the cow shown in Figure 8, and Creutzfeldt-Jakob disease were also associated with prions. Eating meat containing the disease-causing prion can cause infection. A viroid (VEER oid) is a single strand of RNA that has no capsid. Viroids are important infectious disease agents in plants. Viroids have affected economically important plants such as cucumbers, potatoes, avocados, and oranges. Topic: Viral Diseases Keyword: HX4186 Figure 8 Infected cow. This cow, which is unable to stand and walk, is showing signs of mad cow disease. Section 1 Review Compare the properties of viruses with the properties of cells. 4C Describe Stanley s experiment with the tobacco mosaic virus. 3C 3F Name the parts of a virus. 4C List the steps by which viruses replicate. 4C Describe how HIV causes AIDS. 4C Critical Thinking Evaluate the argument that emerging viruses are new viruses. 4C TAKS Test Prep Viruses differ from cells because viruses 4C A can grow. C have homeostasis. B do not metabolize. D lack nucleic acids. SECTION 1 Viruses 441

10 Section 2 Bacteria Objectives List seven differences between bacteria and eukaryotic cells. 4A Describe three different ways bacteria can obtain energy. 4B Describe the external and internal structure of Escherichia coli. 4A Distinguish two ways that bacteria cause disease. Identify three ways that bacteria benefit humans. Key Terms pilus bacillus coccus spirillum capsule antibiotic endospore conjugation anaerobic aerobic toxin Figure 9 Flagella and pili. Bacteria have flagella that provide them with movement and pili that enable adherence to surfaces. 4D 4D Pilus Flagellum Bacterial Structure The prokaryotes referred to in this chapter as bacteria include the organisms that compose the kingdom Eubacteria (Domain Bacteria) and the organisms that compose the kingdom Archaebacteria (Domain Archaea). Bacteria differ from eukaryotes in at least seven ways. 1. Internal compartmentalization. Bacteria are prokaryotes. Unlike eukaryotes, prokaryotes lack a cell nucleus. Bacterial cells have no internal compartments or membrane systems. 2. Cell size. Most bacterial cells are about 1 µm in diameter; most eukaryotic cells are more than 10 times that size. 3. Multicellularity. All bacteria are single cells. Some bacteria may stick together or may form strands. However, these formations are not truly multicellular because the cytoplasm in the cells does not directly interconnect, as is the case with many multicellular eukaryotes. Also, the activities of the cells are not specialized. 4. Chromosomes. Bacterial chromosomes consist of a single circular piece of DNA. Eukaryotic chromosomes are linear pieces of DNA that are associated with proteins. 5. Reproduction. Bacteria reproduce by binary fission, a process in which one cell pinches into two cells. In eukaryotes, however, microtubules pull chromosomes to opposite poles of the cell during mitosis. Afterward, the cytoplasm of the eukaryotic cell divides in half, forming two cells. 6. Flagella. Bacterial flagella are simple structures composed of a single fiber of protein that spins like a corkscrew to move the cell. Eukaryotic flagella are more-complex structures made of microtubules that whip back and Magnification: 69,230 forth rather than spin. Some bacteria also have shorter, thicker outgrowths called pili (PIHL ee) (singular, pilus), shown in Figure 9. Pili enable bacteria to attach to surfaces or to other cells. 7. Metabolic diversity. Bacteria have many metabolic abilities that eukaryotes lack. For example, bacteria perform several different kinds of anaerobic and aerobic processes, while eukaryotes are mostly aerobic Proteus mirabilis organisms. 442 CHAPTER 20 Viruses and Bacteria

11 Bacterial Cell Shapes A bacterial cell is usually one of three basic shapes, as shown in Figure 10: bacillus (buh SIHL uhs), a rod-shaped cell; coccus (KAHK us), a round-shaped cell; or spirillum (spy RIHL uhm), a spiral cell. A few kinds of bacteria aggregate into strands. Species that form filaments are indicated by the prefix strepto-, while species that form clusters are indicated by the prefix staphylo-. Members of the kingdom Eubacteria have a cell wall made of peptidoglycan, a network of polysaccharide molecules linked together with chains of amino acids. Outside the cell wall and membrane, many bacteria have a gel-like layer called a capsule. Members of the kingdom Archaebacteria often lack cell walls. Cell walls Eubacteria can have two types of cell walls, distinguished by a dye staining technique called the Gram stain. One group is called Gram-negative, and the other Gram-positive. Gram staining is important in medicine because the two groups of eubacteria differ in their susceptibility to different antibiotics. Antibiotics are chemicals that interfere with life processes in bacteria. Thus, Gram staining can help determine which antibiotic would be most useful in fighting an infection. Endospores Some bacteria form thick-walled endospores (EHN doh spohrz) around their chromosomes and a small bit of cytoplasm when they are exposed to harsh conditions. These conditions can be the depletion of nutrients, a drought, or high temperatures. Endospores can survive environmental stress and may germinate years after they were formed, releasing new, active bacteria. Pili Pili enable bacteria to adhere to the surface of sources of nutrition, such as your skin. Some kinds of pili enable bacteria to exchange genetic material through a process called conjugation. Conjugation (kahn juh GAY shuhn) is a process in which two organisms exchange genetic material. In prokaryotes, pili from one bacterium adhere to a second bacterium, and genetic material is transferred from the first bacterium to the second bacterium. Conjugation enables bacteria to spread genes within a population. Magnification: 117,300 Reviewing Information Prepare flash cards for each of the Key Terms in this chapter. On each card, write the term on one side and its definition on the other side. Use the cards to review meanings of the Key Terms. Topic: Bacteria Keyword: HX4018 Figure 10 Bacterial shapes. Bacteria are usually one of three shapes. Magnification: 2,295 Bacillus (rod-shaped) E. coli Coccus (round-shaped) Micrococcus luteus Spirillum (spiral-shaped) Spirillum volutans SECTION 2 Bacteria 443

12 Real Life Big, big bacteria In 1999, scientists announced the discovery of the largest bacteria ever discovered. Thiomargarita namibiensis was found off the coast of Namibia. More than 100 times larger than the previously known largest bacterium, T. namibiensis is 0.5 mm wide. Figure 11 Photosynthetic bacterium. Anabaena is a photosynthetic cyanobacterium in which individual cells adhere in filaments. The two large orange-colored cells are encased in a structure where nitrogen fixation occurs. Obtaining Energy Over 4,000 species of bacteria have been named, and probably many more haven t yet been discovered. Bacteria occur in the widest possible range of habitats and play key ecological roles in nearly all of them. As you may recall from an earlier chapter, bacteria thrive in hot springs, frigid arctic seas, and groundwater. They are even found at high pressures in the deep sea and inside solid rock. Bacteria can be classified in several different ways. Classifying bacteria by the different ways in which they obtain energy, for example, gives a good general sense of the great diversity among bacteria. Bacteria can also be classified according to their phylogenetic relationships. By comparing the sequence of their ribosomal RNA, scientists have determined that there are at least 12 phyla of eubacteria and four phyla of archaebacteria. Photosynthesizers A significant fraction of the world s photosynthesis is carried out by bacteria. Photosynthetic bacteria can be classified into four major groups based on the photosynthetic pigments they contain: purple nonsulfur bacteria, green sulfur bacteria, purple sulfur bacteria, and cyanobacteria. Green sulfur bacteria and purple sulfur bacteria grow in anaerobic (oxygen-free) environments. They cannot use water as a source of electrons for photosynthesis and instead use sulfur compounds, such as hydrogen sulfide, H 2 S. Purple nonsulfur bacteria use organic compounds, such as acids and carbohydrates, as a source of electrons for photosynthesis. Of particular importance are the cyanobacteria, which often clump together in large mats of filaments. Recall that cyanobacteria are thought to have made the Earth s oxygen atmosphere. Each filament is a chain of cells encased in a continuous jellylike capsule. Many cyanobacteria, such as species of Anabaena, shown in Figure 11, are capable of fixing nitrogen. Chemoautotrophs Bacteria called chemoautotrophs (KEE moh AW toh trohfs) obtain energy by removing electrons from inorganic molecules such as ammonia, NH 3, and hydrogen sulfide, H 2 S, or from organic molecules such as methane, CH 4. In the presence of one of these hydrogen-rich chemicals, chemoautotrophic bacteria can manufacture all their own amino acids and proteins. Chemoautotrophic bacteria that live in the soil, such as Nitrosomonas and Nitrobacter, are of great importance to the environment and to agriculture. They have an important role in the nitrogen cycle called nitrification. Nitrification, as you may recall from an earlier chapter, is the process in which bacteria oxidize ammonia into nitrate. Nitrate is the form of nitrogen most commonly used by plants. 444 CHAPTER 20 Viruses and Bacteria

13 Heterotrophs Most bacteria are heterotrophs. Together with fungi, heterotrophic bacteria are the principal decomposers of the living world; they break down the bodies of dead organisms and make the nutrients available to other organisms. Many are aerobic, that is, they live in the presence of oxygen. Some other bacteria can live without oxygen. Other activities of heterotrophic bacteria may be helpful or harmful to humans. For example, more than half of our antibiotics are produced by several species of Streptomyces, a filamentous bacterium found in soil. On the other hand, one species of Staphylococcus can secrete a poison into food. This poison causes nausea, diarrhea, and vomiting in people who eat the Staphylococcus-contaminated food. Species of the symbiotic bacteria Rhizobium are by far the most important of all nitrogen-fixing organisms. Rhizobium species are heterotrophic bacteria that usually live within lumps on the roots of legumes (plants such as soybeans, beans, peas, peanuts, alfalfa, and clover), as shown in Figure 12. Farmers take advantage of Rhizobium s nitrogen-fixing abilities when they rotate their crops every few years and grow legumes, which replenish the soil with nitrogencontaining compounds. Magnification: 1,440 Figure 12 Nitrogen-fixing bacteria. The bacteria inside the lumps on these soybean roots contain Rhizobium, a nitrogen-fixing bacteria. Drilling for Buried Bacteria One of the least known ecosystems on Earth consists of bacteria that live at the bottom of the ocean, buried deep in sediment. These microorganisms have been recovered from depths as great as 800 m below the ocean floor. In this completely dark environment, they rely on a variety of chemoautotrophic and heterotrophic processes to obtain energy and nutrients. Found at every site where scientists have looked for them, deep-ocean bacteria are extremely abundant. Scientists estimate that they comprise between 10 and 30 percent of the Earth s biomass. Ocean Drilling Program Researchers in the Ocean Drilling Program (ODP) are collecting sediment from several sites in the Pacific Ocean. They drill into the ocean floor from the ODP s research ship, JOIDES Resolution. The ship can drill in waters over 8,200 m deep. Texas A&M University runs the ship and stores the sediment samples. As the drilling proceeds, sediment is pulled up through the drill pipe and kept under sterile, anaerobic conditions until the bacteria in them can be studied. Scientists are identifying bacteria from different depths and learning how these bacteria affect Earth s oceans and atmosphere. Topic: Ocean Drilling Keyword: HXX4019 SECTION 2 Bacteria 445

14 Up Close Escherichia coli Scientific name: Escherichia coli Size: Up to 1 µm Habitat: Inhabits the intestines of many mammals Mode of nutrition: Heterotrophic Characteristics Cell structure E. coli is a Gramnegative eubacterium. It has a rigid cell wall composed of peptidoglycan. Outside the cell wall lies the outer membrane, which is composed of lipids and polysaccharides. Cell wall Outer membrane Cell membrane Genetic material Like all bacteria, E. coli has a single DNA molecule in the form of a loop. E. coli has approximately 5,000 genes. DNA Ribosome Flagellum Locomotion By rotating its slender, whiplike flagella, E. coli propels itself through its environment. Peptidoglycan Pili Reproduction Most bacteria reproduce by binary fission, a process by which a single cell divides into two identical new cells. E. coli can divide as often as every 20 minutes. Adherence Like many Gram-negative bacteria, E. coli has pili short, thin, hairlike appendages. Pili can adhere to surfaces, including the surfaces of intestinal-lining cells. Pili also join bacterial cells prior to conjugation. 446 CHAPTER 20 # Chapter Viruses Title and Bacteria

15 Pathogenic Bacteria In order to understand infectious diseases, think of your body as a treasure chest full of resources. Your body has protein, minerals, fats, carbohydrates, and vitamins. You may want to keep and use these resources, but so do many other organisms, including the bacteria on and in your body. Bacteria have evolved various means of obtaining these resources from you. In some cases, the competition for the resources in your body can result in your becoming ill. Bacteria Can Metabolize Their Host Heterotrophic bacteria obtain nutrients by secreting enzymes that break down complex organic structures in their environment and then absorbing them. If that environment is your throat or lungs, this can cause serious problems. For example, tuberculosis, a disease of the lungs, is caused by Mycobacterium tuberculosis, shown in Figure 13. Tuberculosis was once one of the most common causes of death. In most cases, infection occurs when tiny droplets of moisture that contain the bacteria are inhaled. Some bacteria settle in the lungs, where they grow using human tissue as their nutrients. The bacteria may also spread to other parts of the body. Symptoms may include coughing up sputum and blood, chest pain, fever, fatigue, weight loss, and loss of appetite. If left untreated, death may occur as quickly as within 18 months but more commonly within 5 years. Other important bacterial diseases are described in Table 2. Figure 13 Tuberculosis in a lung. The red-stained structures in this light micrograph are Mycobacterium tuberculosis, which cause tuberculosis. Table 2 Important Bacterial Diseases Disease Description of illness Bacterium How the disease is transmitted or caused Anthrax Fever, severe difficulty breathing Bacillus anthracis Inhalation of spores Bubonic plague Fever, bleeding, lymph nodes that form swellings called buboes; often fatal Yersinia pestis Bite of an infected flea Cholera Severe diarrhea and vomiting; fatal if not treated Vibrio cholerae Drinking contaminated water Dental cavities Destruction of minerals in tooth Streptococus mutans Dense collections of bacteria in mouth Lyme disease Rash, pain, swelling in joints Borrelia burgdorferi Bite of an infected tick Tuberculosis Fever, difficulty breathing Mycobacterium Inhalation Typhus Headache, high fever Rickettsia Bite of infected flea or louse SECTION 2 Bacteria 447

16 Not all bacteria are lethal. For example, some bacteria cause everyday health problems, such as acne. Acne occurs in about 85 percent of teenagers. Bacteria, such as Propionibacterium acnes, normally grow in an oil gland of the skin. They metabolize a certain kind of oil produced by those glands. During puberty the oil glands increase the amount of oil produced, and the bacterial population on the skin increases greatly. The bacteria grow in the pores where the oil normally flows, forming pimples and blackheads. Figure 14 The effect of bacterial toxins. This species of Streptococcus secretes a toxin that destroys red blood cells. The agar contains red blood cells and clearly shows a zone around the bacteria where the toxin has destroyed the red blood cells. Bacterial Toxins The second way bacteria cause disease is by secreting chemical compounds into their environment. These chemicals, called toxins, are poisonous to eukaryotic cells, as shown in Figure 14. Toxins can be secreted into the body of an infected person or into a food in which bacteria are growing. When bacteria grow in food and produce toxins, the toxins can cause illness in humans who eat those contaminated foods. This kind of illness is called an intoxication. For example, Staphylococcus aureus causes the most common type of food poisoning. The symptoms include nausea, vomiting, and diarrhea. This type of poisoning is painful but is seldom fatal. Another type of intoxication that is fatal occurs when food is not canned properly. Sometimes canned food is not heated enough to kill endospore-forming bacteria, such as Clostridium botulinum. The bacteria can then grow and produce a deadly toxin that affects the nervous system. A person who eats food that contains this toxin then becomes ill with a disease called botulism, whose symptoms include double vision and paralysis. People with botulism may die because they are unable to breathe. Some bacteria are responsible for other diseases reported in the news, such as E. coli O157:H7, the cause of several outbreaks of food poisoning in the United States. E. coli normally lives in our intestines. However, if it acquires DNA that codes for the toxin through conjugation, it can produce the toxin. E. coli poisoning is associated with raw or improperly cooked ground beef. Most bacteria can be killed by boiling water or various chemicals. Using hot, soapy water to prevent contamination of our food utensils and food supply is one way of preventing disease. Many commercial antibacterial products can also be used to prevent bacterial contamination in the kitchen and in industrial food factories. 448 CHAPTER 20 Viruses and Bacteria Biowarfare Biowarfare is the deliberate exposure of people to biological toxins or pathogens such as bacteria or viruses. The United States government is justifiably concerned about the use of bioweapons biological toxins or pathogens suitable for mass infection against military personnel overseas and against civilians within the United States. Biologists are working on new approaches to recognize the onset of an attack with a bioweapon, to treat infected people, and to slow the spread of any outbreak of disease.

17 Antibiotics In 1928, the British bacteriologist Alexander Fleming noticed a fungus of the genus Penicillium growing on a culture of S. aureus. He saw that bacteria did not grow near the fungus. He concluded that the fungus was secreting a substance that killed the bacteria, as shown in Figure 15. Fleming isolated the substance and named it penicillin. In the early 1940s, scientists found that penicillin was effective in treating many bacterial diseases, such as pneumonia. Different antibiotics interfere with different cellular processes. Because these processes do not occur in viruses, antibiotics are not effective against them. Other antibiotics, such as tetracycline and ampicillin, have been discovered in nature or imitated chemically. Antibiotic-Resistant Bacteria In recent years, some bacteria have become resistant to antibiotics. Susceptible bacteria are eliminated from the population, and resistant bacteria survive and reproduce, thus passing on their resistance traits. Mutations for antibiotic resistance arise spontaneously in bacterial populations as errors in DNA replication. There are many individuals in a bacterial population, and bacteria multiply very rapidly (doubling their numbers in as few as 20 minutes). Therefore, a mutation that gives the bacteria a selective advantage can quickly spread throughout a population. Antibiotic Misuse Mutations that confer resistance to antibiotics are strongly favored in bacterial populations being treated with an antibiotic. Usually, if the full course of the antibiotic is administered, all the targeted bacteria are killed and there is no chance for a resistant strain to develop. If antibiotic treatment ends prematurely, some of the bacteria may survive. Which ones? The ones most resistant to the antibiotic. A patient who does not take the full course of a prescribed antibiotic is setting the stage for the development of antibioticresistant bacteria. Multiple-antibiotic Resistance A related problem can arise in a patient being treated with two or more antibiotics at the same time. This practice selects for bacteria that have acquired several antibiotic-resistance genes. A number of strains of Staphylococcus aureus associated with severe infections of hospital patients (so-called hospital staph) have appeared in recent years. These strains are resistant to penicillin and a wide variety of other antibiotics, so infections caused by these strains are very difficult to treat. Recently, concern has arisen over the common use of antibacterial soaps. Antibacterial soaps are marketed as a means of protecting people from harmful bacteria. Their routine use, however, may favor bacteria resistant to the antibacterial agents in the soap. Ultimately, routine use of antibacterial soaps could reduce our ability to treat common bacterial infections. Fungus Bacteria Figure 15 Antibiotics are naturally produced. Alexander Fleming saw a plate of agar very similar to this one. Notice how the bacteria do not grow next to this fungus. SECTION 2 Bacteria 449

18 Importance of Bacteria Despite the misery that some bacteria cause humans in the form of disease and food spoilage, much of what bacteria do is extremely important to our health and economic well-being. Figure 16 Swiss cheese. In making Swiss cheese, bacteria grow in the cheese and produce gas. As the cheese hardens these pockets of gas remain, giving the cheese its characteristic holes. Figure 17 Industrial fermenter. Bacteria can be used to produce useful chemicals such as in this fermenter. Food and Chemical Production Many of the foods that we eat are processed by specific kinds of bacteria. For example, many fermented foods are produced with the assistance of bacteria, as shown in Figure 16. These foods include pickles, buttermilk, cheese, sauerkraut, olives, vinegar, sourdough bread, and even some kinds of sausages. Humans are able to use different bacteria to produce different kinds of chemicals for industrial uses, as shown in Figure 17. For example, different kinds of Clostridium species can make either acetone or butanol. These chemicals can be used to produce a large variety of other useful chemicals. Genetic engineering companies use genetically engineered bacteria to produce their many products, such as drugs for medicine and complex chemicals for research. Mining and Environmental Uses of Bacteria Mining companies can use bacteria to concentrate desired elements from low-grade ore. Low-grade ore has a low percentage of the desired mineral, but it also has sulfur compounds. Chemoautotrophic bacteria can convert the sulfur into a soluble compound, leaving the desired mineral behind. The sulfur compound can be washed away with water, leaving only the desired mineral. This technique can be used to harvest copper or uranium. Bacteria metabolize different organic chemicals and are therefore used to help clean up environmental disasters such as petroleum and chemical spills. Powders containing petroleummetabolizing bacteria are used to help clean oil spills. Section 2 Review Construct a table that lists the seven ways bacteria differ from eukaryotic cells. 4A List the structures found in E. coli. 4A Identify the relationship between photosynthesis, heterotrophic metabolism, and chemoautotrophic metabolism. 4B Describe the relationship between metabolism, toxins, bacteria, and disease. 4B 4C 4D List three ways bacteria are helpful. 4D 450 CHAPTER 20 Viruses and Bacteria Critical Thinking Defending a Theory How does the growth of antibiotic resistance in bacteria support the theory of evolution by natural selection? 7B TAKS Test Prep inhaling a bacterium? A cholera B botulism C E. coli food poisoning D tuberculosis Which disease is caused by 4D

19 Study ZONE CHAPTER HIGHLIGHTS Key Concepts 1 Viruses Viruses consist of segments of a nucleic acid contained in a protein coat, and some have an envelope. Viruses do not have all of the characteristics of life and are therefore not considered to be alive. Viruses replicate inside living cells. They enter a cell by injecting their genetic material into the cell, by slipping through tears in the plant cell wall, or by binding to molecules on the cell surface and triggering endocytosis. Viruses replicate through a lytic cycle or a lysogenic cycle. HIV replicates inside immune system cells, eventually destroying them, leaving the host without adequate defense against disease. Emerging viruses are geographically isolated viruses that cause disease in humans. Viroids are infectious RNA molecules that cause disease in plants, and prions are infectious proteins that cause disease in certain animals. 2 Bacteria Bacteria differ from eukaryotes in their cellular organization, cell structures, and metabolic diversity. Bacteria can be classified into two groups according to their cell wall structure. Gram staining can be used to distinguish these two groups. Bacteria can transfer genes to one another by conjugation. Bacteria are grouped according to their ribosomal RNA sequences and the way they obtain energy. Bacteria cause disease by metabolizing nutrients in their host or by releasing toxins, which damage their host. Bacterial disease can usually be fought with soap, chemicals, and antibiotics. Bacteria are used to make foods, antibiotics, and chemicals; to fix nitrogen; to clean the environment; and to cycle important chemicals in the environment. Key Terms Section 1 virus (434) pathogen (434) capsid (435) envelope (435) glycoprotein (435) bacteriophage (435) lytic (436) provirus (436) lysogenic (437) prion (441) viroid (441) Section 2 pilus (442) bacillus (443) coccus (443) spirillum (443) capsule (443) antibiotic (443) endospore (443) conjugation (443) anaerobic (444) aerobic (445) toxin (448) CHAPTER 20 Highlights 451

20 Performance ZONE CHAPTER REVIEW Using Key Terms 1. A type of virus that infects bacteria is a(n) a. viroid. c. bacteriophage. b. glycoprotein. d. emerging virus. 2. The basic components of all viruses are a nucleic acid and a(n) 4C a. endospore. c. protein coat. b. glycoprotein. d. icosahedron. 3. E. coli can move by using its a. pili. b. nucleus. c. peptidoglycan. d. flagella. 4. A bacteriophage kills its host cell during 4C a. a lytic cycle. b. conjugation. c. a lysogenic cycle. d. assembly of the capsid. 5. For each pair of terms below, explain the differences in their meanings. a. capsid, envelope b. virus, provirus c. prion, viroid d. bacillus, coccus Understanding Key Ideas 6. Unlike cells, viruses do not a. grow. b. have homeostasis. c. metabolize. d. All of the above 7. What evidence led Stanley to conclude that TMV is not a living organism? 3F a. The extract of TMV crystallized. b. TMV is made of RNA and protein. c. TMV reproduces only in cells. d. The virus poisons tobacco plants. 4A 4B 4C 8. HIV infects and destroys 4C a. skin cells. c. immune cells. b. red blood cells. d. bacterial cells. 9. Bacteria 4A a. always have flagella. 4C b. are smaller than viruses. c. have aerobic or anaerobic metabolism. d. have a nucleus. 10. Bacteria that do not require sunlight and obtain energy by removing electrons from hydrogen-rich chemicals are called 4A a. heterotrophs. b. photosynthetic bacteria. c. cyanobacteria. d. chemoautotrophs. 11. Environmental spills of petroleum are sometimes cleaned up using 11C 11D a. viroids. c. bacteria. b. prions. d. bacteriophages. 12. Identify the pilus in the photo below. C 13. If cold viruses invade your body, your body s immune system may destroy most but not all of these viruses. How does your body s immune system affect the evolution of the cold viruses? 4C 7B 14. Are the bacteria collected by the Ocean Drilling Program photosynthetic? How do you know? 4A 15. Concept Mapping Make a concept map describing the relationships of bacteria and viruses to diseases. Try to include the following terms in your map: bacteria, viruses, pathogen, emerging viruses, antibiotics, and toxin. 3E A B 4A 452 CHAPTER 20 Review