I. BACTERIAL PATHOGENESIS
C. VIRULENCE FACTORS THAT DAMAGE THE HOST
1. Producing Cell Wall Components (Pathogen-Associated Molecular Patterns or PAMPs) that Bind to Host Cells Causing them to Synthesize and Secrete Inflammatory Cytokines and Chemokines
c. Gram-positive cell wall components: lipoteichoic acids and peptidoglycan cell wall fragments
The overall purpose of this Learning Object is:
1) to learn how various cells involved in body defenses are able to recognize conserved molecules from the gram-positive cell wall and subsequently produce cytokines that initiate innate immune defenses such as the inflammatory response, the complement pathways, and the coagulation pathway;
2) to learn how innate immune defenses such as the inflammatory response, the complement pathways, and the coagulation pathway work to remove remove gram-positive bacteria and protect the body; and
3) to learn how innate immune defenses such as the inflammatory response, the complement pathways, and the coagulation pathway can be harmful to the body if there is an excessive production of cytokines.
LEARNING OBJECTIVES FOR THIS SECTION
In this section on Bacterial Pathogenesis we are looking at virulence factors that damage the host. Virulence factors that damage the host include:
1. The ability to produce cell wall components (Pathogen-Associated Molecular Patterns or PAMPs) that bind to host cells causing them to synthesize and secrete inflammatory cytokines and chemokines;
2. The ability to produce harmful exotoxins.
3. The ability to induce autoimmune responses.
We will now look at the ability of bacteria to produce cell wall components that bind to host cells and cause them to synthesize and secrete inflammatory cytokines.
The Ability to Produce Cell Wall Components (Pathogen-Associated Molecular Patterns or PAMPs) that Bind to Host Cells causing them to Synthesize and Secrete Inflammatory Cytokines and Chemokines
c. Gram-Positive Cell Wall Components: Lipoteichoic Acids and Peptidoglycan Monomers
In order to protect against infection, one of the things the body must initially do is detect the presence of microorganisms. The body does this by recognizing molecules unique to microorganisms that are not associated with human cells. These unique molecules are called pathogen-associated molecular patterns (PAMPs). (Because all microbes, not just pathogenic microbes, possess PAMPs, pathogen-associated molecular patterns are sometimes referred to as microbe-associated molecular patterns or MAMPs.)
Molecules unique to bacterial cell walls, such as peptidoglycan monomers, teichoic acids, LPS, mycolic acid, and mannose, are PAMPs that bind to pattern-recognition receptors (PRRs) on a variety of defense cells of the body causing them to synthesize and secrete a variety of proteins called cytokines (def). These cytokines can, in turn promote innate immune defenses such as inflammation, fever, and phagocytosis. The binding of PAMPs to PRRs also leads to activation of the complement pathways (def) and activation of the coagulation pathway (def).
Cytokines such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), and interleukin-8 (IL-8) are known as inflammatory cytokines (def) because they promote inflammation. Some cytokines, such as IL-8, are also known as chemokines (def). Chemokines promote an inflammatory response by enabling white blood cells to leave the blood vessels and enter the surrounding tissue, by chemotactically attracting these white blood cells to the infection site, and by triggering neutrophils (def) to release killing agents for extracellular killing.
The mechanism is as follows:
1. The lysis of gram-positive bacteria causes peptidoglycan monomers (the building blocks of peptidoglycan; see Fig. 2) and lipotechoic acids to be released from the gram-positive cell wall.
2. These peptidoglycan fragments and lipoteichoic acids, in turn, bind to toll-like receptors (TLRs) (def) such as TLR-2 and TLR-6 that are specific for these cell wall components and are found on the surface of body defense cells called macrophages (def).
3. Binding of the cell wall components to the TLRs of the macrophages triggers them to release various defense regulatory chemicals called cytokines, including tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), inflammatory chemokines such as IL-8, and platelet-activating factor (PAF) (see Fig. 1). The cytokines then bind to cytokine receptors on target cells and initiate an inflammatory response (def). They also activate both the complement pathways (def) and the coagulation pathway (def) (see Fig. 1), in a manner similar to endotoxin (LPS) from the gram-negative cell wall.
4. The binding of peptidoglycan fragments or lipoteichoic acids to their TLRs on the surfaces of phagocytic white blood cells called neutrophils (def) causes them to release proteases (def) and toxic oxygen radicals (def) for extracellular killing. Chemokines such as interleukin-8 (IL-8) also stimulate extracellular killing. In addition, cytokines stimulate the synthesis of a vasodilator called nitric oxide.
During minor local infections with few bacteria present, low levels of peptidoglycan monomers and lipoteichoic acids are released leading to moderate cytokine production by defense cells such as monocytes, macrophages (def) and dendritic cells (def) and, in general, promoting body defense by stimulating inflammation and moderate fever, breaking down energy reserves to supply energy for defense, activating the complement pathway (def) and the coagulation pathway (def), and generally stimulating immune responses (see Fig. 1). Also as a result of these cytokines, circulating phagocytic white blood cells such as neutrophils (def) and monocytes stick to the walls of capillaries, squeeze out and enter the tissue, a process termed diapedesis (def). The phagocytic white blood cells such as neutrophils then kill the invading microbes with their proteases and toxic oxygen radicals. These defenses will be covered in greater detail in Units 4 and 5.
However, during severe systemic infections with large numbers of bacteria present, high levels of these gram-positive cell wall components are released resulting in excessive cytokine production by the defense cells and this can harm the body (see Fig. 2). In addition, neutrophils (def) start releasing their proteases and toxic oxygen radicals that kill not only the bacteria, but the surrounding tissue as well. Harmful effects include high fever, hypotension (def), tissue destruction, wasting, acute respiratory distress syndrome (ARDS), disseminated intravascular coagulation (DIC), and damage to the vascular endothelium. This can result in shock (def), multiple system organ failure (MOSF), and often death.
Keep in mind that a primary function of the circulatory system is perfusion, the delivery of nutrients and oxygen via arterial blood to a capillary bed in tissue. This, in turn, delivers nutrients for cellular metabolism and oxygen for energy production via aerobic respiration to all of the cells of the body.
Sepsis is an infection that leads to a systemic inflammatory response resulting in physiologic changes occuring at the capillary endothelial level. This systemic inflammatory response is referred to as Systemic Inflammatory Response Syndrome or SIRS.
Based on severity, there are three sepsis syndromes based on severity:
1. Sepsis. SIRS in the setting of an infection.
2. Severe sepsis. An infection with end-organ dysfunction as a result of hypoperfusion, the reduced delivery of nutrients and oxygen to tissues and organs via the blood.
3. Septic shock. Severe sepsis with persistent hypotension (def) and tissue hypoperfusion (def) despite fluid resuscitation.
We will now take a look at the underlying mechanism of SIRS that can result in septic shock.
Systemic Inflammatory Response Syndrome (SIRS) Resulting in Septic ShockDuring a severe systemic infection, an excessive inflammatory response triggered by overproduction of inflammatory cytokines such as TNF-alpha, IL-1, IL-6, IL-8, and PAF in response to PAMPs often occurs.
The release of inflammatory cytokines eventually leads to vasodilation of blood vessels. Vasodilation (def) is a reversible opening of the junctional zones between endothelial cells (def) of the blood vessels and results in increased blood vessel permeability. Normally, this fights the infection by enabling plasma, the liquid portion of the blood, to enter the surrounding tissue.The plasma (def) contains defense chemicals such as antibody molecules (def), complement proteins (def), lysozyme (def), and beta defensins (def). Increased capillary permeability also enables white blood cells to adhere to the inner capillary wall, squeeze out of the blood vessels, and enter the tissue to fight infection, a process called diapedesis (def).
Excessive productions of cytokines during a systemic infection results in the following events:
1.During diapedesis (def), phagocytic WBCs called neutrophils (def) adhere to capillary walls in massive amounts. Chemokines such as IL-8 activate extracellular killing by neutrophils, causing them to release proteases (def) and toxic oxygen radicals (def) while still in the capillaries. These are the same toxic chemicals neutrophils use to kill microbes, but now they are dumped onto the vascular endothelial cells to which the neutrophils have adhered. These events result in damage to the capillary walls and leakage of blood into surrounding tissue (see Fig. 3).
2. Prolonged vasodilation (def) and the resulting increased capillary permeability causes plasma (def) to leave the bloodstream and enter the tissue. Prolonged vasodilation also leads to decreased vascular resistance within blood vessels that, in turn, contributes to a drop in blood pressure (hypotension). This contributes to hypoperfusion (def).
3. At high levels of TNF, vascular smooth muscle tone and myocardial contractility are inhibited. This results in a marked hypotension (def). Cytokine-induced overproduction of nitric oxide (NO) by cardiac muscle cells and vascular smooth muscle cells can also lead to heart failure.
4. Neutrophil-induced damage to the capillaries, as well as prolonged vasodilation, results in blood and plasma leaving the bloodstream and entering the surrounding tissue. This can lead to a decreased volume of circulating blood (hypovolemia) (def). This contributes to hypoperfusion (def).
5. Activation of the blood coagulation pathway and concurrent down-regulation of anticoagulation mechanisms causes clots called microthrombi to form within the blood vessels throughout the body. This is called disseminated intravascular coagulation (DIC) (def). These microthrombi block the capillaries and interfere with perfusion (def). Activation of neutrophils also leads to their accumulation and plugging of the vasculature.
6. The increased capillary permeability as a result of vasodilation in the lungs, as well as neutrophil-induced injury to capillaries in the alveoli (def)leads to acute inflammation, pulmonary edema (def), and loss of gas exchange in the lungs. Thiscondition is called acute respiratory distress syndrome (ARDS) (def). As a result, the blood does not become oxygenated.
7. Hypoperfusion (def) and capillary damage in the liver results in impaired liver function and a failure to maintain normal blood glucose levels. Overuse of glucose by muscle and a failure of the liver to replace glucose can lead to a drop in blood glucose level below what is needed to sustain life.
8. Reduced perfusion can also leads to kidney and bowel injury.
9. The combination of hypotension (def), hypovolemia (def), DIC (def), ARDS (def), and the resulting hypoperfusion (def) leads to acidosis (def). Without oxygen, cells switch to fermentation and produce lactic acid that lowers the pH of the blood. A blood pH range between 6.8 and 7.8 is needed for normal cellular metabolic activities in humans. Changes in the pH of arterial blood extracellular fluid outside this range lead to irreversible cell damage.
10. Collectively, this cascade of:
- hypotension, hypovolemia, and DIC that result in marked hypoperfusion;
- ARDS that prevents oxygenation of the blood;
- drop in blood glucose level from liver dysfunction;
- acidosis that results in cell death;and
- cardiac failure
leads to:
- end-organ ischemia (def) (ischemia is a restriction in blood supply that results in damage or dysfunction of tissues or organs);
- multiple system organ failure (MSOF) (def); and
- death.
SIRS is summarized in Fig. 4.
For more on SIRS and Septic Shock, see Septic Shock.
Septicemia (def) is a condition where bacteria enter the blood and cause harm. There are approximately 750,000 cases of septicemia per year in the U.S. and the mortality rate is between 20% and 50%. Over 210,000 people a year in the U.S. die from septic shock. Approximately 45% of the cases of septicemia are due to gram-positive bacteria, 45% are a result of gram-negative bacteria, and 10% are due to fungi (mainly the yeast Candida).
Another example of damage from gram-positive cell wall components is gram-positive bacterial meningitis (def). The same inflammatory events lead to identical effects (def) in the brain and the decreased delivery of oxygen and glucose to the cells of the brain results in damage and death of brain tissue.
One such example is the pneumococcus, Streptococcus pneumoniae (inf). When S. pneumoniae enters the alveoli (def) of the lungs and is lysed by antibiotics or body defenses, glycopeptide cell wall fragments and techoic acids bind to receptors on endothelial cells, the alveolar epithelium, and leukocytes causing the release of TNF-alpha, Il-1, and chemokines. This leads to increased vascular permeability that enables serous fluids, red blood cells, and leukocytes to enter the air spaces of the lung where gas exchange occurs. This prevents normal gas exchange and the person drowns on his or her own serous fluids (def). From the lungs, S. pneumoniae often invades the blood, crosses the blood-brain barrier, and enters the meninges.
Pathogenic strains of Staphylococcus aureus producing leukocydin (def) and protein A (def), including MRSA (def), cause an increased inflammatory response. Protein A, a protein that blocks opsonization (def) and functions as an adhesin (def), binds to cytokine receptors for TNF-alpha (def). It mimics the cytokine and induces a strong inflammatory response. As the inflammatory response attracts neutrophils to the infected area, the leukocydin causes lysis of the neutrophils (def). As a result, tissue is damaged and the bacteria are not phagocytosed.
Gram-positive bacteria such as Staphylococcus and Enterococcus, along with the normal flora gram-negative bacteria mentioned above, are among the most common causes of nosocomial infections (def). The three most common gram-positive bacteria causing nosocomial infections are Staphylococcus aureus, coagulase-negative staphylococci (def), and Enterococcus species. Collectively, these three bacteria accounted for 34% of all nosocomial infections in the U.S. between 1990 and 1996. There are over two million nosocomial infections per year in the U.S.
Highlighted Bacterium: Staphylococcus aureusClick on this link, read the description of Staphylococcus aureus, and be able to match the bacterium with its description on an exam.
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
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