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Pathogens

Pathogens

BRIAN HOYLE

Pathogens are organisms, frequently microorganisms, or components of these organisms, that cause disease. Microbial pathogens include various species of bacteria, viruses, and protozoa. Many diseases caused by microbial pathogens, and the frequency of these diseases, are a national security issue.

Pathogens and disease. A disease is any condition caused by the presence of an invading organism or a toxic component that damages the host. In humans, diseases can be caused by the growth of microorganisms such as bacteria, viruses, and protozoa. Bacterial growth, however, is not mandatory to cause disease. For example, some bacterial pathogens cause disease by virtue of a toxic component of the bacterial cell such as lipopolysaccharide. Finally, the damaging symptoms of a disease can be the result of the attempts by the host's immune system to rid the body of the invader. One example is the immune-related damage caused to the lungs of those afflicted with cystic fibrosis, as the body unsuccessfully attempts to eradicate the chronic infections caused by Pseudomonas aeruginosa.

Not all pathogens cause diseases that have the same severity of symptoms. For example, an infection with the influenza virus can cause the short term aches and fever that are hallmarks of the flu, or it can cause more dire symptoms, depending on the type of virus that causes the infection. Bacteria also vary in the damage caused. For example, the ingestion of food contaminated with Salmonella enteritica causes intestinal upset. But, consumption of Escherichia coli O157:H7 causes a severe disease, which can permanently damage the kidneys and which can even be fatal.

Types of bacterial pathogens. There are three categories of bacterial pathogens. Obligate pathogens are those bacteria that must cause disease in order to be transmitted from one host to another. These bacteria must also infect a host in order to survive, in contrast to other bacteria that are capable of survival outside of a host. Examples of obligate bacterial pathogens include Mycobacterium tuberculosis and Treponema pallidum.

Opportunistic pathogens can be transmitted from one host to another without having to cause disease. However, in a host whose immune system is not functioning properly, the bacteria can cause an infection that leads to a disease. In those cases, the disease can help the bacteria spread to another host. Examples of opportunistic bacterial pathogens include Vibrio cholerae and Pseudomonas aeruginosa.

Finally, some bacterial pathogens cause disease only accidentally. Indeed, the disease actually limits the spread of the bacteria to another host. Examples of these "accidental' pathogens include Neisseria meningitides and Bacteroides fragilis.

Spread of pathogens. Pathogens can be spread from person to person in a number of ways. Not all pathogens use all the available routes. For example, the influenza virus is transmitted from person to person through the air, typically via sneezing or coughing. But the virus is not transmitted via water. In contrast, Escherichia coli is readily transmitted via water, food, and blood, but is not readily transmitted via air or the bite of an insect.

While routes of transmission vary for different pathogens, a given pathogen will use a given route of transmission. This has been used in the weaponization of pathogens. The best-known example is anthrax. The bacterium that causes anthraxBacillus anthracis can form an environmentally hardy form called a spore. The spore is very small and light. It can float on currents of air and can be breathed into the lungs, where the bacteria resume growth and swiftly cause a serious and often fatal form of anthrax. As demonstrated in the United States in the last few months of 2001, anthrax spores are easily sent through the mail to targets. As well, the powdery spores can be released from an aircraft. Over a major urban center, modeling studies have indicated that the resulting casualties could number in the hundreds of thousands.

Contamination of water by pathogens is another insidious route of disease spread. Water can look crystal clear despite the presence of millions of bacteria in each milliliter. Viruses, which are much smaller, can be present in even higher numbers without affecting the appearance of the liquid. Thus, water can be easily laced with enough pathogens to cause illness.

Food-borne pathogens cause millions of cases of disease and hundreds of deaths each year in the United States alone. Frequently the responsible microbes are bacteria, viruses, or protozoa that usually reside in the intestinal tract of humans or other creatures. Examples of such microorganisms include Escherichia coli O157:H7, Campylobacter jejuni, and rotavirus.

Pathogens can be transmitted to humans through contact with animals, birds, and other living creatures that naturally harbor the microorganism. The agent of anthraxBacillus anthracis naturally dwells in sheep. Other examples include Brucella abortic (Brucellosis), Coxiella burnetti (Q fever), and viruses that cause hemorrhagic fevers such as Ebola and Marburg.

Pathogenic mechanisms. Microorganisms have various strategies to establish an infection in a host. Some micro-organisms recognize molecules on the surface of the host cell, and use these as receptors. The binding of bacteria or viruses to receptors brings the microorganism in close contact with the host surface.

The nature of the interaction between the host receptor molecule and the attachment molecule on the surface of the bacteria, virus, or protozoan has in some cases been defined, even to the genetic level. The use of recombinant DNA technologywhere a target section of genetic material is removed from one organism and inserted into a certain region of the genetic material of another organism, in a way that does not affect the expression of the geneallows the genetic manipulation of a microorganism so as to enhance its ability to cause an infection. Alternatively, inserting a gene that codes for a toxin into a bacterium that is a normal inhabitant of an environment like the intestinal tract could produce a formidable pathogen. This altered bacteria would readily associate with host cells, but would also carry the toxin.

Viruses almost always damage the host cells. Because viruses cannot reproduce on their own, they rely on the replication mechanism of the host cell to make more copies of themselves (i.e., they are obligate pathogens). Then, the new viral particles will exit the cell and search for another cell to infect. This exit is often very physically damaging to the host cell. Thus, viral infections can be detrimental because of the loss of function of host cells.

Some viral pathogens are capable of causing a disease long after they have infected a host. This delayed response occurs because the viral genetic material becomes incorporated into the genetic material of the host. Thereafter, the viral genetic material is replicated along with that of the host, using the replication enzymes and other machinery of the host. But, in response to a number of signals, the viral material can be excised from the host material and form the template for the manufacture and assembly of new virus particles. A prominent example of such a virus is the Human Immunodeficiency Virus, which is acknowledged to be the cause of Acquired Immunodeficiency Syndrome.

Because viruses must infect other cells in order to replicate, they have developed means of escaping (at least for a time) the defensive responses of the host. This efficiency of attack has not escaped the attention of molecular biologists bent on the malicious use of viruses. By inserting gene coding for a toxic compound into a viral genome, particularly into the genome of an infectious virus (i.e., influenza or cold viruses) the virus becomes a bioweapon. For example, scientists in the former Soviet Union attempted to construct an influenza virus that contained the gene coding for cobra toxin.

FURTHER READING:

BOOKS:

Fields, Bernard N., Peter M. Howley, and Diane E. Griffin, eds. Virology. Philadelphia: Lippincott Williams & Wilkins, 2001.

Shnayerson, Michael, and Mark J. Plotkin. The Killers Within: The Deadly Rise of Drug Resistant Bacteria. New York: Little Brown & Company, 2002.

Smith, H., C. J. Dornan, G. Dougan, et al., eds. The Activities of Bacterial Pathogens In Vivo. River Edge, NJ: World Scientific, 2001.

ELECTRONIC:

Centers for Disease Control and Prevention. "Disease Information." Special Pathogens Branch. July 26, 2002. <http://www.cdc.gov/ncidod/dvrd/spb/mnpages/disinfo.htm> (28 December 2002).

SEE ALSO

Biocontainment Laboratories
Decontamination Methods
Infectious Disease, Threats to Security

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Pathogens

Pathogens

Forensic analysis often involves the determination of the circumstances surrounding an illness outbreak or death. Medical examiners search for pathogens in body tissues and fluids to determine if the cause of a death was due to an infectious process.

Pathogens are organisms, frequently microorganisms or components of these organisms, that cause disease. Microbial pathogens include various species of bacteria, viruses, and protozoa. Many diseases caused by microbial pathogens, and the frequency of these diseases, are a national security issue.

A disease is any condition caused by the presence of an invading organism, or a toxic component, that damages the host. In humans, diseases can be caused by the growth of microorganisms such as bacteria, viruses, and protozoa. Bacterial growth, however, is not mandatory to cause disease. For example, some bacterial pathogens cause disease by virtue of a toxic component of the bacterial cell such as lipopolysaccharide. Finally, the damaging symptoms of a disease can be the result of the attempts by the host's immune system to rid the body of the invader. One example is the immune-related damage caused to the lungs of those afflicted with cystic fibrosis, as the body unsuccessfully attempts to eradicate the chronic infections caused by Pseudomonas aeruginosa (a cause of pneumonia).

Not all pathogens cause diseases that have the same severity of symptoms. For example, an infection with the influenza virus can cause the short term aches and fever that are hallmarks of the flu, or can cause more dire symptoms, depending on the type of virus that causes the infection. Bacteria also vary in the damage caused. For example, the ingestion of food contaminated with Salmonella enteritica causes intestinal upset. But, consumption of Escherichia coli O157:H7 causes a severe disease, which can permanently damage the kidneys and which can even be fatal.

There are three categories of bacterial pathogens. Obligate pathogens are those bacteria that must cause disease in order to be transmitted from one host to another. These bacteria must also infect a host in order to survive, in contrast to other bacteria that are capable of survival outside of a host. Examples of obligate bacterial pathogens include Mycobacterium tuberculosis (tuberculosis) and Treponema pallidum (syphilis).

Opportunistic pathogens can be transmitted from one host to another without having to cause disease. However, in a host whose immune system is not functioning properly, the bacteria can cause an infection that leads to a disease. In those cases, the disease can help the bacteria spread to another host. Examples of opportunistic bacterial pathogens include Vibrio cholerae (cholera) and Pseudomonas aeruginosa (bacterial pneumonia).

Finally, some bacterial pathogens cause disease only accidentally. Indeed, the disease actually limits the spread of the bacteria to another host. Examples of these "accidental" pathogens include Neisseria meningitides (bacterial meningitis) and Bacteroides fragilis (normal intestinal flora that can cause serious infection if it gets into the bloodstream, usually through intestinal ulceration or trauma).

Pathogens can be spread from person to person in a number of ways. Not all pathogens use all the available routes. For example, the influenza virus is transmitted from person to person through the air, typically via sneezing or coughing. But the virus is not transmitted via water. In contrast, Escherichia coli is readily transmitted via water, food, and blood, but is not readily transmitted via air or the bite of an insect.

While routes of transmission vary for different pathogens, a given pathogen will use a given route of transmission. This has been used in the weaponization of pathogens. The best-known example is anthrax . The bacterium that causes anthraxBacillus anthracis can form an environmentally hardy form called a spore. The spore is very small and light. It can float on currents of air and can be breathed into the lungs, where the bacteria resume growth and swiftly cause a serious and often fatal form of anthrax. As demonstrated in the United States in the last few months of 2001, anthrax spores are easily sent through the mail to targets. As well, the powdery spores can be released from an aircraft. Over a major urban center, modeling studies have indicated that the resulting casualties could number in the hundreds of thousands.

Contamination of water by pathogens is another insidious route of disease spread. Water remains crystal clear until there are millions of bacteria present in each milliliter. Viruses, which are much smaller, can be present in even higher numbers without affecting the appearance of the liquid. Thus, water can be easily laced with enough pathogens to cause illness.

Food-borne pathogens cause millions of cases of disease and hundreds of deaths each year in the United States alone. Frequently the responsible microbes are bacteria, viruses, or protozoa that usually reside in the intestinal tract of humans or other creatures. Examples of microorganisms include Escherichia coli O157:H7, Campylobacter jejuni, and rotavirus.

Pathogens can be transmitted to humans through contact with animals, birds, and other living creatures that naturally harbor the microorganism. The agent of anthraxBacillus anthracis naturally dwells in sheep. Other examples include Brucella abortic (Brucellosis), Coxiella burnetti (Q fever), and viruses that cause hemorrhagic fevers such as Ebola and Marburg.

Microorganisms have various strategies to establish an infection in a host. Some microorganisms recognize molecules on the surface of the host cell, and use these as receptors. The binding of bacteria or viruses to receptors brings the microorganism in close contact with the host surface.

The nature of the interaction between the host receptor molecule and the attachment molecule on the surface of the bacteria, virus, or protozoan has in some cases been defined, even to the genetic level. The use of recombinant DNA technologywhere a target section of genetic material is removed from one organism and inserted into a certain region of the genetic material of another organism, in a way that does not affect the expression of the geneallows the genetic manipulation of a microorganism so as to enhance its ability to cause an infection. Alternatively, the addition of a gene that codes for a toxin into a bacterium that is a normal inhabitant of an environment like the intestinal tract could produce a formidable pathogen. This altered bacteria would readily associate with host cells, but would also carry the toxin.

Viruses almost always damage the host cells. Because viruses cannot reproduce on their own, they rely on the replication mechanism of the host cell to make more copies of themselves (i.e., they are obligate pathogens). Then, the new viral particles will exit the cell and search for another cell in which to infect. This exit is often very physically damaging to the host cell. Thus, viral infections can be detrimental because of the loss of function of host cells.

Some viral pathogens are capable of causing a disease long after they have infected a host. This delayed response occurs because the viral genetic material becomes incorporated into the genetic material of the host. Thereafter, the viral genetic material is replicated along with that of the host, using the replication enzymes and other machinery of the host. But, in response to a number of signals, the viral material can be excised from the host material and form the template for the manufacture and assembly of new virus particles. A prominent example of such a virus is the human immunodeficiency virus (HIV), which is acknowledged to be the cause of acquired immunodeficiency syndrome, or AIDS.

see also Bacterial biology; Biosensor technologies; Bioterrorism; Prions; Toxins.

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Viroids and Virusoids

Viroids and Virusoids

Viruses are infectious agents consisting of a nucleic acid genome made of DNA or RNA, a protein coat, and sometimes lipids. They are able to replicate only inside cells, and the viral genome contains genes coding for proteins. Viroids and virusoids are also infectious agents, but they differ from viruses in several ways. For instance, they have a single-stranded circular, RNA genome. Their genomes are very small and do not code for proteins. Viroids replicate autonomously inside a cell, but virusoids cannot. Rather, virusoid replication requires that the cell is also infected with a virus that supplies "helper" functions.

Viroids

Viroids infect plant cells, and more than twenty-five kinds in two families are known. Viroid RNA is 246 to 375 nucleotides long and it folds to form rodlike structures with nucleotide base pairing (in which A pairs with U, C pairs with G). The potato spindle tuber viroid and tomato plant macho viroid, both members of the family Pospiviroidae, replicate in the cell nucleus. A cellular enzyme, RNA polymerase, copies the circular RNA of the viroid to make a linear, repeated copy of the genome in complementary or "negative sense." This RNA is copied again to make another linear, repeated "positive sense" RNA. Cellular enzymes cut this second copy of RNA at each place where the genome begins a repetition, yielding multiple copies of the genome. These copies then reassume a circular shape to make new viroid RNA.

The members of the second family of viroids, the Avsunviroidae, replicate in cell chloroplasts. Two are known, the avocado sunblotch viroid and the peach latent mosaic viroid. In both of these species, RNA polymerase makes a long, linear negative sense RNA. This RNA contains a catalytic ribozyme sequence, which cleaves itself. The negative sense RNA resumes its customary circular shape and is copied to form linear positive sense RNA. The ribozyme again cleaves this RNA, yielding linear genomic units that again recircularize, forming viroids.

The differing replication strategies of these two groups reflect different evolutionary origins. Viroids move through the plant in the phloem and plasmodesmata, which are part of the plant's circulatory system. They propagate by mechanical means, vegetative reproduction, and possibly via seeds and insects. They cause plant diseases following interactions with proteins. For example, when they interact with an enzyme that impairs protein synthesis, the growth of the host plant may be stunted. This can have severe economic consequences.

Virusoids

Virusoid genomes are 220 to 388 nucleotides long. A virusoid genome does not code for any proteins, but instead serves only to replicate itself. Virusoids can replicate in the cytoplasm and possess a ribozyme activity. RNA replication is similar to that of viroids, but each requires that the cell be infected with a specific "helper" virus. Five virusoids are known, and the helper viruses for these are all members of the Sobemovirus family. An example of a "helper" virus is the subterranean clover mottle virus, which has an associated virusoid. Virus enzymes may aid replication of the virusoid RNA. The virusoid is incorporated into the virus particle and transmitted as a "satellite," a separate nucleic acid not part of the viral chromosome. Replication of the helper virus is independent of the virusoid.

Virusoids belong to a larger group of infectious agents called satellite RNAs, which are found in bacteria, plants, fungi, invertebrates, and vertebrates. Some satellite RNAs encode proteins; these are called satellite viruses and, like virusoids, must coinfect with a helper virus in order to replicate. One satellite RNA infecting humans is the hepatitis delta virusoid. It has a circular, single-stranded RNA genome of 1,700 nucleotides. Its helper is the hepatitis B virus, which is associated with liver disease. Coinfection with both agents results in a more severe infection.

see also Ribozyme; Rna Polymerases; Virus.

Shaun Heaphy

Bibliography

Internet Resource

Viroids and Virusoids. University of Leicester, Department of Microbiology & Immunology. <http://www-micro.msb.le.ac.uk/335/viroids.html>.

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Pathogenic Organisms

PATHOGENIC ORGANISMS

Pathogenic organisms are life forms that cause human disease. They range in size and complexity, and include molecules like proteinaceous particles (prions); viruses that are visible under an electron microscope; bacteria, fungi, and protozoan parasites that are sometimes visible to the naked eye; and multicellular parasites like tapeworms that may be many meters long. Many live in natural ecosystems, while others are commensal or parasitic on animals and/or humans. Only a few cannot survive independently of human hosts.

Pathogenic organisms can harm human health in several ways, including consuming nutriment intended for their host (tapeworms); producing poisonous metabolic products (staphylococcus, diphtheria, botulism toxin, and many others); destroying vital organs and tissues (prions, polio, rabies viruses); or interfering with body chemistry (toxic fungus). A few cause cancer (e.g. campylobacter). Their capacity to harm varies widely: The rabies virus virtually always kills its human victims, but seems to live harmlessly in many species of bats. Some obscure microorganisms that do no harm to healthy people can cause debilitating and ultimately fatal infections in persons whose immune defenses have been disrupted by the AIDS (acquired immunodeficiency syndrome) virus.

Rapid advances in microbiology, immunology, pharmacology, and other relevant sciences throughout the twentieth century have led to increasingly effective control measures against many pathogens. However, in biomass alone, microorganisms outweigh higher life forms, including humans, by several orders of magnitude, and they may number many billions of species. Eradication, or even elimination, of most pathogens from human ecosystems is thus not feasible. Humans must therefore learn to live in harmony with them as best they can. The best strategies generally are enhanced immunity and, where possible, avoidance of exposure to them and their harmful effects.

John M. Last

(see also: Communicable Disease Control )

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Pathogens

Pathogens

A pathogen is an agent that causes disease. The agent usually is a microorganism, such as a fungus, bacterium, or virus. The most numerous and prominent pathogens of plants are the fungi, but many plant diseases are also caused by bacteria and viruses. Although the diseases caused by phytoplasmas are similar to those caused by viruses, these pathogens are actually a kind of bacterium. A few diseases are caused by viroids, agents that are similar to but are even simpler than viruses. Other pathogens include nematodes (roundworms), which attack many types of plants, and 2,500 species of angiosperms that live parasitically on other plants. Relatively few of the angiosperms are economically important pathogens.

A pathogen usually initiates disease by parasitizing a host, that is, taking its organic nutrients. However, in a few cases the host actually benefits by the presence of the parasite. Mycorrhizal fungi attack roots and live parasitically in the roots. But infected roots are much more effective than nonmycorrhizal roots in obtaining mineral nutrients, especially phosphorus. Where the level of phosphorus in the soil is low, the plants with mycorrhizal roots are much healthier. Since the parasite actually benefits the plant, it does not cause disease and is not considered a pathogen.

Types of Pathogens

Fungi.

Like all eukaryotic organisms, fungal cells have nuclei, a well-defined endoplasmic reticulum with ribosomes, and cell organelles such as mitochondria. The fungal body consists of filamentous strands called hyphae that collectively make up a mycelium. Sometimes the hyphae become compressed, forming a tissue such as that found in a mushroom fruiting body.

Fungi are classified in the kingdom Fungi, separate from all other organisms. There are many different groups within the kingdom, and most groups have prominent plant pathogens. Of particular note are the Ascomycetes, Fungi Imperfecti, and some of the Basidiomycetes. Ascomycetes produce sexual spores called ascospores that are vital in survival between hosts. They also produce asexual spores called conidia that play a major role in the spread of disease during the growing season. The Fungi Imperfecti usually are ascomycetes that have lost the ascospore (sexual) stage. Sometimes they produce ascospores but have been classified according to the coni-dial stage because of its importance in the disease cycle. It is valid to classify a fungus based either on its perfect (sexual) state or on its imperfect (asexual) state.

The Basidiomycetes are extremely common in nature, but only the rusts and smuts are notable plant pathogens. While most basidiomycetes produce basidiospores in a fruiting body such as a mushroom, in rusts and smuts the basidiospores are produced from a specialized, overwintering spore called a teliospore. The rust fungi are especially common and usually have a complex sexual cycle with four spore stages and two different hosts required for completion of the sexual cycle. Rusts also produce an asexual spore called a uredospore that is responsible for spread of disease during the growing season.

A fourth group of fungi, the Oomycetes, has long been recognized to be very different from other fungi in both morphology and chemistry. They produce overwintering sexual spores called oospores and asexual motile spores (zoospores) that spread disease during the season. The current trend is to place these fungi in a kingdom other than Fungi. The Oomycetes contain the Pythium and Phytophthora species and the downy mildews, plant pathogens that are of worldwide importance. Regardless of classification, these pathogens will continue to be treated much the same as true fungi by those who work with plant diseases.

The potato late blight disease caused by Phytophthora infestans that resulted in famine in Ireland in 1845 and 1846 first brought the attention of the world to plant diseases. At that time many thought that fungi arose spontaneously in diseased plants and did not themselves cause disease. However, publications by the German scientist Anton de Bary beginning in 1853 convincingly demonstrated the prominent role fungi play as plant pathogens.

Bacteria.

The plant pathogenic bacteria are rod-shaped eubacteria. They are prokaryotes, having a chromosome but no nucleus. The cytoplasm has ribosomes but no endoplasmic reticulum and no organelles. The cells have a cell wall and may or may not have flagella .

The first bacterium shown to cause a plant disease, fire blight of pome fruits, was reported by Thomas Burrill in Illinois in 1878. However, it was the research by Erwin F. Smith from 1890 to 1915 that demonstrated the importance of these agents as plant pathogens.

Viruses.

Viruses are nucleoprotein macromolecules . They contain genetic information, either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA), which is covered by protein subunits. Plant pathogenic viruses usually contain RNA rather than DNA. Since viruses are not cellular organisms, they express lifelike characteristics only when within a susceptible host cell. Additionally, since they are not cellular, they do not obtain organic nutrients directly from the host. Instead, the RNA or DNA of the virus directs the metabolic machinery of the host cell to use organic nutrients present in the cell. Various chemical reactions lead to symptom expression as well as production of new virus particles.

DISTINGUISHING CHARACTERISTICS OF VARIOUS PATHOGENS
Character Fungi Bacteria Viruses Phytoplasmas Nematodes
Body type Hyphae make up a mycelium (eukaryotic) Cells with cell walls (prokaryotic) Nucleoprotein (not cellular) Cells with no cell walls (prokaryotic) Worms with organ systems, males and females
Inoculum Overwinter by ascospores, teliospores, and oospores; spread of disease by conidia, zoospores, uredospores Cells Virus particles Cells Larvae
Dissemination Wind and splashing rain Splashing rain Transmission by insects, mechanically, or planting stock Transmission by insects Soil movement, running soil water
Penetration Direct by appressorium and penetration peg; some through wounds or natural openings Through natural openings (stomata) or wounds Through wounds (insects) or planting stock Through wounds (insects) Direct with stylet mouthpart
Host-parasite relation Inter- and intracellular; intercellular with haustoria Intercellular Intracellular in parenchyma or phloem tissue Intracellular in phloem tissue Intercellular with stylet inserted in host cells

Tobacco mosaic disease was shown in the 1890s to be caused by a sub-microscopic infectious agent later determined to be a virus, now called tobacco mosaic virus.

Viroids.

A viroid is a small, infectious piece of RNA. Unlike a virus, it has no protein, but it behaves as a plant parasite much the same as a virus. In 1967, Theodor Diener and William Raymer reported on the basic characteristics of the agent causing spindle tuber of potato, and in 1971 Diener named these agents "viroids." About twenty diseases have been shown to be caused by this type of pathogen.

Phytoplasmas.

Phytoplasmas are prokaryotic cellular organisms. They represent a separate group of bacteria, having no cell wall or flagella. Much like viruses, they are transmitted by insects and cause phloem necrosis-type diseases. These diseases, having yellows witches'-broom-type symptoms, were thought to be caused by viruses until 1967 when Yoji Doi and others in Japan showed that the disease-inducing agents are mycoplasma-like organisms. Some of these pathogens, especially the aster yellows phytoplasma, have a wide host range and attack plants in many families.

For many years these plant pathogens were identified as mycoplasma-like organisms, but they now are called phytoplasmas. Although viruses and phytoplasmas are very different biological agents, similarities in transmission and in host-parasite interactions and symptoms make it easy to understand why researchers before 1967 thought all these diseases were caused by viruses.

Nematodes.

Plant pathogenic nematodes (roundworms) are usually found in soil. These plant pathogens have a stylet (spearlike) mouthpart that is used to penetrate and feed on roots. Although root knot nematode diseases have been well known since the 1850s, it was not until the period between 1920 and 1940 that research by many investigators showed the full significance of these agents as plant pathogens.

Angiosperms.

Like typical flowering plants, parasitic angiosperms have stems, leaves, flowers, and seeds. They do not have true roots but produce a structure that penetrates stems and unites with the vascular system of the host. The leaves may or may not have chloroplasts , but these pathogens are completely dependent on the host for water and mineral nutrients. Dwarf mistletoe is a prominent pathogen of coniferous forest trees, and the dodders attack many crops worldwide. Leafy mistletoe, known as a popular household Christmas decoration, attacks hardwood trees but is seldom a leading pathogen.

Insects.

Insects do not merely feed on plants; they often produce toxins and growth substances that cause diseaselike symptoms. Some of the biological interactions are similar to those that occur with nematodes. However, there are hundreds of thousands, perhaps millions, of insect species, and their life cycles and behavior may be very complex. Although the phenomenon may be much the same, insect problems are worked on by experts (entomologists), and the insects usually are not thought of as pathogens. They are, however, major vectors of diseases, including those caused by viruses and phytoplasmas and some fungi and bacteria.

How Pathogens Cause Disease

Pathogens impede normal growth of plants in many ways. They attack and kill seeds and seedlings (damping off diseases). They invade and kill roots, preventing absorption of water and mineral nutrients (root rots). They invade and plug xylem tissue, preventing movement of water and minerals to leaves and growing points (vascular wilts). They kill leaves, preventing photosynthesis and production of carbohydrates (leaf spots and blights, downy mildews, powdery mildews, and rusts). The phloem tissue may be invaded and killed, preventing translocation of the carbohydrate produced in photosynthesis to other parts of the plant (phloem necrosis). After the crop has been produced, pathogens may rot fruits and vegetables in transit or storage or in the marketplace (fruit and vegetable rots). A few pathogens cause disease by inducing abnormal growth, thus stunting normal growth (galls).

Damping Off.

Many fungi that live in the soil invade and kill seeds or seedlings. This is called damping off. Beyond the seedling stage, a plant is no longer susceptible to damping off. There is no resistance but seed treatments with fungicides usually provide effective control.

Root Rots.

Roots may be rotted by many fungi living in the soil. The resulting lack of water and mineral nutrients stunts growth and causes a general yellowing due to lack of chlorophyll. There usually is little resistance, and chemical controls are ineffective. Crop rotation may hold crop losses to acceptable levels.

Vascular Wilts.

Vascular wilts, diseases of the xylem tissue, are caused by fungi and bacteria. Fungi causing this disease are soilborne. They infect roots and grow through the plant, colonizing the xylem. Bacteria that cause vascular wilts are transmitted by insects or penetrate leaves through stomatal openings. They invade xylem tissue through pits in the xylem vessel cell walls and become systemic in the plant. Vascular wilts caused by both fungi and bacteria reduce movement of water and mineral nutrients to stems and leaves, resulting in symptoms of wilting and yellowing. There is no chemical control but genetic resistance may be helpful. Crop rotation is important in combating vascular wilts.

Leaf Diseases.

Many fungi and bacteria attack leaves, causing leaf spots and blights. The downy and powdery mildews also cover the leaves with fungal structures further reducing photosynthesis. Both of the mildews and the rusts eventually kill leaf tissue, but since all three are obligate parasites , they no longer can obtain nutrients after the leaves are dead. Resistance to these fungal diseases often is available, and chemical controls usually are effective. Most of the fungicides applied to crops are used to control leaf diseases. There are fewer leaf diseases caused by bacteria, and this is fortunate because genetic resistance is seldom available and chemical controls are usually ineffective.

Phloem Necrosis.

Viruses and phytoplasmas are transmitted by insect vectors that feed on leaves. The vectors often feed on the phloem tissue, directly depositing the pathogen in the tissue. These agents then become systemic in the phloem tissue, killing the phloem cells (necrosis) and preventing translocation of organic nutrients throughout the plant. Typical symptoms are stunting, yellowing, mosaic (different shades of green and yellow), and mottling (blotches of different colors). There is no chemical control and often little resistance. Cultural practices such as use of healthy planting stock often limit disease incidence.

Fruit and Vegetable Rots.

Postharvest rots by fungi and bacteria often cause serious losses. Chemical treatments help control these diseases, but more significant prevention tactics include sanitation and use of proper storage conditions, particularly reduced temperature and increased air circulation.

Galls.

Abnormal growth is an extremely common phenomenon and can be caused by many biological agents. The most notable abnormal growth diseases of crops are crown gall, caused by the bacterium Agrobacterium, club root of crucifers caused by the fungus Plasmodiophora brassicae, and root knot caused by the nematode Meloidogyne. Abnormal growth results from action of the same kinds of growth substances that are responsible for normal growth: auxins , cytokinins, and gibberellins.

Recognition and Penetration

Relatively little is known of the biochemistry of recognition of a susceptible host by a pathogen. Fungal and bacterial inoculum (infectious material) is spread at random to both hosts and nonhosts. Mucilagenous substances on the surface of the inoculum facilitates adherence to host surfaces. Some host chemicals that serve as signals leading to penetration are known, and some pathogen chemicals that serve as elicitors in disease development have been identified. In some cases penetration occurs but growth in the host is limited, and disease does not develop. Either the agent does not produce the elicitors that lead to infection, or the host produces chemicals that prevent infection. Since viruses and phytoplasmas are brought to hosts by insect vectors, disease may result from an adaptive sequence in which the vector feeds preferentially on the hosts that are susceptible to the pathogen.

Fungi usually penetrate leaves by production of a specialized structure called an appressorium. As a fungal hypha grows over the surface of a leaf, the hyphal tip mounds up and becomes cemented to the leaf, forming an appressorium. A specialized hypha, called a penetration peg, grows from the appressorium and penetrates the leaf, largely by mechanical pressure. The penetration peg also may produce cutinase and cellulose enzymes that soften the tissue. The leaf epidermis is covered by a cuticle made primarily of a waxy substance called cutin, and the epidermal cell walls have a high cellulose content. Sometimes a fungus penetrates through a stoma, a hole in the lower epidermis of the leaf formed by two guard cells. Even when a fungus penetrates through a stoma, an appressorium is usually produced. Of course, the penetration peg meets no resistance.

Inside the leaf, fungal hyphae grow between cells (intercellular) and through cells (intracellular) to obtain nutrients. When the leaf dies, the fungus is able to obtain nutrients from the dead cells. The fungi that are obligate parasites (downy mildews, powdery mildews, and rusts) grow inter-cellularly and produce haustoria (specialized hyphal structures) that penetrate the host cells. The haustoria produce enzymes and obtain nutrients from the host cells. Eventually the cells die, and these fungi are no longer able to obtain nutrients.

Bacteria that attack leaves are disseminated in splashing rain and penetrate through stomata or wounds. The bacteria are found between cells in the host and never penetrate the living cells. Nutrients leaking from the host cells provide sufficient food for the bacteria. After the death of leaves, the bacteria continue to obtain nutrients from the dead cells.

Viruses and phytoplasmas are usually transmitted by insects. Feeding by the insects deposits these agents into the phloem or parenchyma tissues. Some viruses can be transmitted by workers in the field. Handling plants causes small wounds and transmits small amounts of contaminated sap. Many viruses attack crops that are propagated vegetatively (by bulbs, corms, budding, etc.), and the diseases are transmitted through use of infected planting stock. Both viruses and phytoplasmas are obligate parasites and cannot obtain nutrients from tissues that have died.

Nematodes penetrate roots mechanically by use of the stylet mouth part. Once inside they insert the stylet into parenchyma cells of the cortex and obtain nutrients. Some nematodes have a long stylet and feed on plant roots while the body is outside the root. Plant pathogenic nematodes are all obligate parasites, capable of obtaining nutrients only from living host cells.

Role of Enzymes, Toxins, and Phytoalexins

Because cutin and cellulose provide tough, protective barriers for the plant, cutinase and cellulase enzymes are necessary to the penetration of plant hosts by pathogenic fungi. They break down the cutin in the cuticle and the cellulose in the primary cell wall. Hydrolytic (digestive) enzymes also play important roles in pathogenesis. The organic food in the host is usually in the form of complex carbohydrates, fats, and proteins. To be absorbed by pathogens, they must be broken down to their simpler units: simple sugars, fatty acids, glycerol, and amino acids. Common digestive enzymesamylases, cellulases, lipases, and proteasesproduced by pathogens break down these complex foods.

The middle lamella, the area between cells in parenchyma tissue, has a high pectin content. For many diseases pathogenesis involves production of pectolytic enzymes that break down pectin. This causes dissolution and eventually death of the cells. Damping off, root rots, vascular wilts, and fruit and vegetable rots are caused by pathogens that produce large amounts of pectolytic enzymes.

Several toxins have been shown to be produced by plant pathogenic fungi and bacteria. Most of them are nonhost-specific toxins. They usually kill cells but may act on the permeability of the cytoplasmic membrane. Although they are involved in pathogenesis, in some cases strains of the pathogen that are unable to produce the toxin still can cause disease. In a few cases the toxin is host-specific and only affects that host at normal toxin concentrations.

Most plants are resistant to infection because of the presence of preexisting chemicals. However, there are many cases where chemicals that ward off infection are produced by the host only after the pathogen is present. These chemicals are called phytoalexins. This is a rather common phenomenon, with about three hundred chemicals from thirty different families of plants having been identified as phytoalexins.

see also Agriculture, Modern; Chestnut Blight; Defenses, Chemical; Dutch Elm Disease; Eubacteria; Fungi; Herbicides; Hormones; Interaction, Plant-Fungal; Potato Blight.

Ira W. Deep

Bibliography

Agrios, George N. Plant Pathology, 4th ed. New York: San Diego, CA: Academic Press, 1997.

Bove, Joseph M. "Wall-less Prokaryotes of Plants." Annual Review of Phytopathology 22 (1984): 361-96.

Doi, Yoji, et al. "Mycoplasma- or PLT Group-like Microorganisms Found in the Phloem Elements of Plants Infected with Mulberry Dwarf, Potato Witches'-Broom, Aster Yellows, or Paulownia Witches'-Broom." Annual Phytopathological Society of Japan 33 (1967): 259-66.

Diener, Theodor O. "The ViroidA Subviral Pathogen." American Scientist 71 (1983): 481-89.

Schumann, Gail L. Plant Diseases: Their Biology and Social Impact. St. Paul, MN: American Phytopathological Society Press, 1991.

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pathogens

pathogens Disease‐causing bacteria, as distinct from those that are harmless.

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