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Chapter 13: Evolution and Diversity Among the Microbes

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1 Chapter 13: Evolution and Diversity Among the Microbes
Bacteria, archaea, protists, and viruses: the unseen world Lectures by Mark Manteuffel, St. Louis Community College

2 Learning Objectives Know there are microbes in all three domains
Know that bacteria may be the most diverse of all organisms Know that archaea exploit some of the most extreme habitats Know that protists are single-celled eukaryotes Know that viruses are at the border between living and non-living Know the highlighted examples of organisms within each domain.

3 Section 13-1 Opener This strain of E. coli produces a toxin that causes extreme gastrointestinal discomfort.

4 Most microbes are actually even smaller than an amoeba: a typical bacterium or archaean is about one thousand million billion times smaller than a human (1018), and an influenza virus is about one thousand billion trillion (1024) times smaller than you are (Figure 13-1 The most abundant organisms on earth are too small to see).

5 Cyanobacteria can photosynthesize!
13.1 Microbes are the simplest, but most successful organisms on earth. Bacterial fermentation is used to produce cheese, yogurt, buttermilk, and many types of sausage. Cyanobacteria can photosynthesize! Nitrogen-fixing bacteria play a symbiotic role helping plants. Humans are large organisms, and being large comes with a lot of baggage that we don't usually think about. We need a skeletal system to support our weight against the pull of gravity, and a respiratory system to take in oxygen and get rid of carbon dioxide. We need a circulatory system to move oxygen and carbon dioxide around our bodies, and a digestive system that includes a mouth to take in food and teeth to grind the food into small pieces that can be broken down by enzymes in our stomach and intestines. We even need a nervous system so our brain knows what distant parts of our body are doing. Microbes—the most abundant organisms on earth—don’t have skeletal, respiratory, circulatory, digestive, or nervous systems because they are too small to need them; in fact, “microbe” is a combination of Greek words that means “small life” and the “small” part of the name fits them well.

6 Microbes Are Genetically Diverse
>500,000 kinds Millions more expected to be distinguished! The number of bacteria in one human’s mouth is greater than the total number of people who ever lived. The number of bacteria in a cubic centimeter of soil rivals the number of stars in the galaxy. Microbes are genetically diverse. More than 500,000 kinds of microbes have been identified by nucleotide sequences, and further studies will almost certainly distinguish millions of additional species of microbes.

7 Microbes are abundant! Microbes are abundant.
Surface sea water contains more than 100,000 bacterial cells per milliliter, and diatoms (a protist in the eukarya domain) are as abundant as bacteria. Those densities translate to about 8000 million billion trillion (8 x 1030) individuals of just these two kinds of microbes in the world's oceans. Your own body is a testament to the abundance of microbes: it contains about 100 trillion cells, but only one-tenth of those cells are actually human cells—the remaining 90 trillion cells are the microbes that live in and on you (Figure 13-3 Microbe majority). You’re a minority in your own body.

8 Microbes Can Live Almost Anywhere and Eat Almost Anything
As you read these lines, more than 400 species of microbes are thriving in your intestinal tract, 500 more species call your mouth home, and nearly 200 species live on your skin (Figure  13-2 Microbes are everywhere on earth). The microbes that live in you and on you eat mostly what you eat—some of the bacteria in your mouth and intestine compete with you, trying to digest your food before you can, and the others use the waste products you release after you have broken down the food. Others feed on the leftovers released by the breakdown of your cells during the normal process of cell renewal. Living conditions in the human body are relatively moderate. Other microbes inhabit some of the toughest environments on Earth—in the almost boiling water of hot springs, a mile below the earth's surface, and more than a mile deep in the oceans where hydrothermal vents emit water at 400°C (750°F).

9 The sloppiness of the name “microbes” stems from the fact that microbes are not all evolutionarily related. As a consequence, it is hard to make generalizations about them as we can for other organisms. We can say of all animals, for example, that they have the ability to move, if only during one portion of their life cycle. Microbes, on the other hand, are grouped together simply because they are small, not because they all share a recent common ancestor. In fact, however, microbes occur in all three domains of life—Bacteria, Archaea, and Eukarya—and therefore could not be more widely separated (Figure 13-4 Microbes are highly diverse). In this chapter we focus on the tiny organisms from each of these domains. The microbes found in domains Bacteria and Archaea are prokaryotic, although archaeans have some characteristics that are like prokaryotes and others that are like eukaryotes. Protists are the microbe members of domain Eukarya; viruses, another type of microbe, are not classified into any domain at all because they are only at the borderline of life.

10 Section 13-2 Opener Staphylococcus aureus (green) on the surface of the small instestine.

11 Bacteria and Archaea are both prokaryotes.
Recall prokaryotic cells. Bacteria come in 3 basic shapes. Bacteria may be classified by their shape: some are spherical cells (called cocci), some are rod shaped (referred to as bacilli), and others are spiral-shaped (known as spirilli) (Figure 13-5 Bacteria basics).  Bacteria usually reproduce by binary fission, and in a few hours a single cell can form a culture containing thousands of cells. As a bacterial cell divides, the number of cells doubles every generation, producing a colony of cells, each of which is a clone of the original cell. Colonies of different species of bacteria look different, and experts can often identify a bacterium from its colony characteristics. The familiar human intestinal bacterium Escherichia coli forms beige or gray colonies that have smooth margins and a shiny covering of mucus. Species of Proteus, which are often responsible for spoiling food because they can grow at refrigerator temperatures, form colonies with a surface that looks like a contour map. 11

12 Bacterial Diversity and Movement

13 Microbiologists can identify some bacteria simply by looking at the colors and shapes of their colonies. They can get additional information by examining a single cell under a microscope, but living bacterial cells are transparent, so you can't see them with an ordinary microscope unless they have been dyed. In 1884 Hans Christian Gram, a Danish microbiologist, described a method of dying the cell walls of bacteria to make them visible under a microscope, and a Gram stain is still the first test microbiologists make when they are identifying an unknown bacterium (Figure 13-6 Bacterial IDs). Gram-positive bacteria are colored purple by the stain because they have a thick layer of a protein called peptidoglycan on the outside of the cell wall. In Gram-negative bacteria the layer of peptidoglycan lies beneath a membrane and is not stained by the dye. Penicillin is effective in treating infections by Gram-positive bacteria because it interferes with the formation of peptidoglycan cross-links. Penicillin does not affect Gram-negative bacteria because it does not readily pass through the outer membrane that covers the peptidoglycan layer. Being able to distinguish two groups of bacteria is a big help to microbiologists trying to identify a bacterium, but the peptidoglycan is really there because it is important for the bacterial cells. The extensive interlocking bonds of the long peptidoglycan molecules provide strength to the cell envelope. Many bacteria also have a capsule that lies outside of the cell wall. This capsule may restrict the movement of water out of the cell and allow bacteria to live in dry places, such as on the surface of your skin. In other cases, the capsule may be important in allowing the bacteria to bind to solid surfaces such as rocks or to attach to human cells. Species of bacteria can be distinguished by the sizes, shapes, and colors of the colonies they form and by using gram stain to dye the cell walls. 13

14 13.4 Bacterial growth and reproduction is fast and efficient.
Recall Binary fission – asexual reproduction The time it takes for a bacterium to reproduce can be very short; most bacteria have generation times between 1 and 3 hours, and some are even shorter. Escherichia coli, for example, has a generation time of 20 minutes in optimal conditions, and in just over three hours a single E. coli could give rise to a population of 20 billion cells. 14

15 Bacteria Carry Genetic Information in Two Structures
A circular DNA molecule called the chromosome (1 or more) Circular DNA molecules called plasmids metabolic plasmids resistance plasmids virulence plasmids Bacteria carry genetic information in two structures: The genes that provide instructions for all of the basic life processes of a bacterium are usually located in a circular DNA molecule called the chromosome. Most bacteria have just one chromosome, but some have more than one. A bacterial chromosome is organized more efficiently than a eukaryote chromosome in two ways. First, in bacteria the genes that code for proteins that have related functions—enzymes that play a role in breaking down food for energy, for example—are often arranged right next to each other on the chromosome. This makes it possible to efficiently control the transcription of all of the genes together. Additionally, unlike Eukarya, almost all the DNA in a bacterial chromosome codes for proteins, so bacteria do not use time and energy to transcribe mRNA that will not be translated. As we saw in Chapter 5, as much as 90% of the DNA in the chromosomes of eukaryotes does not code for genes and is edited from mRNA after it has been transcribed and before it is translated to protein. Besides their main chromosome, many bacteria have additional genetic information—circular DNA molecules called plasmids that carry genes for specific functions. These include metabolic plasmids, which have genes enabling bacteria to break down specific substances, such as toxic chemicals; resistance plasmids, which have genes enabling bacteria to resist the effects of antibiotics; and virulence plasmids, which have genes that control how sick an infectious bacterium makes its victim. The strain of E. coli that occasionally sickens patrons of fast-food restaurants carries a virulence plasmid, which magnifies the effects of a gene for a toxin that can be lethal. (E. coli without that virulence plasmid are normal components of the bacterial community of the human intestine.) 15

16 Do bacteria have sex? However, bacteria can also transfer genetic information laterally—in other words, not just to their “offspring,” but to other individuals within the same generation—through any of three different processes called conjugation, transduction, and transformation. Conjugation is the process by which one bacterium transfers some or all of its DNA to another bacterium—even when the two bacteria are different species. Plasmid transfer has given the second bacterium genetic information it did not have before. These genes could allow the bacterium to make an enzyme that allows it to metabolize a new chemical or to defend itself against a new antibiotic. Figure 13-8 part 1 Lateral transfer of genetic information: conjugation. 16

17 13.5 Many bacteria are beneficial.
Do you like the taste of yogurt? You can thank bacteria for that—Lactobacillus acidophilus and several other species of bacteria are added to milk to create yogurt. As the bacterial cells use the milk sugar for energy, the by-product is lactic acid, which reacts with the milk proteins to produce the characteristic taste and texture of yogurt. If you buy a brand of yogurt that is labeled as containing live cultures, you are consuming living bacterial cells as you eat the yogurt (Figure 13-9 Yogurt contains beneficial bacteria). Bacteria are used to produce many other foods, such as cheese, and bacteria and yeasts are used in the production of beer, wine, and vinegar. Industrial microbiology is a multi-billion dollar industry. 17

18 You Owe Your Life to Bacteria
Your normal flora benign bacteria that is your first line of defense against infection by harmful bacteria A disease-causing bacterium must colonize your body before it can make you sick, and your body is already covered with harmless bacteria. If the population of harmless bacteria is dense enough, it will stop invading bacteria. Probiotic therapy a method of treating infections by deliberately introducing benign bacteria But you can thank bacteria for more than just tasty snacks—you owe your life to bacteria. Hundreds of species of bacteria grow in and on your body; these microbes are called your normal flora. The normal flora take up every spot on your body that a disease-causing bacterium could adhere to and consume every potential source of nutrition, making it difficult for a disease-causing bacterium to gain a foothold. Thus, maintaining a robust population of these benign bacteria is your first line of defense against infection by harmful bacteria. Probiotic therapy is a method of treating infections by deliberately introducing benign bacteria in numbers large enough to swamp the harmful forms. Lactobacillus acidophilus, which is a normal inhabitant of the human body, is used to treat gastrointestinal upsets, such as traveler’s diarrhea, and urinary tract infections. In addition to growing so vigorously that it crowds out harmful bacteria, L. acidophilus releases lactic acid, which interferes with the growth of other bacteria and prevents them from adhering to the walls of the urinary tract and bladder. 18

19 13.6 Metabolic diversity among the bacteria is extreme.
One important feature that makes bacterial diversity possible is that bacteria can metabolize almost anything. Some of them can even use energy from light to make their own food. Microbiologists place bacteria into trophic (feeding) categories that reflect their metabolic specialization. 19

20 Chemical organic feeders (chemoorganotrophs) are bacteria that consume organic molecules, such as carbohydrates. You probably see the products of organic feeders every time you take a shower—they are responsible for the pink deposits on the shower curtain and the floor of the shower (Figure part 1 Resourceful feeders). Most of the bacteria that live in and on your body are also organic feeders—some compete with you to metabolize the food you eat. Others digest things you can't eat. 20

21 Chemical inorganic feeders (chemolithotrophs, meaning “rock feeders”) are able to use inorganic molecules like ammonia, hydrogen sulfide, hydrogen, and iron as sources of energy. The most prevalent [OK to add “prevalent”?] inorganic feeders are the iron bacteria responsible for the brown stains that form on plumbing fixtures in regions where tap water contains high levels of iron. Sulfur bacteria, too, are associated with iron bacteria, and are responsible for the slimy black deposits that you will probably find if you lift the stopper out of the drain in your sink. It’s hard to understand how bacteria can live in that environment because there is little that we would consider food available there; in iron and other inorganic molecules, these bacteria utilize a completely different type of “food.” On a larger scale, inorganic feeders are responsible for acid mine drainage. The ore that is being mined usually makes up only a small part of the total amount of rock that is removed from a mine. The portion that does not contain ore is discarded on the ground surface in piles called “tailings.” This material is often rich in minerals such as pyrites (iron-sulfides). It’s not of any nutritional value to us, but inorganic feeders can gain energy by oxidizing these minerals and, in the process, releasing compounds that combine with rainwater to produce strong acids, such as sulfuric acid. When this acidic water drains into streams, it can kill fish and aquatic plants and insects. Figure part 2 Resourceful feeders. 21

22 Photoautotrophs and the Oxygen Revolution
“Light feeders” (photoautotrophs) contain chlorophyll and use the energy from sunlight to convert carbon dioxide to glucose via photosynthesis. The floating mats of gooey green material that you see in roadside ditches are a type of photoautotroph called cyanobacteria. The cyanobacteria that live now closely resemble the first photosynthetic organisms that appeared on earth about 2.6 billion years ago. That date was the start of a major global change; up until then the Earth's atmosphere contained no free oxygen—instead, air consisted almost entirely of nitrogen and carbon dioxide. Cyanobacteria were the first organisms that could use solar energy to build organic compounds from the carbon dioxide and, in the process, break down water molecules to release free oxygen. The accumulation of oxygen released by cyanobacteria is called the Oxygen Revolution. Oxygen—which humans depend on—now makes up about 21 percent of the volume of air, and cyanobacteria still release important quantities of oxygen into the atmosphere. Figure part 3 Resourceful feeders. 22

23 13.7 Bacteria cause many human diseases.
Pathogenic Bacteria The number of pathogenic (disease-causing) bacteria is tiny compared to the total number of species of bacteria, but some pathogens kill millions of people annually despite advances in medicine and sanitation. Some bacteria are always pathogenic (the ones that produce cholera, plague, and tuberculosis), but others (the ones responsible for acne, step throat, scarlet fever, and “flesh eating” necrotizing fasciitis) are normal parts of the communities of bacteria that live in or on humans. These bacteria become pathogenic only under special circumstances. 23

24 Lyme disease and education:
Is caused by bacteria carried by ticks. Antibiotics can cure the disease if administered within a month after exposure. If untreated, Lyme disease can cause arthritis, heart disease and nervous disorders. The best defense is a public education about avoiding tick bites and seeking treatment. Figure 15.15


26 13. 8 Bacteria evolve drug resistance quickly. How
13.8 Bacteria evolve drug resistance quickly. How? Rapid reproduction Misuse and abuse of antibiotics 26

27 Excessive use of antibiotics in medicine and agriculture has made several pathogenic bacteria resistant to every antibiotic, and infections caused by these bacteria are nearly impossible to treat. The use of antibiotics in agriculture is another reason for the spread of antibiotic resistance. Low concentrations of antibiotics are routinely added to the food fed to cattle, hogs, chickens, and turkeys. This can be beneficial in the short-term, promoting growth and minimizing disease in the crowded conditions of commercial meat production. But in the long run it can have disastrous consequences as the practice can lead to selection for bacteria resistant to the antibiotics. The antibiotics can also be passed through the food chain to humans (Figure Antibiotics are used in agriculture). Data gathered by the Union of Concerned Scientists indicate that agriculture in the United States uses about 25 million pounds of antibiotics each year—about eight times more than is used for all human medicine! 27

28 Section 13-3 Opener A steamy pool in Rotorua, North Island, New Zealand, heated by volcanic vents and home to heat-tolerant archaea. 28

29 13.10 Archaea thrive in habitats too extreme for most other organisms.
Extremophiles: Halophiles thrive in salty environments. Acidophiles thrive in acid environments. Thermophiles love extreme heat. Methanogens thrive without oxygen and give off methane. Archaeans are famous for their ability to live in places where life would seem to be impossible, such as in water of 100C or more around the hydrothermal vents that emerge from the sea floor. This water would be boiling if it were at sea level, but 2000 meters below the surface the water pressure reaches 200 atmospheres and water cannot boil. Most organisms die at temperatures between 40 and 50C because their protein molecules are denatured, so the ability of archaeans to survive at 100C is truly remarkable. 29

30 But not all archaeans are extremophiles—archaeans live everywhere that bacteria do, and many of them live in places that you would find perfectly comfortable yourself. In fact, you are home to many archaeans, and on occasion some of them make their presence all too obvious. Beans are notorious for their tendency to produce gas in the intestine, but the beans themselves are not really to blame—archaea are the culprits. Beans contain a couple of carbohydrates with chemical bonds that cannot be broken down very well by any human enzymes (Figure Helping you digest tough bonds).  Methane-producing archaea, on the other hand have no such difficulties, producing an enzyme that targets those bonds. As a result, it is the archaeans in your intestine that digest most of these carbohydrates. But in the process of breaking these bonds, they produce gases that, as they escape the digestive system, can cause considerable distress. 30

31 Prokaryotes and Chemical Recycling
Prokaryotes are important in the breakdown of organic wastes and dead organisms and cycling of chemicals into the air and soil. Prokaryotes and Bioremediation Bioremediation is the use of organisms to remove pollutants from water, air, and soil. A familiar example is use of prokaryotic decomposers in sewage treatment. Also used to clean up oil spills.

32 In this trickling filter system, bacteria and fungi growing on the rocks remove much of the organic material dissolved in sewage sludge.

33 Enormous Potential for Industries: Bioremediation
Degrade hydrocarbon Clearing mineral deposits from pipes in the cooling systems of power plants Bioengineers and biotechnologists believe that extremophile archaeans and bacteria that thrive at high temperatures and pressures and metabolize toxic substances have enormous potential for industries that have to carry out activities under those conditions. Recent experiments have demonstrated the ability of some archaeans to efficiently degrade hydrocarbons, making it possible for them to be used in the removal of sludge that accumulates in oil refinery tanks and potentially in contaminated environments such as oil slicks. Other archaea show promise in clearing mineral deposits from pipes in the cooling systems of power plants (Figure Practical applications for archaea).  33

34 13.12 The first Eukaryotes were protists.
Section 3-4 Opener This diatom, a species of Actinocyclus, is a single-celled protist with a cell wall made of supportive silica. The first Eukaryotes were protists.

35 Ancient protists For the first two billion years of life on Earth, organisms were really, really small; bacteria and archaeans were less than 10 μm across—one-eighth of the diameter of a human hair. But in rocks about 1.9 billion years old, we find fossils of new kinds of organisms that are ten times larger (Figure Ancient protists). These are a group of organisms called akritarchs (a name that can be translated as "confusing old things"). They were the first eukaryotes. 35

36 13.13 There are animal-like protists, fungus-like protists, and plant-like protists.

37 Animal-like Protists Propel themselves Appear to hunt for prey
Paramecia Some protists propel themselves around their environment quickly and appear to hunt for prey. These animal-like protists, which include Paramecia, are the ciliates and get their name from the cilia (hair-like projections) that cover their body surface and propel the cells through the water. Paramecium feed by a process called phagocytosis: cilia in a funnel-shaped structure called the gullet create an inward flow of water that carries bacteria and other small particles of food with it. These particles accumulate at the inner end of the gullet, where a portion of the plasma membrane bulges inward and eventually breaks free, forming a food vacuole that drifts into the interior of the cell. Enzymes and hydrochloric acid enter the food vacuole from the cytoplasm, and in this acid environment the engulfed food items are broken down into molecules that diffuse out of the vacuole. When all of the digestible material has been consumed, the undigested contents of the vacuole are expelled. 37

38 The Ciliates have animal-like qualities.

39 Protists That Resemble Fungi
Slime molds Establish multicellular sheet-like colonies on surfaces Oozing masses of gooey material that flow, engulfing bacteria, fungi, and small bits of organic material as they go Some protists resemble fungi, establishing multicellular sheet-like colonies on surfaces such as the grout in shower stalls. Called slime molds, these protists generally spread without any individual cells moving but rather by adding new cells at the edges of the colony. But plasmodial slime molds are oozing masses of gooey material that flow, engulfing bacteria, fungi, and small bits of organic material as they go. The streaming of a slime mold in its feeding phase is easy to observe with a microscope, and a slime mold can flow around, over, or through almost anything—it can even flow through the holes in a window screen and reassemble itself on the other side! If you have a lawn with flower beds, you may have seen an irregularly shaped blob of yellow material in a moist, shaded spot—this was a plasmodial slime mold. What's most remarkable about a plasmodial slime mold is that it is a single cell, but has multiple nuclei. Slime molds divide by binary fission, just like other cells, but in this case a cell may cover an area of several square centimeters! Nonetheless, all of the nuclei in the cell undergo mitosis simultaneously. 39

40 Slime molds resemble fungi in both appearance and function.

41 Plant-like Protists Grow in water and resemble plants
Include brown algae Also called seaweeds Still another group of protists grow in water and resemble plants. These include brown algae (although they are not algae and often aren’t even brown) that grows in the tidal zone and the giant kelp that grows in water 30 meters deep. Another name for these protists is seaweeds, and this term is close to the mark because almost all brown algae are marine. But they are not plants, they are large groups—or colonies—of protists. 41

42 Some protists resemble both plants and animals.

43 Diatoms Among the plant-like protists are the diatoms.
These organisms are usually single-celled and live in ponds, lakes, and rivers as well as the oceans. They are so small that 30 individuals of some species could be lined up across the width of a human hair. A characteristic feature of the diatoms is that they are enclosed in a shell made of silica (Figure Diatoms).   Many species of diatoms float in the water, forming part of the phytoplankton—the microscopic organisms that fix carbon dioxide and release oxygen. They can reach densities of hundreds of thousands of cells per liter, and account for about one quarter of the photosynthetic production of oxygen on Earth. Small fish and shrimp-like organisms called copepods feed on phytoplankton, and these small predators are eaten by larger predators, which are eaten by still larger predators. Thus the diversity of life in the marine habitat relies on diatoms and the other microbes that make up the phytoplankton. Diatoms 43

44 The evolutionary link between unicellular and multicellular organisms were probably colonial protists. Unicellular protist 1 Colony 2 Locomotor cells Food-synthesizing cells Gamete Early multicellular organism with specialized, interdependent cells 3 Somatic cells Later organism with gametes and somatic (non-reproductive) cells Figure 15.24

45 Seaweeds Are large, multicellular marine algae
Grow on rocky shores and just offshore Are often edible (b) Red algae (a) Green algae (c) Brown algae

46 13.14 Some protists can make you very sick.
Recall the heterozygous defense against malaria… A parasite is an organism that lives in or on an organism called a host and damages it. A parasitic protist called Plasmodium that is transmitted by a mosquito is responsible for a worldwide epidemic disease—malaria. Malaria occurs in tropical parts of Africa, Asia, and Latin America and it is common in the eastern Mediterranean as well. Between 350 million and 500 million people have clinical cases of malaria, and about one million people die of malaria each year. Malaria is the leading cause of death for children under 5 in sub-Saharan Africa; an African child dies of malaria about every 30 seconds. 46

47 Section 13-5 Opener HIV in action. Viruses in different stages of budding. 47

48 13.15 Viruses are not exactly living organisms.
A virus is not a cell, and that is why viruses do not fit into one of the three domains of life. A virus particle (called a virion) consists of genetic material inside a container made of protein. Some viruses also contain a few enzymes. That's all there is to a virus. The protein container is called the capsid, and the genetic material can be either DNA or RNA. Some viruses wrap themselves in a bit of the plasma membrane of the host cell as they are released. A virus of this type is called an enveloped virus—the flu virus is an example. Non-enveloped viruses are enclosed only by the protein container—the virus that causes the common cold is an example of a non-enveloped virus. Figure The simple structure of viruses. A virus is not alive, but it can carry out some of the same functions as living organisms, provided that it can get inside a cell. 48

49 A virus particle does not carry out any metabolic processes, and it does not control the inward or outward movement of molecules to make conditions inside the virus particle different from conditions outside. Viruses just wait for a chance to insert their genetic material into a living cell. Viruses identify the cells that they can infect by recognizing the structure of glycoprotein molecules on the surfaces of cells. Every cell in your body has these molecules. They are embedded in the plasma membrane and extend outward from the cell. Your immune system uses these proteins to identify the cells as part of you—as self rather than non-self—and it does not react to proteins that it recognizes as self. Viruses have cracked the glycoprotein code, and when they find a cell with the appropriate glycoprotein on its surface, they bind to that cell's plasma membrane and insert their genetic material into the cell (Figure Making more viruses). Inside the host cell, the viral DNA or RNA takes over the cellular machinery and uses it to produce more viruses. Viruses carry out nearly all of their activities by hijacking materials and organelles in the host cell. Viral proteins are synthesized in exactly the same way as host cell proteins—mRNA binds to ribosomes and tRNA matches the correct amino acid to each mRNA codon. The mRNA comes from the virus, but the protein-building machinery all comes from the host cell, as does the ATP required to synthesize the new viral protein. 49

50 13.16 Viruses are responsible for many health problems.
Why do flu viruses change quickly? DNA vs. RNA viruses: RNA replication doesn’t involve as much editing as DNA replication. RNA viruses are continually mutating. Many diseases are caused by viruses. Some viral diseases, such as the common cold, are not usually serious, but others have been responsible for worldwide epidemics (called pandemics). The influenza pandemic in 1918 and 1919 killed at least 20 million people, and possibly as many as 50 million. The current AIDS epidemic has infected nearly 40 million people with an annual mortality of about 3 million. 50

51 The glycoproteins on the surface of a virus determine what host species the virus can infect and which tissues of the host the virus can enter. Influenza A viruses (the ones that cause outbreaks of flu every year), for example, have two types of glycoprotein that have different functions. One glycoprotein matches that of a host cell, and allows the virus to enter the cell. The other glycoprotein allows the virus to get back out of the cell, releasing new virus particles that can infect other cells. Figure A virus’s surface proteins. The influenza virus has surface glycoproteins that allow it to bind to and exit a host cell. 51

52 Bird Flu (H5N1) So far requires close contact with infected flocks of birds or by eating birds that had died of the virus. WHO and national health agencies are preparing for a worldwide pandemic. A new flu is making headlines. Since 1997, one strain of bird flu has spread from Hong Kong through most of Asia and into Turkey, France, Germany, and England. This bird flu is the most recent avian influenza virus to also infect humans. The virus readily infects birds, and tens of millions of chickens, turkeys, and ducks have been killed in attempts to eradicate the infection. And because the host-entry glycoproteins do not bind well to human cells, the virus does not infect people easily and does not readily spread from human to human. So far nearly all the people who have been infected by the avian influenza virus have contracted it by close contact with infected flocks of birds or by eating birds that had died of the virus. Only a half-dozen cases are known in which the virus appears to have been transmitted from one person to another. However, avian flu can be deadly when it does infect humans—more than half of the humans cases reported have ended in death. The World Health Organization and national health agencies are preparing for a worldwide pandemic of this bird flu virus that also can infect humans. 52

53 What role does a pig play in the transmission of virus from a bird to a human?
The cells of pigs have glycoproteins that allow both human and bird viruses to bind to them. If human and bird flu mix, the new strain could be passed from human to human. Influenza A is an example of a virus that can move from one species to another, and all of the influenza pandemics in the past century have begun with the transmission of a bird influenza virus to humans. Here’s how “species-jumping” can occur. 53

54 Swine Flu (H1N1) Swine flu does spread easily from human to human, but it is not as lethal as the bird flu. Pig virus + bird flu virus might produce a new form of the virus that carries the genes that make the bird flu lethal to humans AND the gene that codes for the host-entry glycoprotein (human to human transmission). Since 1997, one strain of bird flu has spread from Hong Kong through most of Asia and into Turkey, France, Germany, and England. This bird flu is the most recent avian influenza virus to also infect humans. The virus readily infects birds, and tens of millions of chickens, turkeys, and ducks have been killed in attempts to eradicate the infection. And because the host-entry glycoproteins do not bind well to human cells, the virus does not infect people easily and does not readily spread from human to human. So far nearly all the people who have been infected by the avian influenza virus have contracted it by close contact with infected flocks of birds or by eating birds that had died of the virus. Only a half-dozen cases are known in which the virus appears to have been transmitted from one person to another. However, avian flu can be deadly when it does infect humans—more than half of the humans cases reported have ended in death. The World Health Organization and national health agencies are preparing for a worldwide pandemic of this bird flu virus that also can infect humans. 54

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