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Transcript
Medical Microbiology Microbial Pathogenicity Lecture #14 Bio3124 Pathogens as Parasites • pathogens are parasites – organisms that live on or within a host organism, metabolically dependent on the host – Parasitism: • Ectoparasite: parasite lives on the host • Endoparasite: parasite lives in the host Parasitism and disease • Infection – growth and multiplication of parasite on or within host • Infectious disease – disease resulting from infection • Pathogen: any parasitic organism that causes infectious disease – primary (frank) pathogen – causes disease by direct interaction with healthy host – opportunistic pathogen – part of normal flora, causes disease when gains access to other tissue sites or when host is immunocompromised • Pathogenicity – ability of a parasite to cause disease Host-parasite relationship and disease outcome Disease state depends on: – number of organisms present – degree of virulence of pathogen – virulence factors • e.g., capsules, pili, toxins – host’s defenses or degree of resistance Virulence: degree/intensity of pathogenicity • determined by, – Invasiveness: ability to spread to adjacent tissues – Infectivity: ability to establish focal point of infection – pathogenic potential: degree to which pathogen can cause damage to host • Toxigenicity: ability to produce toxins • Immunopathogenicity: ability to trigger exaggerated immune responses Measuring virulence • lethal dose 50 (LD50) – number of pathogens that will kill 50% of an experimental group of hosts in a specified time • Infectious dose 50 (ID50) – number of pathogens that will infect 50% of an experimental group of hosts in a specified time Infection Cycle • Mode of entry depends on pathogen • Mucosal surfaces, wounds, insect bites • Infection cycle Route a pathogen takes to spread • Spread via direct contact • Indirect contact – Contact with fomites – Horizontal transmission via vectors • Mosquitoes—Yellow fever, malaria • Reservoir for disease organism – May not show disease symptoms Virulence Factors • Virulence genes – Help pathogen to invade host • Toxins, attachment proteins, capsules • Pathogenicity islands – Section of genome • Contain multiple virulence genes – Often encode related functions » protein secretion system, toxin production – Horizontally transmitted • Often flanked by tRNA genes; phage or plasmid genes • Often have GC content different from rest of genome Virulence Factors • Several factors contribute – Protein secretory systems • Examples:Type II, type III and type IV – Adhesins: host attachement & colonization – Toxins • Exotoxins – – – – – Membrane active toxins Protein synthesis inhibitors Cell signaling inhibitors Superantigens proteases • Endotoxins – Immune avoidance factors Role of protein secretory pathways in virulence • PS Type II (retractable) – Subunits in inner, outer and periplasmic space – G subunit polymerize/depolymerize – Extends/retracts past outer membrane through complex D – like a piston pushes out the secreted proteins to periplasmic space – Ex. Cholera toxin • PS Type II mechanism resemble pili type IV used for twitching motility Type III protein secretory system • many G- bacteria, live in close association with their hosts • secrete regulatory proteins via injectisome directly into host cells – to modulate host cell activities – evolutionary resemblance to flagellum • increase virulence potential – Avoids receptor use – Avoids dilution of secreted proteins outside pathogen Ken Miller talks about PSIII and flagellum Salmonella SPI-1 and SPI-2 are type III secretory systems • 12 pathogenicity islands in S. typhi • SPI-1, a type III secretory system • Injects 13 different toxins (effector proteins) • Subvert signaling, remodel cytoskeleton • Induce membrane ruffles, take S.typhi • SPI-2: alter vesicle trafficking – Prevent phogosome-lysosome fusion – Pathogen avoids innate immunity Injectisome: a type III secretory virulence factor Toxin secretion by type IV secretory system • Resemble conjugation apparatus of gram negative bacteria • Bordetella pertussis toxin secreted through general SecA pathway to periplasm • Type IV collects toxin in periplamic space • Exports across outer membrane General SecA dependent secretory system Adhesins: Microbial Attachment • Human body expels invaders – – – – Mucosa, dead skin constantly expelled Liquid expelled from bladder Coughing, cilia in lungs Expulsion of intestinal contents • Adhesins: surface proteins, glycolipids, glycoproteins – assist in attachment and colonization of host tissues • Pili (fimbriae) • Hollow fibrils with tips to bind host cells Adhesins: Pili type I • e.g. Pyelonephritis pili of uropathogenic E.coli • attachment to P-blood group antigen • upper uninary tract infection • Pili assemble on outer membrane • First, general SecA dependent secretion to periplasm • PapG,E,F & major subunit Pilin A • PapD chaperon sorting/delivery to PapC • Secretion and pilus formation • PapG recognizes the digalactoside on P-blood group antigen of host kidney cells Adhesins: Pili type IV • Found on P. aeruginosa, V. cholera, pathogenic E. coli & N. meningitidis • Mediates attachment and twitching motility • Resemble type II secretory system • Pil A is major structural pilin • PilC,Y1 tip attachment proteins • Assembly: PilA preprotein signal sequence removed by PilD • PilQ mediates export across outer membrane • PilF/T mediates energy dependent assembly/disassembly of pilus Type IV pili: bacterial attachment and motility Exotoxins • soluble, heat-labile, proteins • usually released into the surroundings as bacterial pathogen grows • most exotoxin producers are gram-positive • often travel from site of infection to other tissues or cells where they exert their effects More About Exotoxins • Some toxin genes born on plasmids or prophages • the most lethal substances known • highly immunogenic • can stimulate production of neutralizing antibodies (antitoxins) • can be chemically inactivated to form immunogenic toxoids – e.g., tetanus toxoid Membrane-disrupting exotoxins Alpha toxin of S. aureus • Forms 7-membered oligomeric beta-barrel • Cause cytoplasmic leakage Phospholipase of Clostridium perfringens • removes charged head group of phospholipids in host-cell plasma membranes – membrane destabilized, cell lyses and dies – Also called α-toxin or lecithinase AB type Exotoxins Composed of two subunits • “A” subunit – responsible for toxic effect – ADP-ribosyltion of target proteins eg. diphtheria toxin – Cleave 28S rRNA, eg. Shiga toxin • “B” subunit – binds to target cell, delivers A subunit Diphtheria exotoxin • B subunit mediates receptor binding • Endocytosis and fusion membrane vesicles eg. ER or endosomes • B recycles back to membrane • “A” escapes and enters cytoplasm • In the cytoplasm A catalyses ADPribosylation of EF2, halts translation • Cell death ensues Diphtheria toxin targets EF2 disrupts translation Anthrax toxin: a deadly protease • Anthrax toxin composed of, – Protective antigen (B subunit): delivers EF and LF (A subunits) – Edema factor raises cAMP levels • Causes fluid secretion, tissue swelling – Lethal factor cleaves protein kinases • Blocks immune system from attacking Bacillus anthracis Animation: anthrax toxin mode of action Superantigens • Are bacterial and viral proteins that can activate T-cells • in the absence of a real bacterial antigen mediate the binding of MHC-II and T-cell receptors (almost 30% of Tcell population) • eg. Staphylococcal enterotoxin B (SEB) • Massive activation results in producing lots of cytokines • Results in tissue damage and shock and multi-organ failure Animation: Superantigens Endotoxins • lipopolysaccharide in gram-negative cell wall can be toxic to specific hosts – called endotoxin because it is bound to bacterium and released when organism lyses and some is also released during multiplication – toxic component is the lipid portion, lipid A • heat stable • toxic (nanogram amounts) • weakly immunogenic • generally similar, despite source Immune avoidance mechanisms • Once inside host cell, how to avoid death? – Cell ingests pathogens in phagosome • Some pathogens use hemolysin to break out – Shigella dysenteriae, Listeria monocytogenes – Phagosome fuses with acidic lysosome • Some pathogens secrete proteins to prevent fusion – Salmonella, Chlamydia, Mycobacterium, Legionella • Some pathogens mature in acidic environment – Coxiella burnetii—Q fever Surviving within the Host Survival inside phagocytic cells • escape from phagosome before fusion with lysosome – microbes use actin-based motility to move within and spread between mammalian host cells Burkholderia pseudomallei forming actin tails and protrude through membrane and extend infection to nearby cells Surviving within the Host • Outside host cell, how to avoid death? – Complement, antibodies bind pathogen • Some pathogens secrete thick capsule – Streptococcus pneumoniae, Neisseria meningitidis • Some pathogens make proteins to bind antibodies – Staphylococcus aureus cell wall Protein A » Binds Fc fragment » Antibodies attach “upside down” » Prevents opsonization • Some pathogens cause apoptosis of phagocytes