Bacterial Pathogenesis Flashcards
What are intrinsic characteristics of bacteria that contribute to pathogenicity?
Virulence Factors
What is a pathogenicity island?
Large segments of DNA that carry virulence genes acquired by horizontal gene transfer. They are activated by stimulus or quorum sensing.
What are some of the main bacterial virulence factors?
- Adherence
- Enzymes
- Glycocalyx & Biofilms
- Toxins
What are the two ways in which bacteria adhere to hosts or surfaces?
Adhesins & Biofilms
What are the mechanisms for bacterial host evasion?
- Defense against antibodies
- Antigenic masking to block phagocytic recognition
- Other antiphagocytic measures
- Inhibition of chemotaxis
What are adhesins?
Surface projections that bind to receptors on the host cell surface, such as fimbriae and pili. These structures may be part of the glycocalyx (capsule or slime layer).
What functions do adhesins serve?
- Promote attachment and colonization, but not necessarily invasion
- Complete with normal flora for nutrients
- Promote avoidance of host defenses
- Stimulate inflammatory response
What is a biofilm?
An aggregate of microorganisms adhered to each other and resist clearance. They may form on living or non-living surfaces and reduce the efficacy of antibiotics.
What is quorum sensing?
Regulation of gene expression based on signal compounds released by large numbers of nearby cells. These genes promote growth and organism survival.
Describe the process by which a biofilm forms.
Free-floating bacteria attach to a surface. Bacteria begin to college and are held together by exopolsaccharide (EPS). EPS enables the biofilm to develop a complex structure. Alignate is a compound in EPS that inhibits innate immune defenses.
What are the common invasins produced by bacteria to promote spreading and invasion?
- Hyaluronidase
- Collagenase
- Streoptokinase/Stapholykinase
- Neuraminidase
What is the function of the hyaluronidase?
Cleaves hyaluronic acid, which is a component of the epithelial matrix, to cause tissue penetration
What invasin converts inactive plasminogen to plasmin, which digests fibrin and prevents blood clotting?
Streptokinase/Staphylokinase
What is the function of collagenase?
Breaks down collage in connective tissues
What is the function of neuraminidase?
Degrades sialic acid in the epithelial cells of the intestinal mucosa
What are the common degradative enzymes produced by bacteria?
- Lecithinase
- Phospholipase
- Leukocidin/Streptolysin
- Hemolysins
What degrative enzyme lyses phagocytes and their granules?
Leukocidin/Streptolysin
How do exotoxins differ from endotoxins?
Exotoxins are actively released out while endotoxins are components of the outer leaflet of the outer membrane and released during cell death.
Exotoxins are generally heat liable and very toxic, while endotoxins are heat stable and weakly toxic.
Only G- bacteria produce endotoxins.
Describe the process of receptor-mediated endocytosis of A=B toxins.
The B subunit binds to a receptor on the cell. The A+B structure is internalized in an endosome. The low pH of the endosome separates the A+B subunits. The A subunit is released from the endosome and the B subunit is recycled to the surface.
What toxin causes hyperactivation of the G protein to produce cAMP, causing loss of Cl- and H2O?
Cholera Toxin
What is the mechanism of superantigens?
Non-specifically bind T cells, activating overwhelming cytokines and proinflammatory cytokines. May lead to toxic shock.
What component of the G- cell membrane is responsible for endotoxin toxicity?
Lipid A (part of LPS)
Describe the mechanism by which Lipid A affects the body.
LPS is released when a G- cell dies. Free floating LPS is bound by a LPS binding protein, which then binds a macrophage. The macrophage then releases proinflammatory cytokines. These cytokines then activate the coagulation pathway, result in prostaglandins and leukoreins, and activation of complement pathways. This may result in damage to blood vessels and epithelium, acute respiratory distress, shock, and organ system failure.
