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Mechanisms in Pathogenesis

PathogenicityAbility of an organism to cause disease to a host that it infects

VirulenceThe degree of pathogenicity- the relative capacityof a pathogen to invade and harm host cells

AvirulenceNot pathogenic

Molecular Biology and Biotechnology: A Comprehensive Desk ReferenceRobert A. Meyers (editor) VCH Publishers, New York

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Virulence Continuum

Highly virulent

Opportunistic Pathogens

Francisella tularensisRabbit fever

Pseudomonas aeruginosaLung infections of cystic fibrosis patients

Virulence factors

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How Many Cells Does it Take to Cause an Infection?

Most organisms require minimum number before infectious

Infectious capacity of lethal pathogens can be expressed in terms of number of organisms required to kill 50% of animals challenged with infection - LD50

Infectious capacity of non-lethal pathogens can be expressed as number of organisms required to cause disease in 50% of animals challenged with infection - ID50

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Sequence of Pathogenesis Microbe must gain access to host

Attachment to host tissues critical to gaining access

Must also penetrate or evade host defenses Skin Immune cellsMust compete with Normal Flora

Become established

Cause damage

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Portals of Entry - Mucous MembranesMucous membranes Line respiratory, GI, & genitourinary tracts plus conjunctiva (eye)

Respiratory tract most accessible and commonly infectedMicrobes inhaled in droplets and dust particles

GI tract infected by contaminated food, waterMost organisms killed by gastric acid, bile, or digestive enzymesSome species can survive and cause disease

Genitourinary tract STD’s, UTI’s, may require abrasion, some only require attachment

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Portals of Entry - Parenteral Route (other than through GI and Respiratory Route)

Involves depositing organisms directly beneath surface of skin

Requires penetration or injury to skin

May be mechanical means of trauma Surgery, bite, cuts, punctures, injections

May be due to physiological breach Drying, swelling, chapped lips

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Portals of Entry - Skin Largest organ in body, first line of defense

Impenetrable by most organisms if unbroken

Some organisms can infect through hair follicles or sweat gland ducts (e.g., Staphylococcus aureus)

Few organisms can bore through skin (not bacteria)Hookworm

Some fungi can grow in keratin layer or infect skin itself (e.g., tinea pedis) (Low aw)

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Adhesion FactorsSpecialized structures or attachment proteins

Viruses & bacteria have lipoproteins & glycoproteins

Ligands:Bind to complementary receptors on host cells

Adhesins is a bacterial term

Attachment proteins is a viral term

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Adhesion Factors Found on fimbriae (pili), flagella & glycocalyxes of pathogenic bacteria

Neisseria gonorrhoeae STD – causes gonorrheaAdhesins on piliStick to cells lining urethra & vagina

Bordetella pertussisWhooping cough

Filamentous hemagglutinin antigen binds to membranes of cells of upper

respiratory tract… cells then produce toxins that kill ciliary cells of host and allow invasion

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Extracellular Enzymes Many pathogens produce enzymes that degrade structural molecules in the host.

Maintain infection, invade the host further, and avoid immune system

HyaluronidaseDegrades hyaluronic acid

CollagenaseDegrades collagen

CoagulaseCoagulates blood proteinsPromotes blood clots (Staph)

KinasesDigest blood clots

HemolysinsCause lysis of red blood cells

Others… keratinase, mucinase

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Toxins Damage tissues or incite the host immune responses that cause further damage.

Distinction between extracellular enzymes and toxins is not always clear. Some toxins are enzymes and some enzymes have toxins and some toxins have enzymatic activity.

Toxemia:

Toxins enter the bloodstream and are carried to other parts of the body.

Fever, cardiovascular effects, diarrhea, shock, damage to nervous system, cell membranes, blood cells, and blood vessels

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Exotoxins

Produced mostly by Gram-positive and some Gram-negative bacteria

Many toxin genes carried on plasmids or prophages (e.g., Corynebacterium diphtheriae and phage β)

Damage host cells by inhibiting specific metabolic function

Cytotoxins - kill host cells or inhibit function

Neurotoxins - interfere with normal nerve impulse transmission

Enterotoxins - Affect epithelial cells of GI tract mainly cause diarrhea

Proteins or peptides, most are enzymes, soluble, and diffusable… High toxicity!

Generally these do not cause fever

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Exotoxins Often the exotoxin causes the disease and not the bacterium

Killing of bacteria is insufficient to alleviate disease, must clear toxin

Requires host production of antibodies that neutralize toxin

ANTITOXIN

Immunization against disease caused by toxins use inactivated toxins

TOXOIDS

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A B ToxinsTwo separate componentsSubunit A is responsible for the enzymatic activity Subunit B is binds to a specific receptor on the host cell membrane and transfers subunit A across the membrane

Cholera toxin, diphtheria toxin, and pertussis toxin

www-structmed.cimr.cam.ac.uk/.../SLT/ABtoxin.gif

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Diphtheria Toxin - CytotoxinCorynebacterium diphtheriae

Two subunits: A (catalytic) & B (binding)

Toxin produced and secreted

B subunit(s) attaches to target cell

Toxin internalized, translocates to cytosol

A subunit catalyzes reactions inhibiting protein synthesis or some other damaging reaction

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Vibrio EnterotoxinVibrio cholereaAB toxin family

B subunit binds to epithelial cells of intestine

A subunit catalyzes formation of cAMP from ATP

cAMP signals cells to discharge large amounts of fluids into intestinal lumen

Causes extremely watery diarrhea… leads to dehydration very quickly

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Botulinum Toxin - Neurotoxin

Produced by Clostridium botulinum when spores germinate

Released upon lysis of cells in log phase

Binds to motor nerve cells, prevents release of acetylcholine

Causes paralysis, cause of death is suffocation

Once toxin is bound to neuron, no treatment is effective

Most potent toxin known to date, 10 ng (ng = 10-9 g) sufficient to kill

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Tetanus Toxin - Neurotoxin

Produced by Clostridium tetani Binds to motor neurons

Prevents transmission of nerve impulses that allow muscles to relax

Causes uncontrollable muscle contraction

Muscle contractions can be violent enough to break bones

Also extremely potent toxin - only 50 ng required to kill

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EndotoxinsGram negative cell wall has an outer membrane composed of lipopolysaccharide, phospholipids and proteins.Lipid A

Lipid portion of LPS

Lipid A is released when bacterial cells die or are digested by phagocytic cells.

Lipid A causes the host to release chemicals that incite fever, inflammation, diarrhea, hemorrhaging, shock, and blood coagluation

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Endotoxin ShockLow toxicity butfatal in high concentrations

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Mechanism of Endotoxin ActionBacteria engulfed by macrophage, digested in lysosome

Endotoxin induces macrophage to make interleukin 1 (IL-1)

IL-1 travels to the hypothalamus and triggers production of prostaglandin

Prostaglandin causes increase in body temperature

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Endotoxin-Induced Shock

Sepsis causes macrophages to produce tumor necrosis factor (TNF)Designed to eliminate tumor cells

TNF causes damage to capillaries that increases permeability

Fluid loss accompanied by dangerous drop in blood pressure - SHOCK

Serious affect on kidneys, lungs, and GI tract

Requires large amount of endotoxin, massive bacterial infection

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Antiphagocytic Factors

3. Avoid Phagocytes

1. Inhibit Phagocytosis

4. Survival Inside of Phagocytes

2. Kill or Injure Phagocytes

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Process in which phagocytes engulf and digest microorganisms and other cellular debris

What is phagocytosis?

Material is taken into the cell in specialize vesicles called phagocytesLysozymes fuse and release digestive enzymes

An important defense against infection

Bacterial cells

Pseudopodium

Macrophagetissue

Neutrophilsblood

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Avoid Phagocytes

Confined to regions inaccessible to phagocytes

Does not illicit an overwhelming inflammatory response

Inhibits phagocyte chemotaxis

Covers the antigenic surface with host cell proteins

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Inhibit Phagocytosis

Polysaccharide capsules

Group A streptococci M protein and pili

Pseudomonas aeruginosa biofilm slime

E. coli O polysaccharide associated with LPS

CapsulesProtect the bacterial cellEvade the host’s immune systemSlipperyRetard digestion

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Kill or Injure Phagocytes

Pseudomonas aeruginosa exotoxin A kills macrophages

Pathogenic staphylococci produce leukocidin

Pathogenic streptococci produce streptolysin

Gram-positive pyogenic cocci, produce hemolysins

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Survival Inside of Phagocytes

Escape from the phagosome

Survival inside the phagolysosome

Inhibition of fusion of the phagocytic lysosomes with the phagosome

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Antigens

Small molecules, simple molecules, and large but repetitive molecules can evade the immune system.

Whole cells, viruses and complex molecules elicit the immune response.

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Antigenic Variation Some bacterial species periodically alter their surface antigens to avoid the host antibody response

Alter proteins in fimbriae or fibriae tips.

Alter the outer membrane proteins.

Alter capsule proteins.Neisseria gonorrhoeae changes fimbrial proteins.

Delicately controlled expression of the genes.

Many different strains of Salmonella typhimurium with unique cell wall (O) antigens or flagellar (H) antigens.

There are greater than 100 strains of Streptococcus pneumoniae based on capsule antigens.

textbookofbacteriology.net

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Streptococcus

Diverse group of Gram positive, facultatively anaerobic cocci arranged in pairs or chains.

Group ABacterial pharyngitis, scarlet & rheumatic fever

-hemolysis

Utilize several virulence factors

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M proteinsAttachment & interferes with host immune response

Hyaluronic acid capsuleCamouflages the bacteriumStreptokinases

Dissolves blood clots

PeptidasesDegrades proteins involved in immune response

Pyrogenic toxinsStimulate fever, rash & shock

StreptolysinsLyse erythrocytes, leukocytes & platelets

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Neisseria gonorrhoeaeA Gram-negative coccus

Adhere to columnar epithelial cells

Fimbriae antigenic variation

textbookofbacteriology.net

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Types of Infection

1. Localized Infection2. Systemic Infection3. Focal Infection4. Mixed Infection (Polymicrobial Disease)5. Primary and Secondary Infections6. Acute Infections7. Chronic Infections8. Asymptomatic (Subclinical) Infections

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Types of Infection (continued)

9. Communicable

10. Contagious – Highly communicable

11. Non-communicable

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Process of Clinical Infection

4 General Stages

1. Incubation period

2. Prodromal Stage

3. Period of Invasion

4. Convalescent Period

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How Microbes are Transferred

Reservoir: primary habitat of the pathogen in the natural world

Source: Where pathogen actually came from

Carrier: Usually person with asymptomatic infection that spreads disease (e.g.“Typhoid Mary) but can be incubation and convalescent carriers as well

“Passive Carrier”- you could be one!

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How Microbes are Transferred(More terms)

Vector: Living organism that transfers pathogen (e.g., mosquitoes, ticks, etc.)2 types

1) Biological- participates in the pathogens life cycle

2) Mechanical- not part of pathogen’s life cycle… simply carries pathogen mechanically (e.g., horseflies)

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How Microbes are Transferred(More terms)Vehicle: inanimate material that transmits

infectious agents (e.g., air, water)

Common vehicle: a single material that is a source of infection for many individuals

Fomite: inanimate objects that harbors and transmits pathogens (e.g., doorknobs)

Epidemiology: the study of the frequency and distribution of disease and other health-related factors in defined human populations

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Nosocomial Infections

Why so prevalent?

Some patients come in with infections… at least some are going to communicable

Almost all patients are compromised to some degree (e.g., through surgery etc.)

Many portals of infection are vulnerable and compromised

Many “passive” carriers are present

Many fomites are present

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Nosocomial Infections (continued)

Genitourinary tract- 39% E. coli, Klebsiella, Pseudomonas

Respiratory – 18%Surgical Infections- 17%Skin – 8%Septicemia – 6%Other – gastroenteritis, meningitis, etc.

12%

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Nosocomial Infections (continued)

Organisms Involved- Mainly opportunists!

Bacteria: Gram-negatives includeE. coli, Klebsiella, Pseudomonas

Gram-positives include Staphylococcus and Streptococcus

Fungi: Candida albicans