Unit 7 - Virulence and Pathogenicity_no_figs

8/6/2019 Unit 7 - Virulence and Pathogenicity_no_figs

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Transmission of InfectiousTransmission of InfectiousDisease: Role of VirulenceDisease: Role of Virulence

Factors in BacterialFactors in BacterialInvasivenessInvasiveness



G eneral Definitions



Normal FloraNormal Flora

Microorganisms that are associated with a host,established at a particular anatomical locationand don¶t harm their hostImportant part of innate immunity ± Compete for space and nutrients with harm harmful

organisms

When displaced from normal site to other areasmay cause disease

± Example: Staphylococcus aureus in nose and throatcan enter a wound and cause serious infections



Commensalism - the microorganism(commensal) benefits, while the host is

neither harmed nor helped ± The microorganism shares the same foodsource with the host

± The microorganism is not directlydependent on the metabolism of the host

± The microorganism causes no particularharm to host



± Ex amplethe common nonpathogenic strain of

Escherichia coli lives in the human

colon;this facultative anaerobe uses o x ygencreating an anaerobic environment inwhich obligate anaerobes (e.g.b acteroids ) can grow.The bacteroids benefit but the E. coli

derives no obvious benefit or harm.



M utualism ± There is some reciprocal benefit to both partners ± The microorganism (mutualist) and host are metabolically

dependent upon each other ± Syntropism is a mutually beneficial relationship in which

each organism provides one or more growth factors,nutrients or substrates for the other organism;

also referred to as cross-feeding or the satellite phenomenonEx ample: Intestinal flora synthesizing vitamin K and Bcomple x vitamins for host



P arasitism - relationship in which a

symbiont harms or lives at the expense of ahostHost - the body of the animal in which theparasite lives; ± it provides the microenvironment that

shelters and supports the growth andmultiplication of the parasitic organism



O pportunist: ± Lack ability to cause disease during health, but

may invade following illness or suppression



Nosocomial infectionsNosocomial infections

Infections acquired during hospitalizationCaused by opportunists that make up our

normal floraE ndogenous disease - a disease causedby the host¶s own microbiota becausethe host¶s resistance has dropped



Gnotobiotic animalsGnotobiotic animals

Germ-free animalsDerived from caesarian section and

rearing in a strict environmentMake valuable models for investigatingimmune responses and prevention of

opportunistic infections



P athogenicityP athogenicity

Ability of microbes to cause disease in thehost it infects



VirulenceVirulence

Extent of pathogenicityCapacity to cause disease

± Related to severity of disease in the host



Communicable diseasesCommunicable diseases

ContagiousTransmissible from one host to another



Endemic diseaseEndemic disease

P ersistent disease in a definedgeographical area

± Examples:RhinovirusesMalaria



P arasite - usually the smaller of the twoorganisms; it is metabolically dependent onthe hostE ctoparasite - lives on the surface of thehost



Endoparasite - lives within the hostFinal host - the host on (or in) which the

parasite either gains sexual maturity or reproducesIntermediate host - a host that serves as atemporary but essential environment for parasite development



Transfer host - a host that is not necessary

for development but that serves as a vehiclefor reaching the final hostReservoir host - an organism (other than ahuman) that is infected with a parasite thatcan also infect humans



Infectious Disease Caused byInfectious Disease Caused byP arasitesP arasites

Infection - the state occurring when aparasite is growing and multiplying onor within a host



Infectious disease - a change from a state of health as a result of an infection by aparasitic organism ± Adverse affect on the function of part or all of the

host body as a result of the presence of a parasiteor its toxic products

P athogen - any parasitic organism thatproduces an infectious diseaseP athogenicity - the ability of a parasiticorganism to cause a disease



LatencyLatency

When no symptoms present themselvesIntermittent latency ± Herpes simplex virus (HS V)

Lies dormant in neural tissue and is induced to replicateby various factors including intense sunlight and stress

Quiescent latency ± P ersistence over very long periods of time (many

years )Varicella zoster

± Chickenpox in children ± Shingles in the elderly



O utcome of HostO utcome of Host- -P arasiteP arasiteRelationshipsRelationships



± Dependent on three factors:The actual number of parasites in the bodyHow virulent the parasite isHow effective the host¶s resistance is toinfection

The final outcome of hostThe final outcome of host- -parasiteparasiterelationshipsrelationships



Disease often incurred if parasite isparticularly virulent, even if the host hashigh resistanceDisease also incurred if virulence isn¶tparticularly intense, but the host¶sresistance is weakened

± Age ± Illness ± Immunosuppressive drugs



What DeterminesWhat DeterminesVirulence?Virulence?



Virulence ± a measure of pathogenicintensity determined by three properties of

the pathogen: ± Invasiveness ± organism¶s ability topenetrate the first line of defense andspread to adjoining tissues in the host

± Infectivity ± organism¶s ability to establisha primary site of infection

± P athogenic potential ± organism¶s abilityto induce morbid symptoms, determinedby toxigenicity



What is toxigenicity?What is toxigenicity?

Refers to organisms ability to producetoxic compounds (toxins ) that are harmfuland cause diseaseToxins = specific metabolic products of pathogen ± Exotoxins

± Endotoxins ± Leukocidins ± Hemolysins



LDLD5050 and IDand ID 5050

LD50

± Lethal dose 50

± Number of pathogens required to kill 50% of subjects in a given time period

ID50

± Infectious dose 50 ± Number of pathogens required to infect 50%

of subjects



Components InfluencingComponents InfluencingInfectious DiseaseInfectious Disease



Transmissibility of the pathogen - involves initialtransport to the host ± Direct contact - coughing, sneezing, body contact

(shaking hands, kissing) ± Indirect contact ± deposition into an

environmental medium including soil, water, foodand bedding

Inanimate object that harbor and transmit disease =fomites

± Vectors - living organisms that transmit a

pathogen from one host to another Mosquitoes (Malaria, West Nile Virus)Tick (Rocky Mountain spotted fever, Lyme disease)Flea ( P lague)



Zoonoses ± Disease which are mainly prevalent in an

animal population but can be transferred tohumans

RabiesP laqueBrucellosisTularemia



Attachment and colonization (multiplication) by the pathogen ± P athogen must be able to adhere to and colonize host cells

and tissue;this is mediated by special molecules or structures calledadhesins

± E xamples of adhesinsFilamentous hemagglutinin of Bordetella pertussis (bindsto cilia of RT)Fimbriae, piliG lycocalyx (dental plaque)LectinCapsuleS layer Slime layer Teichoic and lipoteichoic acids



± Adhesins bind to host cell receptorsE xamples: fibronectin, laminin,vitronectin, sialoproteins (found in

host¶s extracellular matrix) ± P athogen must compete with normal

microbiota for essential nutrients



Invasion and Evasion of Invasion and Evasion of Host Defense MechanismsHost Defense Mechanisms



Invasion of the pathogen ± Requires penetration of the host¶s

epithelial cells or tissues ± Invasiveness = features that enable

pathogens to escape immunedetection/defenses and cause overt

infection



P ortals of EntryP ortals of Entry

Each infectious organism has its own portal of entry and portal of exit from the hostEntry:

± Respiratory tract via nose and throatM tuberculosis can survive in dried sputum for weeks

± Gastrointestinal tract via mouthS. typhi multiply only I cells of intestinal mucosa

± Skin and mucus membranes ± Genitourinary system (STD ) ± Blood (insects, transfusions, needles )

Exit: Usually the same as the portals of entry



P athogen-associated mechanismsinvolve the production of lyticsubstances that may: ±Attack the ground substance and

basement membranes of integuments

and intestinal linings ±Depolymerize carbohydrate-proteincomplexes between cells or on cellsurfaces

±Disrupt cell surfaces and extracellular matrix



P assive mechanisms of entry involve:

±Breaks, lesions, or ulcers in themucous membranes

±Wounds, abrasions, or burns on the

skin surface ±Arthropod vectors that penetrate

when feeding ±Tissue damage caused by other

organisms ±E ndocytosis by host cells



±G

aining of access to deeper tissues ± P enetration into the circulatory system

G rowth and multiplication of the pathogen -pathogen must find an appropriate environment ± pH ± O xygen tension ± Nutrient availabililty (including specialized growth factors) ± Intracellular growth

Myc oba c terium tuber c ulosis in alveolar macrophages ± E xtracellular growth

P yogenic infections ± pus formation in extracellular spaces(pyogenic streptococci ± S. pyogenes)



Dissemination by EvasionDissemination by Evasion

P athogens spread throughout the hosttissue and sometimes even gain entryinto lymphatic capillaries and theneventually into the circulation ± Localized infection = limited ± Systemic dissemination = multiple organs

Enhanced by microbial products andenzymes (virulence factors )



Capsules and fimbriae ± antiphagocytic, requiresantibodies for opsonizationLeukocidins - extracellular enzymes that killphagocytic leukocytes by causing degranulation of

lysosomesHemolysins - extracellular enzymes that killerythrocytes by forming a pore (iron released) ± Streptolysin O ± inactivated by oxygen, beta hemolysis

when incubated anaerobically (S. pyogenes) ± Streptolysin S ± not oxygen sensitive, beta hemolysis under

aerobic conditionsIf phagocytosed, acts as a leukocidin and kills macrophages



Coagulase ± clots fibrinogen in plasma,protecting pathogen from phagocytosisand other host defenses

Collagenase ± Degrades collagen inconnective tissue promoting thespread of the pathogen

Deoxyribonuclese ± reduces viscosityof exudates, improving pathogenmobility



E lastase ± degrades laminin in E CME xotoxin B ± aka pyrogenic exotoxin b

± proteaseHydrogen peroxide and ammonia ±damage to respiratory and urogenitalsystems



Hyaluronidase ± degrades hyaluronicacid in intercellular ground substancesholding cells together ± increasesmobilityIgA protease ± Degrades antibody (IgAisotype) into Fab and Fc fragmentsLecithinase ± degradation of lecithin inP M (phosphatidylcholine)



P rotein A ± binds Ig G at Fc, preventsC¶ activation

Streptokinase (fibrinolysin,staphylokinase)± binds plasminogenand induces production of plasmin,and digestion of fibrin clots ±increasing mobility of pathogen fromclotted areas



TOX INSTOX INS



Toxigenicity - the capacity of an organism toproduce a toxin ± Intoxications ± diseases caused as a result

of toxin produced inside the body ± Toxin - a specific substance, often a

metabolic product of the organism, thatdamages the host in some specifiedmanner

± Toxemia - symptoms caused by toxins if

they enter the blood of the host



Two main categories of toxinsTwo main categories of toxins

ExotoxinsEndotoxins



EXO TOX INSEXO TOX INS



Characteristics of ExotoxinsCharacteristics of Exotoxins

± Soluble, heat-labile proteins producedby and released from an organismduring growth and replication

± May damage the host at some remotesite

± G enes that encode exotoxins arelocated on chromosomes, plasmidsand bacteriophages



± Make up most lethal substances yet identified ± Have specific disease associations and specific

effects in the hostO ften named after the disease that they cause (e.g.

cholera toxin, diphtheria toxin) ± Stimulate strong immune responses

Highly immunogenicInduce production of neutralizing antibodies (antitoxins)

± Inactivated by a variety of chemicals to form

immunogenic toxoids (e.g. formaldehyde, iodine) ± Do not cause fever in the host with exception of superantigens



± Subclassification depends on howthey exert their effects:

Neurotoxins damage nervous tissueE nterotoxins damage the small intestineCytotoxins do general tissue damagecAM P -producing increae cAM P levels

Superantigens ± trigger massive T cellstimulation and cytokine production (TNF-alpha)



Mechanism of Action of ExotoxinsMechanism of Action of Exotoxins

Inhibition of protein synthesisImpairment of nerve synapse functionMembrane transport disrupted (e.g. electrolyteimbalance resulting in a flow os water into theintestine )Damage to plasma membrane

Increased cAM P levels (cyclic adenosinemonophosphate )Cytokine production induced by superantigens



Two Types of MembraneTwo Types of MembraneDisrupting E xotoxinsDisrupting E xotoxins

1: A channel forming (pore-forming ) type of exotoxin that inserts itself into the normal hostcell membrane and makes an open channel

(pore ) ± Exotoxin attaches to the cholesterol portion of the host

cell plasma membrane ± Examples: Leukocidins (pneumococci, streptococci,

and staphylococci ) and Hemolysins (streptococci )

(See Figure 34.6)



2: A phospholipid-hydrolyzingphospholipase exotoxin destroysmembrane integrity. ± The exotoxins removes the charged polar

head groups from the phospholipid part of thehost cell membrane.

± This destabilizes the membrane and causesthe host cell to lyse.

± Example: gas gangrene caused byC lostridium perfringens E-toxin



Exotoxin Transport MechanismsExotoxin Transport Mechanisms

A close look at the AB Model ± Exotoxins lack an independent cell entry function and

must bind to specific receptors on host cell membranes

to be transported across the membrane ± The ³A´ subunit of the toxin exerts its toxic effect (the Asubunit has the enzyme activity that causes damage tothe host )

± The ³B´ subunit binds to the host receptor (e.g.ganglioside receptors are used for different exotoxins ±GM1 for cholera toxin; GT1 for tetanus toxin; GD1 for botulinum toxin )



± Together the A and B subunits form a dimericexotoxin that will bind to host membranes andenter using two different routes:

P oreReceptor-mediated endocytosis (involving clathrin-coated pits )

± Example: diphtheria toxin ± protein synthesis inhibitionCUR L = component of uncoupling of receptor ligandEF2 is consumed in reaction and is not available for translation



P roperties of NeurotoxinsP roperties of Neurotoxins

Can be ingested as a preformed toxin or made in the body

Affect the nervous system directly and thesmall intestine indirectlyExamples: ± Staphylococcal enterotoxin B (SEB ) ± B acillus cereus emetic toxin (vomiting )



± Botulinum toxinFlaccid paralysis caused by inhibiton of acetylcholine activity at neuromuscular junction

Ingestion of preformed toxinInfant botulism: spores germinate in the gut (dust,honey )



± Tetanus toxinSpores germinate in necrotic tissueSpastic paralysis caused by blocking inhibitory

nerve impulses that cause constant stimulation of somatic motor neurons



P roperties of EnterotoxinsP roperties of Enterotoxins

Directly affect intestinal mucosa ± Vomiting, dysentery and diarrheaElicit massive fluid secretion into intestineWater and electrolytes move across intestingalcells into the lumen of the gutExamples: ± Cholera toxin ± Escherichia coli

± Staphylococcal food poisoning ± Shigella dysentery ± Enterohemorrhagic and enteroinvasive E. coli



P roperties of SuperantigensP roperties of Superantigens

P yrogenic (fever-inducing ) toxinsBind to MHC class II on macrophages

and the variable region on subsets of Tcell receptors of CD4+ TH1 cells(inflammataory T cells )Induce macrophages to produce TNF-alpha and Interleukin-1 (I L-1 =endogenous pyrogen )



TNF-alpha ± Aka cachectin ± Associated with chronic infection and weight loss ( cachexia ) ± If TNF alpha is injected into experimental animals, it

reproduces shock characteristic of Gram negative infections ± TNF-alpha also activates nitric oxide synthase (N O S ) gene

transcriptionNO regulated blood pressure (relaxes amooth muscle cells of blood vessels causing B P to decrease )If BP drops too low, hypovolemic shock results (toxic shock,septic shock )

± P ros: promotion of phagocytosis and increases woundhealing

± Cons: fever, rash (scarlet fever, toxic shock syndrome ),inflammation, shock, death



See Figure 32.5 ± Superantigens stimulate T cells bearing a

particular V F element of the T-cell receptor,

regardless of the nature of the VE receptor element. ± They bind directly to the outer surface of the

MHC class II molecule without requiring

processing by the antigen presenting cells. ± Net effect: Stimulation of between 2-20% of total T cell repertoire!!!!!!!



Examples of superantigens: ± Staphylococcal enterotoxins (SEA, SEB,

SEC ) ± Exfoliatin toxin (Staphylococcus, scalded skin

syndrome ) ± P yrogenic toxisn of staph and strep

± Toxic shock syndrome toxin (TSST-1 ) -Staphylococcus





Characteristics of EndotoxinsCharacteristics of Endotoxins

Lipopolysaccharide ( LP S ) in Gram negativeouter membranes of cell wallWhen microbe lyses (dies ) or replicates,endotoxin is releasedLipid A = lipid portion that is toxicHeat stable (even after autoclaving! )Weakly immunogenic (does not stimulate astrong adaptive immune response )Strongly mitogenic for B lymphocytes (B cellsproliferate non-specifically )Share same structure/composition generally



Can cause systemic effects: ± Fever ± pyrogenic ± Septic shock (hypovolemic shock ) ± Disseminated intravascular coagulation (DIC ) ± blood

coagulation/clotting ± Diarrhea ± Inflammation ± Weakness ± Intestinal hemorrhage ± Lysis of fibrin = fibrinolysis (difficulty with clotting ± internal

hemorrhaging ) ± Circulatory changes (oxygen deprivation to organs ) ± O rgan failure/respiratory distress/mental delirium ± Activation of Hageman Factor (blood clotting factor X II)





±May affect macrophages andmonocytes by binding to special L P S-binding proteins in the plasma that, inturn, bind to receptors on the

macrophages and monocytestriggering cytokine release thatproduces endotoxin effects

±Activation of Hageman Factor



Hageman Factor Initiates Four Hageman Factor Initiates Four Humoral SystemsHumoral Systems

Four general categories of humoraleffects activated: ± Coagulation ± Complement activation ± Fibrinolysis ± Activation of the Kininogen system



Coagulation ± Blood clotting cascade is initiated ± Coagulation, thrombosis (blood clots n

vessels ) ± Accute disseminated intravascular

coagulation

± Depletion of platelet and clotting factors ±internal hemorrhaging



Complement Activation ± Alternative and classical pathways of

complement activatedMembrane attack complexes (MAC ) formInflammatory mediators produces (histamine ),inducing chemotaxis and acivation of neutrophils



Fibrinolysis ± P lasminogen activation

Generates plasmin from plasminogen degradationof fibrin internal hemorrging

± P lasmin can also activate the complementcascade



Kininogen System Activated ± Series of enzyme reactions result in the release of

bradykinins and other vasoactive peptides

± Results:VasodilationP ainIncreased vascular permeability (fluid portion of bloodleaks into interstitial spaces )Hypovolemic/circulatory shockDeath



Teichoic acid of Gram positive cell wallscan trigger similar resposes as LP S of Gram negative cells

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Unit 7 - Virulence and Pathogenicity_no_figs