Micro Test 3: Chapter 15

Signs and Symptoms of Disease.

1. A disease is any condition in which the normal structure or functions of the body are damaged or impaired. a. Physical injuries or disabilities are not classified as disease b. there can be several causes for disease, including infection by a pathogen, genetics (as in many cancers or deficiencies), noninfectious environmental causes, or inappropriate immune responses. 2. An infection is the successful colonization of a host by a microorganism. Infections can lead to disease, which causes signs and symptoms resulting in a deviation from the normal structure or functioning of the host. Microorganisms that can cause disease are known as pathogens. 3. The signs of disease are objective, measurable, and can be directly observed by a clinician. a. Vital signs, which are used to measure the body's basic functions, i. include body temperature (normally 37 °C [98.6 °F]) ii. heart rate (normally 60-100 beats per minute) iii. breathing rate (normally 12-18 breaths per minute) iv. blood pressure (normally between 90/60 mm Hg - 120/80 mm Hg). v. Changes in any of the body's vital signs may be indicative of disease. For example, having a fever (a body temperature significantly higher than 37 °C or 98.6 °F) is a sign of disease because it can be measured. b. Other observable conditions may be considered signs of disease. i. the presence of antibodies in a patient's serum (the liquid portion of blood that lacks clotting factors) can be observed and measured through blood tests and, therefore, can be considered a sign. ii. However, it is important to note that the presence of antibodies is not always a sign of an active disease. iii. Antibodies can remain in the body long after an infection has resolved; also, they may develop in response to a pathogen that is in the body but not currently causing disease. 4. Unlike signs, symptoms of disease are subjective. Symptoms are felt or experienced by the patient, but they cannot be clinically confirmed or objectively measured. Examples of symptoms include: a. Nausea b. loss of appetite c. pain d. Such symptoms are important to consider when diagnosing disease, but they are subject to memory bias and are difficult to measure precisely. e. Some clinicians attempt to quantify symptoms by asking patients to assign a numerical value to their symptoms. For example, the Wong-Baker Faces pain-rating scale asks patients to rate their pain on a scale of 0-10. f. An alternative method of quantifying pain is measuring skin conductance fluctuations. These fluctuations reflect sweating due to skin sympathetic nerve activity resulting from the stressor of pain. 5. A specific group of signs and symptoms characteristic of a particular disease is called a syndrome. Many syndromes are named using a nomenclature based on signs and symptoms or the location of the disease. 6. Clinicians must rely on signs and on asking questions about symptoms, medical history, and the patient's recent activities to identify a particular disease and the potential causative agent. Diagnosis is complicated by the fact that different microorganisms can cause similar signs and symptoms in a patient. a. Ex: an individual presenting with symptoms of diarrhea may have been infected by one of a wide variety of pathogenic microorganisms. i. Bacterial pathogens associated with diarrheal disease include Vibrio cholerae, Listeria monocytogenes, Campylobacter jejuni, and enteropathogenic Escherichia coli (EPEC). ii. Viral pathogens associated with diarrheal disease include norovirus and rotavirus. iii. Parasitic pathogens associated with diarrhea include Giardia lamblia and Cryptosporidium parvum. b. Ex: fever is indicative of many types of infection, from the common cold to the deadly Ebola hemorrhagic fever. Finally, some diseases may be asymptomatic or subclinical, meaning they do not present any noticeable signs or symptoms. For example, most individual infected with herpes simplex virus remain asymptomatic and are unaware that they have been infected

Stages of Pathenogenesis. To cause disease, a pathogen must successfully achieve four steps or stages of pathogenesis: exposure (contact), adhesion (colonization), invasion, and infection.

1. Exposure (contact) An encounter with a potential pathogen is known as exposure or contact. a. An anatomic site through which pathogens can pass into host tissue is called a portal of entry. These are locations where the host cells are in direct contact with the external environment. Major portals of entry are identified in and include the skin, mucous membranes, and parenteral routes. b. Pathogens can also enter through a breach in the protective barriers of the skin and mucous membranes. Pathogens that enter the body in this way are said to enter by the parenteral route. For example, the skin is a good natural barrier to pathogens, but breaks in the skin (e.g., wounds, insect bites, animal bites, needle pricks) can provide a parenteral portal of entry for microorganisms. c. In pregnant women, the placenta normally prevents microorganisms from passing from the mother to the fetus. However, a few pathogens are capable of crossing the blood-placental barrier. The gram-positive bacterium Listeria monocytogenes, which causes the foodborne disease listeriosis, is one example that poses a serious risk to the fetus and can sometimes lead to spontaneous abortion. Other pathogens that can pass the placental barrier to infect the fetus are known collectively by the acronym TORCH. d. Transmission of infectious diseases from mother to baby is also a concern at the time of birth when the baby passes through the birth canal. Babies whose mothers have active chlamydia or gonorrhea infections may be exposed to the causative pathogens in the vagina, which can result in eye infections that lead to blindness. To prevent this, it is standard practice to administer antibiotic drops to infants' eyes shortly after birth. 2. Adhesion (colonization) refers to the capability of pathogenic microbes to attach to the cells of the body using adhesion factors, and different pathogens use various mechanisms to adhere to the cells of host tissues. a. Molecules (either proteins or carbohydrates) called adhesins are found on the surface of certain pathogens and bind to specific receptors (glycoproteins) on host cells. Adhesins are present on the fimbriae and flagella of bacteria, the cilia of protozoa, and the capsids or membranes of viruses. Protozoans can also use hooks and barbs for adhesion; spike proteins on viruses also enhance viral adhesion. The production of glycocalyces (slime layers and capsules), with their high sugar and protein content, can also allow certain bacterial pathogens to attach to cells. b. Biofilm growth can also act as an adhesion factor. A biofilm is a community of bacteria that produce a glycocalyx, known as extrapolymeric substance (EPS), that allows the biofilm to attach to a surface. Persistent Pseudomonas aeruginosa infections are common in patients suffering from cystic fibrosis, burn wounds, and middle-ear infections (otitis media) because P. aeruginosa produces a biofilm. The EPS allows the bacteria to adhere to the host cells and makes it harder for the host to physically remove the pathogen. The EPS not only allows for attachment but provides protection against the immune system and antibiotic treatments, preventing antibiotics from reaching the bacterial cells within the biofilm. In addition, not all bacteria in a biofilm are rapidly growing; some are in stationary phase. Since antibiotics are most effective against rapidly growing bacteria, portions of bacteria in a biofilm are protected against antibiotics. 3. Invasion involves the dissemination of a pathogen throughout local tissues or the body. a. Pathogens may produce exoenzymes or toxins, which serve as virulence factors that allow them to colonize and damage host tissues as they spread deeper into the body. Pathogens may also produce virulence factors that protect them against immune system defenses. A pathogen's specific virulence factors determine the degree of tissue damage that occurs. Ex: H. pylori. b. Intracellular pathogens achieve invasion by entering the host's cells and reproducing. Some are obligate intracellular pathogens (meaning they can only reproduce inside of host cells) and others are facultative intracellular pathogens (meaning they can reproduce either inside or outside of host cells). By entering the host cells, intracellular pathogens are able to evade some mechanisms of the immune system while also exploiting the nutrients in the host cell. c. For most kinds of host cells, pathogens use one of two different mechanisms for endocytosis and entry. i. One mechanism relies on effector proteins secreted by the pathogen; these effector proteins trigger entry into the host cell. This is the method that Salmonella and Shigella use when invading intestinal epithelial cells. When these pathogens come in contact with epithelial cells in the intestine, they secrete effector molecules that cause protrusions of membrane ruffles that bring the bacterial cell in. This process is called membrane ruffling. ii. the second mechanism relies on surface proteins expressed on the pathogen that bind to receptors on the host cell, resulting in entry. For example, Yersinia pseudotuberculosis produces a surface protein known as invasin that binds to beta-1 integrins expressed on the surface of host cells. iii. Some host cells, such as white blood cells and other phagocytes of the immune system, actively endocytose pathogens in a process called phagocytosis. Although phagocytosis allows the pathogen to gain entry to the host cell, in most cases, the host cell kills and degrades the pathogen by using digestive enzymes. Normally, when a pathogen is ingested by a phagocyte, it is enclosed within a phagosome in the cytoplasm; the phagosome fuses with a lysosome to form a phagolysosome, where digestive enzymes kill the pathogen. However, some intracellular pathogens have the ability to survive and multiply within phagocytes. Examples include Listeria monocytogenes and Shigella; these bacteria produce proteins that lyse the phagosome before it fuses with the lysosome, allowing the bacteria to escape into the phagocyte's cytoplasm where they can multiply. Bacteria such as Mycobacterium tuberculosis, Legionella pneumophila, and Salmonella species use a slightly different mechanism to evade being digested by the phagocyte. These bacteria prevent the fusion of the phagosome with the lysosome, thus remaining alive and dividing within the phagosome. 4. Infection can be described as local, focal, or systemic, depending on the extent of the infection. a. local infection is confined to a small area of the body, typically near the portal of entry. For example, a hair follicle infected by Staphylococcus aureus infection may result in a boil around the site of infection, but the bacterium is largely contained to this small location. b. focal infection, a localized pathogen, or the toxins it produces, can spread to a secondary location. For example, a dental hygienist nicking the gum with a sharp tool can lead to a local infection in the gum by Streptococcus bacteria of the normal oral microbiota. These Streptococcus spp. may then gain access to the bloodstream and make their way to other locations in the body, resulting in a secondary infection. c. systemic infection an infection becomes disseminated throughout the body. For example, infection by the varicella-zoster virus typically gains entry through a mucous membrane of the upper respiratory system. It then spreads throughout the body, resulting in the classic red skin lesions associated with chickenpox. Since these lesions are not sites of initial infection, they are signs of a systemic infection. a primary infection, the initial infection caused by one pathogen, can lead to a secondaryinfection by another pathogen. For example, the immune system of a patient with a primary infection by HIV becomes compromised, making the patient more susceptible to secondary diseases like oral thrush and others caused by opportunistic pathogens. Similarly, a primary infection by Influenzavirus damages and decreases the defense mechanisms of the lungs, making patients more susceptible to a secondary pneumonia by a bacterial pathogen like Haemophilus influenzae or Streptococcus pneumoniae. Some secondary infections can even develop as a result of treatment for a primary infection. Antibiotic therapy targeting the primary pathogen can cause collateral damage to the normal microbiota, creating an opening for opportunistic pathogens

Primary Pathogens vs Opportunistic Pathogens

1. A primary pathogen can cause disease in a host regardless of the host's resident microbiota or immune system. An opportunistic pathogen, by contrast, can only cause disease in situations that compromise the host's defenses, such as the body's protective barriers, immune system, or normal microbiota. Individuals susceptible to opportunistic infections include the very young, the elderly, women who are pregnant, patients undergoing chemotherapy, people with immunodeficiencies (such as acquired immunodeficiency syndrome [AIDS]), patients who are recovering from surgery, and those who have had a breach of protective barriers (such as a severe wound or burn)

Koch's Postulates

1. Koch's Postulates a. the suspected pathogen must be found in every case of disease and not be found in healthy individuals. b. The suspected pathogen can be isolated and grown in pure culture. c. A healthy test subject infected with the suspected pathogen must develop the same signs and symptoms of disease as seen in postulate 1. d. the pathogen must be re-isolated from the new host and must be identical to the pathogen from postulate 2. 2. Advances in microbiology have revealed some important limitations in Koch's criteria. Koch made several assumptions that we now know are untrue in many cases. a. the first relates to postulate 1, which assumes that pathogens are only found in diseased, not healthy, individuals. b. Koch's second faulty assumption was that all healthy test subjects are equally susceptible to disease. We now know that individuals are not equally susceptible to disease. Koch also assumed that all pathogens are microorganisms that can be grown in pure culture (postulate 2) and that animals could serve as reliable models for human disease. However, we now know that not all pathogens can be grown in pure culture, and many human diseases cannot be reliably replicated in animal hosts

Virulence Factors for Survival in the Host and Immune Evasion

1. Many bacteria produce capsules, which are used in adhesion but also aid in immune evasion by preventing ingestion by phagocytes. the composition of the capsule prevents immune cells from being able to adhere and then phagocytose the cell. In addition, the capsule makes the bacterial cell much larger, making it harder for immune cells to engulf the pathogen. A notable capsule-producing bacterium is the gram-positive pathogen Streptococcus pneumoniae, which causes pneumococcal pneumonia, meningitis, septicemia, and other respiratory tract infections. Encapsulated strains of S. pneumoniae are more virulent than nonencapsulated strains and are more likely to invade the bloodstream and cause septicemia and meningitis. 2. Some pathogens can also produce proteases to protect themselves against phagocytosis. The human immune system produces antibodies that bind to surface molecules found on specific bacteria (e.g., capsules, fimbriae, flagella, LPS). This binding initiates phagocytosis and other mechanisms of antibacterial killing and clearance. Proteases combat antibody-mediated killing and clearance by attacking and digesting the antibody molecules. 3. Some bacterial pathogens produce other virulence factors that allow them to evade the immune system. a. the fimbriae of certain species of Streptococcus contain M protein, which alters the surface of Streptococcus and inhibits phagocytosis by blocking the binding of the complement molecules that assist phagocytes in ingesting bacterial pathogens. b. the acid-fast bacterium Mycobacterium tuberculosis (the causative agent of tuberculosis) produces a waxy substance known as mycolic acid in its cell envelope. When it is engulfed by phagocytes in the lung, the protective mycolic acid coat enables the bacterium to resist some of the killing mechanisms within the phagolysosome. c. Some bacteria produce virulence factors that promote infection by exploiting molecules naturally produced by the host. For example, most strains of Staphylococcus aureus produce the exoenzyme coagulase, which exploits the natural mechanism of blood clotting to evade the immune system. If bacteria release coagulase into the bloodstream, the fibrinogen-to-fibrin cascade is triggered in the absence of blood vessel damage. The resulting clot coats the bacteria in fibrin, protecting the bacteria from exposure to phagocytic immune cells circulating in the bloodstream. d. Kinases have the opposite effect by triggering the conversion of plasminogen to plasmin, which is involved in the digestion of fibrin clots. By digesting a clot, kinases allow pathogens trapped in the clot to escape and spread, similar to the way that collagenase, hyaluronidase, and DNAse facilitate the spread of infection. Examples of kinases include staphylokinases and streptokinases, produced by Staphylococcus aureus and Streptococcus pyogenes, respectively. Antigenic variation is the alteration of surface proteins so that a pathogen is no longer recognized by the host's immune system. For example, the bacterium Borrelia burgdorferi, the causative agent of Lyme disease, contains a surface lipoprotein known as VlsE. Because of genetic recombination during DNA replication and repair, this bacterial protein undergoes antigenic variation. Each time fever occurs, the VlsE protein in B. burgdorferi can differ so much that antibodies against previous VlsE sequences are not effective. It is believed that this variation in the VlsE contributes to the ability B. burgdorferi to cause chronic disease. Another important human bacterial pathogen that uses antigenic variation to avoid the immune system is Neisseria gonorrhoeae, which causes the sexually transmitted disease gonorrhea. this bacterium is well known for its ability to undergo antigenic variation of its type IV pili to avoid immune defenses

Classifications of Disease

1. the World Health Organization's (WHO) International Classification of Diseases (ICD) is used in clinical fields to classify diseases and monitor morbidity (the number of cases of a disease) and mortality (the number of deaths due to a disease). 2. An infectious disease is any disease caused by the direct effect of a pathogen. A pathogen may be cellular (bacteria, parasites, and fungi) or acellular (viruses, viroids, and prions). Some infectious diseases are also communicable, meaning they are capable of being spread from person to person through either direct or indirect mechanisms. Some infectious communicable diseases are also considered contagious diseases, meaning they are easily spread from person to person. Not all contagious diseases are equally so; the degree to which a disease is contagious usually depends on how the pathogen is transmitted. a. Measles is a highly contagious viral disease that can be transmitted when an infected person coughs or sneezes and an uninfected person breathes in droplets containing the virus. b. Gonorrhea is not as contagious as measles because transmission of the pathogen (Neisseria gonorrhoeae) requires close intimate contact (usually sexual) between an infected person and an uninfected person. 3. Diseases that are contracted as the result of a medical procedure are known as iatrogenic diseases. a. Iatrogenic diseases can occur after procedures involving wound treatments, catheterization, or surgery if the wound or surgical site becomes contaminated. b. Ex: an individual treated for a skin wound might acquire necrotizing fasciitis (an aggressive, "flesh-eating" disease) if bandages or other dressings became contaminated by Clostridium perfringens or one of several other bacteria that can cause this condition. 4. Diseases acquired in hospital settings are known as nosocomial diseases. Several factors contribute to the prevalence and severity of nosocomial diseases. a. First, sick patients bring numerous pathogens into hospitals, and some of these pathogens can be transmitted easily via improperly sterilized medical equipment, bed sheets, call buttons, door handles, or by clinicians, nurses, or therapists who do not wash their hands before touching a patient. b. Second, many hospital patients have weakened immune systems, making them more susceptible to infections. c. Compounding this, the prevalence of antibiotics in hospital settings can select for drug-resistant bacteria that can cause very serious infections that are difficult to treat. 5. Zoonotic diseases (or zoonosis) - infectious diseases not transmitted between humans directly but can be transmitted from animals to humans. According to WHO, a zoonosis is a disease that occurs when a pathogen is transferred from a vertebrate animal to a human; however, sometimes the term is defined more broadly to include diseases transmitted by all animals (including invertebrates). a. For example, rabies is a viral zoonotic disease spread from animals to humans through bites and contact with infected saliva. b. Many other zoonotic diseases rely on insects or other arthropods for transmission. Examples include yellow fever (transmitted through the bite of mosquitoes infected with yellow fever virus) and Rocky Mountain spotted fever (transmitted through the bite of ticks infected with Rickettsia rickettsii). 6. In contrast to communicable infectious diseases, a noncommunicable infectious disease is not spread from one person to another. One example is tetanus, caused by Clostridium tetani, a bacterium that produces endospores that can survive in the soil for many years. This disease is typically only transmitted through contact with a skin wound; it cannot be passed from an infected person to another person. Similarly, Legionnaires disease is caused by Legionella pneumophila, a bacterium that lives within amoebae in moist locations like water-cooling towers. An individual may contract Legionnaires disease via contact with the contaminated water, but once infected, the individual cannot pass the pathogen to other individuals. 7. In addition to the wide variety of noncommunicable infectious diseases, noninfectious diseases (those not caused by pathogens) are an important cause of morbidity and mortality worldwide. Noninfectious diseases can be caused by a wide variety factors: a. genetics. Ex: sickle cell anemia is an inherited disease caused by a genetic mutation that can be passed from parent to offspring b. the environment immune system dysfunction

Pathogenicity and Virulence

1. the ability of a microbial agent to cause disease is called pathogenicity, and the degree to which an organism is pathogenic is called virulence. Virulence is a continuum. a. On one end of the spectrum are organisms that are avirulent (not harmful) and on the other are organisms that are highly virulent. Highly virulent pathogens will almost always lead to a disease state when introduced to the body, and some may even cause multi-organ and body system failure in healthy individuals. b. Less virulent pathogens may cause an initial infection but may not always cause severe illness. Pathogens with low virulence would more likely result in mild signs and symptoms of disease, such as low-grade fever, headache, or muscle aches. Some individuals might even be asymptomatic. 2. An example of a highly virulent microorganism is Bacillus anthracis, the pathogen responsible for anthrax. B. anthracis can produce different forms of disease, depending on the route of transmission (e.g., cutaneous injection, inhalation, ingestion). The most serious form of anthrax is inhalation anthrax. After B. anthracis spores are inhaled, they germinate. An active infection develops, and the bacteria release potent toxins that cause edema (fluid buildup in tissues), hypoxia (a condition preventing oxygen from reaching tissues), and necrosis (cell death and inflammation). Signs and symptoms of inhalation anthrax include high fever, difficulty breathing, vomiting and coughing up blood, and severe chest pains suggestive of a heart attack. With inhalation anthrax, the toxins and bacteria enter the bloodstream, which can lead to multi-organ failure and death of the patient. If a gene (or genes) involved in pathogenesis is inactivated, the bacteria become less virulent or nonpathogenic. Virulence of a pathogen can be quantified using controlled experiments with laboratory animals. Two important indicators of virulence are the median infectious dose (ID50) and the median lethal dose (LD50), both of which are typically determined experimentally using animal models. The ID50 is the number of pathogen cells or virions required to cause active infection in 50% of inoculated animals. The LD50 is the number of pathogenic cells, virions, or amount of toxin required to kill 50% of infected animals

Periods of Disease. The five periods of disease (aka stages or phases) include the incubation, prodromal, illness, decline, and convalescence periods.

1. the incubation period occurs in an acute disease after the initial entry of the pathogen into the host (patient). It is during this time the pathogen begins multiplying in the host. However, there are insufficient numbers of pathogen particles (cells or viruses) present to cause signs and symptoms of disease. Incubation periods can vary from a day or two in acute disease to months or years in chronic disease, depending upon the pathogen. Factors involved in determining the length of the incubation period are diverse, and can include strength of the pathogen, strength of the host immune defenses, site of infection, type of infection, and the size infectious dose received. During this incubation period, the patient is unaware that a disease is beginning to develop. 2. the prodromal period occurs after the incubation period. During this phase, the pathogen continues to multiply, and the host begins to experience general signs and symptoms of illness, which typically result from activation of the immune system, such as fever, pain, soreness, swelling, or inflammation. Usually, such signs and symptoms are too general to indicate a particular disease. 3. the period of illness, during which the signs and symptoms of disease are most obvious and severe. 4. the period of decline during which the number of pathogen particles begins to decrease, and the signs and symptoms of illness begin to decline. However, during the decline period, patients may become susceptible to developing secondary infections because their immune systems have been weakened by the primary infection. 5. the final period is known as the period of convalescence. During this stage, the patient generally returns to normal functions, although some diseases may inflict permanent damage that the body cannot fully repair. 6. Infectious diseases can be contagious during all five of the periods of disease. Which periods of disease are more likely to associated with transmissibility of an infection depends upon the disease, the pathogen, and the mechanisms by which the disease develops and progresses. a. For example, with meningitis (infection of the lining of brain), the periods of infectivity depend on the type of pathogen causing the infection. i. Patients with bacterial meningitis are contagious during the incubation period for up to a week before the onset of the prodromal period ii. patients with viral meningitis become contagious when the first signs and symptoms of the prodromal period appear. b. With many viral diseases associated with rashes (e.g., chickenpox, measles, rubella, roseola), patients are contagious during the incubation period up to a week before the rash develops. c. In contrast, with many respiratory infections (e.g., colds, influenza, diphtheria, strep throat, and pertussis) the patient becomes contagious with the onset of the prodromal period. d. Depending upon the pathogen, the disease, and the individual infected, transmission can still occur during the periods of decline, convalescence, and even long after signs and symptoms of the disease disappear. For example, an individual recovering from a diarrheal disease may continue to carry and shed the pathogen in feces for some time, posing a risk of transmission to others through direct contact or indirect contact (e.g., through contaminated objects or food). A. Acute and Chronic Disease. The duration of the period of illness can vary greatly, depending on the pathogen, effectiveness of the immune response in the host, and any medical treatment received. 1. For an acute disease, pathologic changes occur over a relatively short time (e.g., hours, days, or a few weeks) and involve a rapid onset of disease conditions. a. Ex: influenza (caused by Influenzavirus) is considered an acute disease because the incubation period is approximately 1-2 days. b. Infected individuals can spread influenza to others for approximately 5 days after becoming ill. c. After approximately 1 week, individuals enter the period of decline. 2. For a chronic disease, pathologic changes can occur over longer time spans (e.g., months, years, or a lifetime). a. Ex: chronic gastritis (inflammation of the lining of the stomach) is caused by the gram-negative bacterium Helicobacter pylori. H. pylori is able to colonize the stomach and persist in its highly acidic environment by producing the enzyme urease, which modifies the local acidity, allowing the bacteria to survive indefinitely. Consequently, H. pylori infections can recur indefinitely unless the infection is cleared using antibiotics. b. Hepatitis B virus can cause a chronic infection in some patients who do not eliminate the virus after the acute illness. A chronic infection with hepatitis B virus is characterized by the continued production of infectious virus for 6 months or longer after the acute infection, as measured by the presence of viral antigen in blood samples. 3. In latent diseases, as opposed to chronic infections, the causal pathogen goes dormant aka latent for extended periods of time with no active replication. HSV-1, HSV-2, and VZV evade the host immune system by residing in a latent form within cells of the nervous system for long periods of time, but they can reactivate to become active infections during times of stress and immunosuppression. a. herpes (herpes simplex viruses [HSV-1 and HSV-2]) b. chickenpox (varicella-zoster virus [VZV]) an initial infection by VZV may result in a case of childhood chickenpox, followed by a long period of latency. The virus may reactivate decades later, causing episodes of shingles in adulthood. mononucleosis (Epstein-Barr virus [EBV]) EBV goes into latency in B cells of the immune system and possibly epithelial cells; it can reactivate years later to produce B-cell lymphoma

Molecular Koch's Postulates

1. the phenotype (sign or symptom of disease) should be associated only with pathogenic strains of a species. 2. Inactivation of the suspected gene(s) associated with pathogenicity should result in a measurable loss of pathogenicity. 3. Reversion of the inactive gene should restore the disease phenotype. 4. the molecular Koch's postulates have limitations. a. Ex: genetic manipulation of some pathogens is not possible using current methods of molecular genetics. Some diseases do not have suitable animal models, which limits the utility of both the original and molecular postulates

Virulence Factors of Eukaryotic Pathogens

A. Fungal Virulence. Pathogenic fungi can produce virulence factors that are similar to the bacterial virulence factors that have been discussed earlier in this chapter. 1. Candida albicans is an opportunistic fungal pathogen and causative agent of oral thrush, vaginal yeast infections, and cutaneous candidiasis. Candida produces adhesins (surface glycoproteins) that bind to the phospholipids of epithelial and endothelial cells. To assist in spread and tissue invasion, Candida produces proteases and phospholipases (i.e., exoenzymes). One of these proteases degrades keratin, a structural protein found on epithelial cells, enhancing the ability of the fungus to invade host tissue. In animal studies, it has been shown that the addition of a protease inhibitor led to attenuation of Candida infection. Similarly, the phospholipases can affect the integrity of host cell membranes to facilitate invasion. 2. the main virulence factor for Cryptococcus, a fungus that causes pneumonia and meningitis, is capsule production. The polysaccharide glucuronoxylomannan is the principal constituent of the Cryptococcus capsule. Similar to encapsulated bacterial cells, encapsulated Cryptococcus cells are more resistant to phagocytosis than nonencapsulated Cryptococcus, which are effectively phagocytosed and, therefore, less virulent. 3. Fungal toxins are called mycotoxins. a. Claviceps purpurea, a fungus that grows on rye and related grains, produces a mycotoxin called ergot toxin, an alkaloid responsible for the disease known as ergotism. There are two forms of ergotism: gangrenous and convulsive. In gangrenous ergotism, the ergot toxin causes vasoconstriction, resulting in improper blood flow to the extremities, eventually leading to gangrene. A famous outbreak of gangrenous ergotism occurred in Eastern Europe during the 5th century AD due to the consumption of rye contaminated with C. purpurea. In convulsive ergotism, the toxin targets the central nervous system, causing mania and hallucinations. b. the mycotoxin aflatoxin is a virulence factor produced by the fungus Aspergillus, an opportunistic pathogen that can enter the body via contaminated food or by inhalation. Inhalation of the fungus can lead to the chronic pulmonary disease aspergillosis, characterized by fever, bloody sputum, and/or asthma. Aflatoxin acts in the host as both a mutagen (a substance that causes mutations in DNA) and a carcinogen (a substance involved in causing cancer) and has been associated with the development of liver cancer. Aflatoxin has also been shown to cross the blood-placental barrier. A second mycotoxin produced by Aspergillus is gliotoxin. This toxin promotes virulence by inducing host cells to self-destruct and by evading the host's immune response by inhibiting the function of phagocytic cells as well as the pro-inflammatory response. Like Candida, Aspergillus also produces several proteases. One is elastase, which breaks down the protein elastin found in the connective tissue of the lung, leading to the development of lung disease. Another is catalase, an enzyme that protects the fungus from hydrogen peroxide produced by the immune system to destroy pathogens. B. Protozoan Virulence 1. Protozoan pathogens are unicellular eukaryotic parasites that have virulence factors and pathogenic mechanisms analogous to prokaryotic and viral pathogens, including adhesins, toxins, antigenic variation, and the ability to survive inside phagocytic vesicles. 2. Protozoans often have unique features for attaching to host cells. the protozoan Giardia lamblia, which causes the intestinal disease giardiasis, uses a large adhesive disc composed of microtubules to attach to the intestinal mucosa. During adhesion, the flagella of G. lamblia move in a manner that draws fluid out from under the disc, resulting in an area of lower pressure that facilitates adhesion to epithelial cells. Giardia does not invade the intestinal cells but rather causes inflammation (possibly through the release of cytopathic substances that cause damage to the cells) and shortens the intestinal villi, inhibiting absorption of nutrients. 3. some protozoans are capable of antigenic variation. The obligate intracellular pathogen Plasmodium falciparum (one of the causative agents of malaria) resides inside red blood cells, where it produces an adhesin membrane protein known as PfEMP1. This protein is expressed on the surface of the infected erythrocytes, causing blood cells to stick to each other and to the walls of blood vessels. This process impedes blood flow, sometimes leading to organ failure, anemia, jaundice (yellowing of skin and sclera of the eyes due to buildup of bilirubin from lysed red blood cells), and, subsequently, death. Although PfEMP1 can be recognized by the host's immune system, antigenic variations in the structure of the protein over time prevent it from being easily recognized and eliminated. this allows malaria to persist as a chronic infection in many individuals. 4. the virulence factors of Trypanosoma brucei, the causative agent of African sleeping sickness, include the abilities to form capsules and undergo antigenic variation. T. brucei evades phagocytosis by producing a dense glycoprotein coat that resembles a bacterial capsule. Over time, host antibodies are produced that recognize this coat, but T. brucei is able to alter the structure of the glycoprotein to evade recognition. C. Helminth Virulence. Helminths depend heavily on virulence factors that allow them to gain entry to host tissues. 1. For example, the aquatic larval form of Schistosoma mansoni, which causes schistosomiasis, penetrates intact skin with the aid of proteases that degrade skin proteins, including elastin. 2. Some helminths are so large that the immune system is ineffective against them. Others, such as adult roundworms (which cause trichinosis, ascariasis, and other diseases), are protected by a tough outer cuticle. 3. Over the course of their life cycles, the surface characteristics of the parasites vary, which may help prevent an effective immune response. Some helminths express polysaccharides called glycans on their external surface; because these glycans resemble molecules produced by host cells, the immune system fails to recognize and attack the helminth as a foreign body. This "glycan gimmickry," as it has been called, serves as a protective cloak that allows the helminth to escape detection by the immune system. 4. In addition to evading host defenses, helminths can actively suppress the immune system. S. mansoni, for example, degrades host antibodies with proteases. Helminths produce many other substances that suppress elements of both innate nonspecific and adaptive specific host defenses. They also release large amounts of material into the host that may locally overwhelm the immune system or cause it to respond inappropriately.

Virulence Factors of Bacterial and Viral Pathogens Pathogens contain unique virulence factors produced by individual pathogens, which determine the extent and severity of disease they may cause. A pathogen's virulence factors are encoded by genes that can be identified using molecular Koch's postulates. When genes encoding virulence factors are inactivated, virulence in the pathogen is diminished.

A. Virulence Factors of Adhesion an adhesin is a protein or glycoprotein found on the surface of a pathogen that attaches to receptors on the host cell. Adhesins are found on bacterial, viral, fungal, and protozoan pathogens. One example of a bacterial adhesin is type 1 fimbrial adhesin, a molecule found on the tips of fimbriae of enterotoxigenic E. coli (ETEC). Recall that fimbriae are hairlike protein bristles on the cell surface. Type 1 fimbrial adhesin allows the fimbriae of ETEC cells to attach to the mannose glycans expressed on intestinal epithelial cells. B. Bacterial Exoenzymes and Toxins as Virulence Factors. After exposure and adhesion, the next step in pathogenesis is invasion, which can involve enzymes and toxins. 1. Many pathogens achieve invasion by entering the bloodstream, an effective means of dissemination because blood vessels pass close to every cell in the body. The downside of this mechanism of dispersal is that the blood also includes numerous elements of the immune system. Various terms ending in -emia are used to describe the presence of pathogens in the bloodstream. a. the presence of bacteria in blood is called bacteremia. Bacteremia involving pyogens (pus-forming bacteria) is called pyemia. b. when viruses are found in the blood, it is called viremia. c. the term toxemia describes the condition when toxins are found in the blood. d. If bacteria are both present and multiplying in the blood, this condition is called septicemia. Patients with septicemia are described as septic, which can lead to shock, a life-threatening decrease in blood pressure (systolic pressure <90 mm Hg) that prevents cells and organs from receiving enough oxygen and nutrients. Some bacteria can cause shock through the release of toxins (virulence factors that can cause tissue damage) and lead to low blood pressure. Gram-negative bacteria are engulfed by immune system phagocytes, which then release tumor necrosis factor, a molecule involved in inflammation and fever. Tumor necrosis factor binds to blood capillaries to increase their permeability, allowing fluids to pass out of blood vessels and into tissues, causing swelling, or edema. With high concentrations of tumor necrosis factor, the inflammatory reaction is severe and enough fluid is lost from the circulatory system that blood pressure decreases to dangerously low levels. This can have dire consequences because the heart, lungs, and kidneys rely on normal blood pressure for proper function; thus, multi-organ failure, shock, and death can occur. 2. Some pathogens produce extracellular enzymes, or exoenzymes, that enable them to invade host cells and deeper tissues. Exoenzymes have a wide variety of targets. Each of these exoenzymes functions in the context of a particular tissue structure to facilitate invasion or support its own growth and defend against the immune system. a. For example, hyaluronidase S, an enzyme produced by pathogens like Staphylococcus aureus, Streptococcus pyogenes, and Clostridium perfringens, degrades the glycoside hyaluronan (hyaluronic acid), which acts as an intercellular cement between adjacent cells in connective tissue. This allows the pathogen to pass through the tissue layers at the portal of entry and disseminate elsewhere in the body. b. Pathogen-produced nucleases, such as DNAse produced by S. aureus, degrade extracellular DNA as a means of escape and spreading through tissue. As bacterial and host cells die at the site of infection, they lyse and release their intracellular contents including DNA. Masses of extracellular DNA can trap bacteria and prevent their spread. S. aureus produces a DNAse to degrade the mesh of extracellular DNA so it can escape and spread to adjacent tissues. This strategy is also used by S. aureus and other pathogens to degrade and escape webs of extracellular DNA produced by immune system phagocytes to trap the bacteria. c. Enzymes that degrade the phospholipids of cell membranes are called phospholipases. Their actions are specific in regard to the type of phospholipids they act upon and where they enzymatically cleave the molecules. The pathogen responsible for anthrax, B. anthracis, produces phospholipase C. When B. anthracis is ingested by phagocytic cells of the immune system, phospholipase C degrades the membrane of the phagosome before it can fuse with the lysosome, allowing the pathogen to escape into the cytoplasm and multiply. Phospholipases can also target the membrane that encloses the phagosome within phagocytic cells. d. Bacterial pathogens also produce various protein-digesting enzymes, or proteases. Proteases can be classified according to their substrate target (e.g., serine proteases target proteins with the amino acid serine) or if they contain metals in their active site (e.g., zinc metalloproteases contain a zinc ion, which is necessary for enzymatic activity). One example of a protease that contains a metal ion is the exoenzyme collagenase. Collagenase digests collagen, the dominant protein in connective tissue. Collagen can be found in the extracellular matrix, especially near mucosal membranes, blood vessels, nerves, and in the layers of the skin. Similar to hyaluronidase, collagenase allows the pathogen to penetrate and spread through the host tissue by digesting this connective tissue protein. the collagenase produced by the gram-positive bacterium Clostridium perfringens, for example, allows the bacterium to make its way through the tissue layers and subsequently enter and multiply in the blood (septicemia). C. perfringens then uses toxins and a phospholipase to cause cellular lysis and necrosis. Once the host cells have died, the bacterium produces gas by fermenting the muscle carbohydrates. The widespread necrosis of tissue and accompanying gas are characteristic of the condition known as gas gangrene. 3. In addition to exoenzymes, certain pathogens are able to produce toxins, biological poisons that assist in their ability to invade and cause damage to tissues. The ability of a pathogen to produce toxins to cause damage to host cells is called toxigenicity. Toxins can be categorized as endotoxins or exotoxins. a. the lipopolysaccharide (LPS) found on the outer membrane of gram-negative bacteria is called endotoxin. During infection and disease, gram-negative bacterial pathogens release endotoxin either when the cell dies, resulting in the disintegration of the membrane, or when the bacterium undergoes binary fission. The lipid component of endotoxin, lipid A, is responsible for the toxic properties of the LPS molecule. Lipid A is relatively conserved across different genera of gram-negative bacteria; therefore, the toxic properties of lipid A are similar regardless of the gram-negative pathogen. In a manner similar to that of tumor necrosis factor, lipid A triggers the immune system's inflammatory response. If the concentration of endotoxin in the body is low, the inflammatory response may provide the host an effective defense against infection; on the other hand, high concentrations of endotoxin in the blood can cause an excessive inflammatory response, leading to a severe drop in blood pressure, multi-organ failure, and death. A classic method of detecting endotoxin is by using the Limulus amebocyte lysate (LAL) test. In this procedure, the blood cells (amebocytes) of the horseshoe crab (Limulus polyphemus) is mixed with a patient's serum. The amebocytes will react to the presence of any endotoxin. This reaction can be observed either chromogenically (color) or by looking for coagulation (clotting reaction) to occur within the serum. An alternative method that has been used is an enzyme-linked immunosorbent assay (ELISA) that uses antibodies to detect the presence of endotoxin. b. Exotoxins are protein molecules that are produced by a wide variety of living pathogenic bacteria. Although some gram-negative pathogens produce exotoxins, the majority are produced by gram-positive pathogens. In contrast to endotoxin, which stimulates a general systemic inflammatory response when released, exotoxins are much more specific in their action and the cells they interact with. Each exotoxin targets specific receptors on specific cells and damages those cells through unique molecular mechanisms. Endotoxin remains stable at high temperatures and requires heating at 121 °C (250 °F) for 45 minutes to inactivate. By contrast, most exotoxins are heat labile because of their protein structure, and many are denatured (inactivated) at temperatures above 41 °C (106 °F). As discussed earlier, endotoxin can stimulate a lethal inflammatory response at very high concentrations and has a measured LD50 of 0.24 mg/kg. By contrast, very small concentrations of exotoxins can be lethal. For example, botulinum toxin, which causes botulism, has an LD50 of 0.000001 mg/kg (240,000 times more lethal than endotoxin). The exotoxins can be grouped into three categories based on their target: intracellular targeting, membrane disrupting, and superantigens. i. the intracellular targeting toxins comprise two components: A for activity and B for binding. Thus, these types of toxins are known as A-B exotoxins. The B component is responsible for the cellular specificity of the toxin and mediates the initial attachment of the toxin to specific cell surface receptors. Once the A-B toxin binds to the host cell, it is brought into the cell by endocytosis and entrapped in a vacuole. The A and B subunits separate as the vacuole acidifies. The A subunit then enters the cell cytoplasm and interferes with the specific internal cellular function that it targets. Four unique examples of A-B toxins are the diphtheria, cholera, botulinum, and tetanus toxins. a) the diphtheria toxin is produced by the gram-positive bacterium Corynebacterium diphtheriae, the causative agent of nasopharyngeal and cutaneous diphtheria. After the A subunit of the diphtheria toxin separates and gains access to the cytoplasm, it facilitates the transfer of adenosine diphosphate (ADP)-ribose onto an elongation-factor protein (EF-2) that is needed for protein synthesis. Hence, diphtheria toxin inhibits protein synthesis in the host cell, ultimately killing the cell. b) cholera toxin is an enterotoxin produced by the gram-negative bacterium Vibrio cholerae and is composed of one A subunit and five B subunits. The mechanism of action of the cholera toxin is complex. The B subunits bind to receptors on the intestinal epithelial cell of the small intestine. After gaining entry into the cytoplasm of the epithelial cell, the A subunit activates an intracellular G protein. The activated G protein, in turn, leads to the activation of the enzyme adenyl cyclase, which begins to produce an increase in the concentration of cyclic AMP (a secondary messenger molecule). The increased cAMP disrupts the normal physiology of the intestinal epithelial cells and causes them to secrete excessive amounts of fluid and electrolytes into the lumen of the intestinal tract, resulting in severe "rice-water stool" diarrhea characteristic of cholera. c) Botulinum toxin (also known as botox) is a neurotoxin produced by the gram-positive bacterium Clostridium botulinum. It is the most acutely toxic substance known to date. The toxin is composed of a light A subunit and heavy protein chain B subunit. The B subunit binds to neurons to allow botulinum toxin to enter the neurons at the neuromuscular junction. The A subunit acts as a protease, cleaving proteins involved in the neuron's release of acetylcholine, a neurotransmitter molecule. Normally, neurons release acetylcholine to induce muscle fiber contractions. The toxin's ability to block acetylcholine release results in the inhibition of muscle contractions, leading to muscle relaxation. This has the potential to stop breathing and cause death. Because of its action, low concentrations of botox are used for cosmetic and medical procedures, including the removal of wrinkles and treatment of overactive bladder. d) Tetanus toxin, which is produced by the gram-positive bacterium Clostridium tetani. This toxin also has a light A subunit and heavy protein chain B subunit. Unlike botulinum toxin, tetanus toxin binds to inhibitory interneurons, which are responsible for release of the inhibitory neurotransmitters glycine and gamma-aminobutyric acid (GABA). Normally, these neurotransmitters bind to neurons at the neuromuscular junction, resulting in the inhibition of acetylcholine release. tetanus toxin inhibits the release of glycine and GABA from the interneuron, resulting in permanent muscle contraction. The first symptom is typically stiffness of the jaw (lockjaw). Violent muscle spasms in other parts of the body follow, typically culminating with respiratory failure and death. ii. Membrane-disrupting toxins affect cell membrane function either by forming pores or by disrupting the phospholipid bilayer in host cell membranes. Two types of membrane-disrupting exotoxins are hemolysins and leukocidins, which form pores in cell membranes, causing leakage of the cytoplasmic contents and cell lysis. These toxins were originally thought to target red blood cells (erythrocytes) and white blood cells (leukocytes), respectively, but we now know they can affect other cells as well. a) the gram-positive bacterium Streptococcus pyogenes produces streptolysins, water-soluble hemolysins that bind to the cholesterol moieties in the host cell membrane to form a pore. The two types of streptolysins, O and S, are categorized by their ability to cause hemolysis in erythrocytes in the absence or presence of oxygen. Streptolysin O is not active in the presence of oxygen, whereas streptolysin S is active in the presence of oxygen. Other important pore-forming membrane-disrupting toxins include alpha toxin of Staphylococcus aureus and pneumolysin of Streptococcus pneumoniae. b) Bacterial phospholipases are membrane-disrupting toxins that degrade the phospholipid bilayer of cell membranes rather than forming pores. We have already discussed the phospholipases associated with B. anthracis, L. pneumophila, and Rickettsia species that enable these bacteria to effect the lysis of phagosomes. These same phospholipases are also hemolysins. Other phospholipases that function as hemolysins include the alpha toxin of Clostridium perfringens, phospholipase C of P. aeruginosa, and beta toxin of Staphylococcus aureus. c) Some strains of S. aureus also produce a leukocidin called Panton-Valentine leukocidin (PVL). PVL consists of two subunits, S and F. The S component acts like the B subunit of an A-B exotoxin in that it binds to glycolipids on the outer plasma membrane of animal cells. The F-component acts like the A subunit of an A-B exotoxin and carries the enzymatic activity. The toxin inserts and assembles into a pore in the membrane. Genes that encode PVL are more frequently present in S. aureus strains that cause skin infections and pneumonia. PVL promotes skin infections by causing edema, erythema (reddening of the skin due to blood vessel dilation), and skin necrosis. PVL has also been shown to cause necrotizing pneumonia. PVL promotes pro-inflammatory and cytotoxic effects on alveolar leukocytes. This results in the release of enzymes from the leukocytes, which, in turn, cause damage to lung tissue. Superantigens are exotoxins that trigger an excessive, nonspecific stimulation of immune cells to secrete cytokines (chemical messengers). The excessive production of cytokines, often called a cytokine storm, elicits a strong immune and inflammatory response that can cause life-threatening high fevers, low blood pressure, multi-organ failure, shock, and death. The prototype superantigen is the toxic shock syndrome toxin of S. aureus. Most toxic shock syndrome cases are associated with vaginal colonization by toxin-producing S. aureus in menstruating women; however, colonization of other body sites can also occur. Some strains of Streptococcus pyogenes also produce superantigens; they are referred to as the streptococcal mitogenic exotoxins and the streptococcal pyrogenic toxins

Viral Virulence

A. Viruses use adhesins to facilitate adhesion to host cells, and certain enveloped viruses rely on antigenic variation to avoid the host immune defenses. 1. Viral adhesion is mediated by adhesins that are part of the viral capsid or membrane envelope. The interaction of viral adhesins with specific cell receptors defines the tropism (preferential targeting) of viruses for specific cells, tissues, and organs in the body. The spike protein hemagglutinin found on Influenzavirus is an example of a viral adhesin; it allows the virus to bind to the sialic acid on the membrane of host respiratory and intestinal cells. Another viral adhesin is the glycoprotein gp20, found on HIV. For HIV to infect cells of the immune system, it must interact with two receptors on the surface of cells. The first interaction involves binding between gp120 and the CD4 cellular marker that is found on some essential immune system cells. However, before viral entry into the cell can occur, a second interaction between gp120 and one of two chemokine receptors (CCR5 and CXCR4) must occur. Antigenic variation also occurs in certain types of enveloped viruses, including influenza viruses, which exhibit two forms of antigenic variation: antigenic drift and antigenic shift. Antigenic drift is the result of point mutations causing slight changes in the spike proteins hemagglutinin (H) and neuraminidase (N). On the other hand, antigenic shift is a major change in spike proteins due to gene reassortment. This reassortment for antigenic shift occurs typically when two different influenza viruses infect the same host

Treatment of Disease

For a pathogen to persist, it must put itself in a position to be transmitted to a new host, leaving the infected host through a portal of exit. As with portals of entry, many pathogens are adapted to use a particular portal of exit. Similar to portals of entry, the most common portals of exit include the skin and the respiratory, urogenital, and gastrointestinal tracts. Coughing and sneezing can expel pathogens from the respiratory tract. A single sneeze can send thousands of virus particles into the air. Secretions and excretions can transport pathogens out of other portals of exit. Feces, urine, semen, vaginal secretions, tears, sweat, and shed skin cells can all serve as vehicles for a pathogen to leave the body. Pathogens that rely on insect vectors for transmission exit the body in the blood extracted by a biting insect. Similarly, some pathogens exit the body in blood extracted by needles