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
b. Gram-negative cell wall components: LPS (endotoxin)
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-negative 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-negative 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.
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
b. Gram-Negative Cell Wall Components: LPS (Endotoxin) in the Outer Membrane 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.
As mentioned in Unit 1, the lipopolysaccharide (LPS) in the outer membrane of the gram-negative cell wall is also known as endotoxin (def). Gram-negative bacteria release some endotoxin during their normal replication but endotoxin is released in quantity upon death and degradation of the bacterium. The degree of damage from endotoxin is related to the degree of release of the LPS from the bacterium's cell wall.
LPS, especially when in the blood, can cause a number of pathophysiological changes such as:
a. fever production
b. inflammation (def)
c. tissue destruction
d. respiratory distress (def)
e. capillary damage (leading to petechial rash (def), capillary leakage, and hypovolemia (def)
f. intravascular coagulation (def)
g. hypotension (def)
h. decreased cardiac output
i. irreversible shock (def)
j. wasting of the body
k. diarrhea (from endotoxin in intestines)We now know that these pathophysiologic changes are regulated by cytokines. Endotoxin indirectly harms the body when massive amounts are released during severe gram-negative infections. This, in turn, causes an excessive cytokine response.
1. The LPS released from the outer membrane of the gram-negative cell wall typically binds first to a LPS-binding protein circulating in the blood and this complex, in turn, binds to a receptor molecule called CD14 that is found on the surface macrophages (def) (see Fig. 1) located in most tissues and organs of the body.
2. The interaction of the LPS-binding protein with CD14 is thought to promote the ability of the toll-like receptor (def) TLR-4 (def) to respond to the LPS.
3. The interaction between LPS and its TLRs triggers the macrophage to release various defense regulatory chemicals called cytokines, including tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), interleukin-6 (IL-6), and interleukin-8 (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).
4. The binding of of LPS molecules 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 (def) such as interleukin-8 (IL-8) also stimulate extracellular killing. In addition, LPS and cytokines stimulate the synthesis of a vasodilator called nitric oxide.
During minor local infections with few bacteria present, low levels of cell wall PAMPs 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 LPS 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) (def), disseminated intravascular coagulation (DIC) (def), 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-negative bacteria, 45% are a result of gram-positive bacteria (see the next section on gram-positive cell wall components), and 10% are due to fungi (mainly the yeast Candida).
Highlighted Infection: Septicemia and Septic ShockClick on this link, read the description of septicemia and septic shock, and be able to match the infection with its description on an exam.
Another example of damage from LPS is gram-negative 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.
Medically important gram-negative bacteria include such classical pathogens as Neisseria meningitidis (inf), Salmonella typhi (inf), Neisseria gonorrhoeae (see photomicrograph) (inf), and Hemophilus influenzae type b (inf). In addition, many normal gram negative intestinal flora such as Escherichia coli, Proteus, Klebsiella, Enterobacter, Serratia, and Pseudomonas aeruginosa are responsible for a variety of opportunistic infections (inf) including urinary tract infections, wound infections, pneumonia, and septicemia. These bacteria owe much of their damage to LPS.
Highlighted Bacterium: Pseudomonas aeruginosaClick on this link, read the description of Pseudomonas aeruginosa, and be able to match the bacterium with its description on an exam.
These normal flora gram-negative bacilli (along with gram-positive bacteria such as Staphylococcus aureus and Enterococcus faecalis) are among the most common causes of nosocomial infections (def). The four most common gram-negative bacteria causing nosocomial infections are Escherichia coli, Pseudomonas aeruginosa, Enterobacter species, and Klebsiella pneumoniae. Collectively, these four bacteria accounted for 32% 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.
In some bacteria, other molecules in the outer membrane may also play a role in leading to excessive cytokine production and potentially harmful inflammatory sequelae. For example, many of the complications of Lyme disease are related to an excessive inflammatory response. Borrelia bergdorferi (inf), the spirochete responsible for Lyme disease, produces at least 105 different membrane lipoproteins. Compared to other bacteria, this is a very large amount. This suggests that these lipoproteins play a critical role in the pathogenesis of infection for this bacterium and similarly lead to cytokine production and an inflammatory response.
In the outer membrane of the cell wall of N. gonorrhoeae, lipooligosaccharide (LOS) is found rather than lipopolysaccharide (LPS) and it is the LOS that functions as an endotoxin.
Much research is now being done to find ways to counter harmful LPS in the body. Some of these techniques being tried experimentally will be described later in Units 3 and 4, but basically they involve either:
- neutralizing the production of LPS by bacteria;
- neutralizing the LPS once it is produced so it doesn't bind to macrophages (see Fig. 1); or
- neutralizing the cytokines (def) released by the macrophages so the cytokines do not become harmful (see Fig. 1).
- blocking TLR-4 (def), the pattern-recognition receptor for LPS.
| E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free. |
| For
further information on bacterial pathogenesis, see the online Microbiology
Web Textbook at the University of Wisconsin-Madison. |
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