Microbiology Final Exam Review PDF - December 2023

Microbiology Final Exam Review Tuesday, December 10 8AM TR Ch 25, Microbial Pathogenesis Overview Host-pathogen interactions How microbes attach to host cells How toxins subvert host functions How toxins and effectors are deployed Introduction Mammalian Defenses:...

Microbiology Final Exam Review Tuesday, December 10 8AM TR Ch 25, Microbial Pathogenesis Overview Host-pathogen interactions How microbes attach to host cells How toxins subvert host functions How toxins and effectors are deployed Introduction Mammalian Defenses: ○ Elaborate physical, chemical, and immunological defenses protect against disease-causing microbes. Pathogenic Microbes: ○ Exploit weaknesses in defenses, leading to disease. Fundamental Question: ○ How can an organism too small to be seen kill a human a million times larger? Host-Pathogen Interactions History: ○ Humans and ancestors have suffered infections for millions of years. ○ Robert Koch discovered Bacillus anthracis causes anthrax. The Language of Pathogenesis Parasite: ○ An organism that benefits at the host's expense (e.g., bacteria, viruses, fungi, protozoa, worms). ○ Ectoparasites: Live on the host's surface (e.g., Trichophyton rubrum causes athlete's foot). ○ Endoparasites: Live inside the host's body (e.g., Wuchereria bancrofti causes elephantiasis). Infection: ○ Occurs when a pathogen or parasite grows in/on a host. Not all infections cause disease. Pathogens: ○ Primary Pathogens: Cause disease in healthy hosts (e.g., Shigella flexneri). ○ Opportunistic Pathogens: Cause disease in compromised hosts (e.g., Pneumocystis jirovecii in AIDS patients). ○ Latent Infections: Microbes enter a dormant state (e.g., herpesvirus). Pathogenicity and Virulence Pathogenicity: An organism's ability to cause disease, defined by infectivity and virulence. 17  Microbiology Final Exam Review Tuesday, December 10 8AM TR Virulence: ○ Severity of disease (e.g., Ebola virus is highly virulent). ○ Measured by lethal dose (LD50) and infectious dose (ID50). Infection Cycles Cycle: The route of transmission of an infectious organism. ○ Horizontal Transmission: Direct contact (e.g., sneezing, touch). Indirect transmission (e.g., fomites, food, water). ○ Vertical Transmission: From mother to fetus during pregnancy or birth. ○ Vector Transmission: Through arthropods (e.g., mosquitoes transmitting yellow fever). Reservoirs and Portals of Entry Reservoir: Environment or animal that harbors the pathogen. ○ Example: Monkeys for yellow fever virus, birds for eastern equine encephalitis virus. ○ Asymptomatic Carrier: Person who harbors a potential disease agent but shows no symptoms. Portals of Entry: Best suited to the pathogen's mechanism (e.g., oral, respiratory, conjunctiva, wounds, parenteral). Immunopathogenesis Definition: Immune response to a pathogen that causes tissue and organ damage. Study: Both pathogenic mechanisms and immunopathogenesis symptoms should be studied. Effect of Infections on the Microbiome Host's Normal Microbiome: Affected by pathogen growth and immune response. ○ Example: Diarrhea reduces gut microbiota, intestinal pathogens alter nutrients. Virulence Factors Requirements for Disease: 1. Enter host, find niche, avoid defenses, multiply, transmit to new host. Virulence Factors: Encoded by virulence genes (e.g., toxins, attachment proteins, capsules). Molecular Koch's Postulates: 1. Phenotype associated with pathogenic strains. 2. Inactivation of gene leads to loss of virulence. 3. Reversion or replacement restores pathogenicity. Pathogenicity Islands 18  Microbiology Final Exam Review Tuesday, December 10 8AM TR Definition: Clusters of pathogenicity genes encoding virulence functions. Transmission: Horizontally via conjugation or transduction. Anomalies: Unique GC/AT ratio, linkage to tRNA genes, association with phage/plasmid genes. Microbial Attachment First Contact: ○ Attachment or adhesion is the first step toward infection. ○ Adhesin: Microbial factors promoting attachment (e.g., pili, adherence proteins, cell surface molecules). Pili (Fimbriae) Structure: ○ Hairlike appendages with cylindrical shafts composed of pilin protein subunits. ○ Tips contain receptors (FimH) to attach to host cell receptors. Types: ○ Type I: Adhere to carbohydrates on host membranes, producing static attachment. Grow from the outer membrane of certain Gram-negative bacteria. ○ Type IV: Involved in "twitching motility," producing dynamic attachment via assembly and disassembly. Grow from the inner membrane of many Gram-negative bacteria. Assembly of Type I Pili Pyelonephritis-Associated Pilus (Pap): ○ Assembled in a stepwise fashion. ○ Protein subunits fit together like pieces of a jigsaw puzzle. ○ New protein subunits are added to the bottom of the growing structure. Type IV Pili - Model of Pilus Assembly and Disassembly Involves at least a dozen proteins. Functions: ○ Elongation of the pili to attach to a surface. ○ Retraction of the pili depolymerizes from the base, shortening the pilus and pulling the cell forward. Examples: ○ Found in E. coli and N. meningitidis. Nonpilus Adhesins Function: ○ Tighten interactions between bacteria and target cells. Examples: ○ Streptococcus pyogenes: M protein binds to host cell fibronectin (glycoprotein). 19  Microbiology Final Exam Review Tuesday, December 10 8AM TR ○ Bordetella pertussis: Pertactin binds to host cell integrin and also uses pilus to bind to cells. Susceptibility to Infections Factors: 1. Immunocompetence: The ability of an individual to resist pathogens and parasites through the response of their immune system. 2. Receptor Availability: Host receptors dictate susceptibility to pathogens. Host Receptors and Pathogen Susceptibility Specific Structures: ○ Pathogens rely on specific surface structures (receptors) to recognize and attach to appropriate host cells. ○ Person-to-person differences in receptor structures are possible. Example: ○ HIV binds to T-cell surface receptor, CD4, and coreceptor, CCR5. ○ Individuals lacking CCR5 (mutant) are resistant to HIV infection. Toxins Subvert Host Functions Types: 1. Exotoxins: Proteins produced and secreted by various types of bacteria following attachment. Kill host cells and unlock their nutrients. 2. Endotoxin: A part of lipopolysaccharide (LPS) of Gram-negative bacteria. Hyperactivates host immune systems to harmful levels. Exotoxins vs. Endotoxins Property Exotoxins Endotoxins 20  Microbiology Final Exam Review Tuesday, December 10 8AM TR Producing Organism Gram-positive or Gram-negative only Gram-negative Chemical Protein (50 to 1,000 kDa) Lipopolysaccharide (lipid A moiety; 10 kDa) Denatured by Yes, if boiled long enough No Boiling Mode of Action Target specific features of Bind Toll-like receptor 4, activate eukaryotic cells (e.g., cytokine production membrane, protein synthesis) Enzyme Activity Often No Toxicity High (1-microgram quantities) Low (greater than 100-microgram quantities), primary cause of Gram-negative sepsis Immunogenicity Highly antigenic Poorly antigenic Vaccine Toxoids can be made for some Toxoids cannot be made Fever Production Occasionally Yes (Pyrogenicity) Categories of Microbial Exotoxins 1. Plasma membrane disruption 2. Cytoskeleton alterations 3. Protein synthesis disruption 4. Cell cycle disruption 5. Signal transduction disruption 6. Disrupt cell-cell adherence 7. Alter vesicular traffic 8. Inhibit exocytosis 9. Superantigens (activate immune response) Membrane Disruption Types: ○ Pore-Forming Proteins: Insert into membranes by binding cholesterol and membrane receptors (e.g., alpha toxin of Staphylococcus aureus, Panton-Valentine toxin of MRSA, Listeriolysin O of Listeria monocytogenes). ○ Phospholipase: Hydrolyze phospholipids into fatty acids (e.g., Phospholipase C of Clostridium perfringens). General Terms: 21  Microbiology Final Exam Review Tuesday, December 10 8AM TR ○ Hemolysins: Lyse red blood cells (and other cells). ○ Leukocidins: Lyse white blood cells (leukocytes). ○ Some exotoxins function as both hemolysins and leukocidins (e.g., Streptolysin S of Streptococcus pyogenes). Two-Subunit AB Exotoxins Structure: ○ Comprise two subunits (A and B). ○ A Subunit: Associated with toxicity. ○ B Subunit: Binds host cell and delivers A subunit. AB5 Exotoxins: Consist of 5 identical B subunits arranged in a ring with a single A subunit in the center. ADP-Ribosyltransferase Enzymes: ○ Modify protein structure and function. ○ Function: Transfer ADP-ribose group from an NAD molecule to an amino acid residue in a target host protein. Either destroy host protein function (e.g., diphtheria toxin) or lock a targeted enzyme into an active form (e.g., cholera toxin). Cholera Toxin Type: AB5 exotoxin made by Vibrio cholerae. Mode of Action: ○ B subunits bind to intestinal cell membranes, triggering endocytosis of cholera toxin complex. ○ A subunit ADP-ribosylates a host cell target, leading to a sharp increase in cAMP levels. ○ cAMP activates ion transporters, causing water and electrolytes to leave the cell, leading to watery stools (diarrhea). Anthrax Toxin Produced by: Bacillus anthracis. Structure: ○ B Subunit: Called protective antigen (PA). ○ A Subunits: Edema factor (EF) and lethal factor (LF). Mode of Action: ○ EF: An adenylyl cyclase that raises cAMP levels, causing fluid secretion and tissue swelling. ○ LF: A protease that cleaves protein kinase kinase, blocking the host cell's ability to recruit immune cells to fight infection. AB Exotoxins That Target Protein Synthesis Shiga Toxin: 22  Microbiology Final Exam Review Tuesday, December 10 8AM TR ○ Produced by Shigella dysenteriae and E. coli O157:H7. ○ Disrupts protein synthesis by cleaving 28S rRNA in eukaryotic ribosomes, causing acute kidney failure. Diphtheria Toxin: ○ Produced by Corynebacterium diphtheriae. ○ ADP-ribosylates eukaryotic elongation factor 2 (eEF-2), halting protein synthesis and causing cell death. Mode of Action of Diphtheria Toxin Diphtheria Toxin: ○ Produced by Corynebacterium diphtheriae. ○ Causes the respiratory disease diphtheria. ○ Encoded by the toxin gene (dtx), which is part of a phage genome integrated into the bacterial chromosome. Mechanism: ○ The "A" subunit ADP-ribosylates eukaryotic elongation factor 2 (eEF-2), halting protein synthesis and causing cell death. Endotoxin (LPS) Is Made Only by Gram-Negative Bacteria Endotoxin/Lipopolysaccharide (LPS) Composition: ○ Present in the outer leaflet of the Gram-negative outer membrane. ○ Composed of lipid A (the actual endotoxin factor), core glycolipid, and a polysaccharide chain called O antigen. Release and Action: ○ Released as bacteria die. ○ Acts as a microbe-associated molecular pattern (MAMP) molecule. ○ Binds to Toll-like receptors on macrophages or B cells, triggering a massive cytokine release (e.g., TNF-alpha, interferon, IL1). Effects of Cytokine Release Symptoms: ○ Fever: Induced by the release of pyrogenic cytokines. ○ Activation of Clotting Factors: Leads to disseminated intravascular coagulation. ○ Activation of the Alternative Complement Pathway: Enhances inflammation and phagocytosis. ○ Vasodilation: Leads to hypotension (low blood pressure). ○ Shock: Resulting from hypotension. ○ Death: When symptoms are severe. Example: Neisseria meningitidis Description: 23  Microbiology Final Exam Review Tuesday, December 10 8AM TR ○ A Gram-negative diplococcus. ○ Major cause of bacterial meningitis. ○ Sepsis-related endotoxin release: Causes a massive depletion of clotting factors. Leads to internal bleeding displayed as small pinpoint hemorrhages called petechiae (rash) on the patient's hands and feet. Deploying Toxins and Effectors Protein Secretory Systems Type II Protein Secretion: ○ Homologous to type IV pilus biogenesis. Type III Protein Secretion: ○ Homologous to flagellar synthesis. Type IV Protein Secretion: ○ Homologous to DNA transfer by conjugation. Type III Secretion System (T3SS) Mechanism: ○ T3SS genes are usually located within pathogenicity islands inherited via horizontal gene transfer. ○ Found in Salmonella, Yersinia, Shigella, and Escherichia species. ○ Uses a tiny molecular syringe (injectisomes) to inject proteins from the bacterial cytoplasm directly into the host cell. ○ Secretion is triggered by cell-cell contact between host and bacterium. E. coli Pathogens Mechanism: ○ Do not rely solely on host receptors for attachment. ○ Use a T3SS to insert their own receptors into target cells. ○ Tir Proteins: Injected into the host cell. Act as receptors for the bacterial outer membrane protein intimin. Establish a strong attachment. Used By: ○ Enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC). ○ The result of this attachment is the destruction of the microvilli and pedestal formation, avoiding engulfment. Surviving Within the Host Extracellular Immune Avoidance 24  Microbiology Final Exam Review Tuesday, December 10 8AM TR Strategies: 1. Production of a thick polysaccharide capsule. 2. Production of proteins that bind to antibodies. 3. Induction of apoptosis of phagocytic cells. 4. Alteration of surface antigens. Intracellular Immune Avoidance Mechanisms: ○ Many infectious bacteria invade host cells and become intracellular pathogens. ○ Avoid being killed by a phagolysosome using three options: 1. Grow inside the phagolysosome: Thrive under stress. 2. Prevent phagosome-lysosome fusion. 3. Escape the phagosome. Alternative Fates of Intracellular Pathogens Strategies: ○ Employ molecular tricks to misdirect the immune system. ○ Buy time for the microbe to overwhelm the host. Autophagy: ○ Highly regulated internal surveillance mechanism. ○ Eukaryotic cells form vesicles around damaged organelles to scavenge them for nutrients. ○ Used as a universal innate defense mechanism to fight intracellular pathogens. ○ Intracellular pathogens prevent autophagy to prolong their survival. Chapter Summary Pathogens: ○ Any microbial agent of disease. ○ Primary Pathogens: Cause disease in normal hosts. ○ Opportunistic Pathogens: Need immunocompromised hosts. Infection Cycles: ○ Can be direct or indirect. ○ May involve fomites and vectors. Virulence Factors: ○ May be encoded by gene clusters on pathogenicity islands. ○ Acquired by horizontal transmission. Adhesins: ○ Mediate bacterial attachment to host cells. ○ Type I Pili: Static attachment. ○ Type IV Pili: Continually assembled and disassembled. ○ Nonpilus Adhesins: Include pertactin and M protein. Microbial Exotoxins: ○ Fall into nine categories based on their mechanisms of action. 25  Microbiology Final Exam Review Tuesday, December 10 8AM TR ○ Two-subunit AB toxins may use ADP-ribosylation to disrupt signaling processes or protein synthesis in the host. Endotoxin (LPS): ○ Made only by Gram-negative bacteria. Protein Secretion Pathways: ○ Used by pathogens to deliver toxins. ○ Examples: Type II (pilus-like), type III (syringe-like), and type IV (conjugation system-like). Immune Avoidance: ○ Bacteria have evolved various mechanisms to help them survive and thrive within an infected host. ○ Molecular mechanisms for avoiding the immune system include molecular mimicry, interfering with autophagy, and redirecting ubiquitylation signals. 26