Download - Lecture #14 Bio3124
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Lecture #14Lecture #14Bio3124Bio3124
Medical MicrobiologyMedical MicrobiologyMicrobial PathogenicityMicrobial Pathogenicity
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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
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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
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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
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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
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Infection Cycle
• Mode of entry depends on pathogen• Mucosal surfaces, wounds, insect bites
• Infection cycleRoute a pathogen takesto 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
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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
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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
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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
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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
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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
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Injectisome: a type III secretory virulence factor
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General SecA dependent secretory system
Toxin secretion by type IV secretory systemToxin 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
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Adhesins: Microbial AttachmentAdhesins: 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
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Adhesins: Pili type IAdhesins: 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
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Adhesins: Pili type IVAdhesins: 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
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Type IV pili: bacterial attachment and motility
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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
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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
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Membrane-disrupting exotoxinsAlpha 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
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AB type ExotoxinsComposed of two subunits• “AA” subunit – responsible for toxic effect
– ADP-ribosyltion of target proteins eg. diphtheria toxin
– Cleave 28S rRNA, eg. Shiga toxin
• “BB” 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 ADP-
ribosylation of EF2, halts translation • Cell death ensues Diphtheria toxin targets EF2Diphtheria toxin targets EF2
disrupts translationdisrupts translation
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• 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
Anthrax toxin: a deadly proteaseAnthrax toxin: a deadly protease
Bacillus anthracis
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Animation: Animation: anthrax toxin mode of actionanthrax toxin mode of action
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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 T-cell
population)
• eg. Staphylococcal enterotoxin B (SEB)
• Massive activation results in producing lots of cytokines
• Results in tissue damage and shock and multi-organ failure
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Animation: Superantigens
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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
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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
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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
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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