Mechanisms in Pathogenesis Pathogenicity Ability of an organism

Mechanisms in Pathogenesis Pathogenicity Ability of an organism to cause disease to a host that it infects Virulence The degree of pathogenicity- the relative capacity of a pathogen to invade and harm host cells Avirulence Not pathogenic Molecular Biology and Biotechnology: A Comprehensive Desk Reference Robert A. Meyers (editor) VCH Publishers, New York 1

Virulence Continuum Highly virulent Francisella tularensis Rabbit fever Virulence factors Opportunistic Pathogens Pseudomonas aeruginosa Lung infections of cystic fibrosis patients 2

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 - LD 50 Infectious capacity of non-lethal pathogens can be expressed as number of organisms required to cause disease in 50% of animals challenged with infection ID 50 3

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 cells Must compete with Normal Flora Become established Cause damage 4

Portals of Entry - Mucous Membranes Mucous membranes Line respiratory, GI, & genitourinary tracts plus conjunctiva (eye) Respiratory tract most accessible and commonly infected Microbes inhaled in droplets and dust particles GI tract infected by contaminated food, water Most organisms killed by gastric acid, bile, or digestive enzymes Some species can survive and cause disease Genitourinary tract STD’s, UTI’s, may require abrasion, some only require attachment 5

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 6

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) 7

Adhesion Factors Specialized 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 8

Adhesion Factors Neisseria gonorrhoeae Found on fimbriae (pili), flagella & glycocalyxes of pathogenic bacteria STD – causes gonorrhea Adhesins on pili Stick to cells lining urethra & vagina Bordetella pertussis Whooping 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 9

Extracellular Enzymes Maintain infection, invade the host further, and avoid immune system Coagulase Many pathogens produce enzymes that degrade structural molecules in the host. Collagenase Coagulates blood proteins Promotes blood clots (Staph) Degrades collagen Hyaluronidase Degrades hyaluronic acid Kinases Digest blood clots Hemolysins Cause lysis of red blood cells Others… keratinase, mucinase 10

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 11

Exotoxins Proteins or peptides, most are enzymes, soluble, and diffusable… High toxicity! Cytotoxins - kill host cells or inhibit function Neurotoxins - interfere with normal nerve impulse transmission Enterotoxins - Affect epithelial cells of GI tract mainly cause diarrhea 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 12 Generally these do not cause fever

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 13

A B Toxins Two separate components Subunit A is responsible for the Cholera toxin, enzymatic activity diphtheria toxin, Subunit B is binds to a specific receptor and pertussis toxin on the host cell membrane and transfers subunit A across the membrane www-structmed. cimr. cam. ac. uk/. . . /SLT/ABtoxin. gif 14

Diphtheria Toxin - Cytotoxin Corynebacterium 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 15

Vibrio Enterotoxin Vibrio cholerea AB toxin family B subunit binds to epithelial cells of intestine A subunit catalyzes formation of c. AMP from ATP c. AMP signals cells to discharge large amounts of fluids into intestinal lumen Causes extremely watery diarrhea… leads to dehydration very quickly 16

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 17

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 18

Endotoxins Gram 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 19

Endotoxin Shock Low toxicity but fatal in high concentrations www. kcom. edu/. . . /Website/Lects/bact 7. jpg 20

Mechanism of Endotoxin Action Bacteria 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 21

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 22

Antiphagocytic Factors 1. Inhibit Phagocytosis 2. Kill or Injure Phagocytes 3. Avoid Phagocytes 4. Survival Inside of Phagocytes 23

What is phagocytosis? An important defense against infection Process in which phagocytes engulf and digest microorganisms and other cellular debris Material is taken into the cell in specialize vesicles called phagocytes Macrophage Lysozymes fuse and release tissue digestive enzymes Pseudopodium Bacterial cells Neutrophils blood 24

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 25

Inhibit Phagocytosis Polysaccharide capsules Group A streptococci M protein and pili Capsules Protect the bacterial cell Evade the host’s immune system Slippery Retard digestion Pseudomonas aeruginosa biofilm slime E. coli O polysaccharide associated with LPS 26

Kill or Injure Phagocytes Pseudomonas aeruginosa exotoxin A kills macrophages Pathogenic staphylococci produce leukocidin Pathogenic streptococci produce streptolysin Gram-positive pyogenic cocci, produce hemolysins 27

Survival Inside of Phagocytes Inhibition of fusion of the phagocytic lysosomes with the phagosome Survival inside the phagolysosome Escape from the phagosome 28

Antigens Whole cells, viruses and complex molecules elicit the immune response. Small molecules, simple molecules, and large but repetitive molecules can evade the immune system. 29

Antigenic Variation Alter proteins in fimbriae or fibriae tips. Alter the outer membrane proteins. Alter capsule proteins. Delicately controlled expression of the genes. Many different strains of Salmonella typhimurium with unique cell wall (O) antigens or flagellar (H) antigens. Some bacterial species periodically alter their surface antigens to avoid the host antibody response Neisseria gonorrhoeae changes fimbrial proteins. There are greater than 100 strains of Streptococcus pneumoniae based on capsule antigens. textbookofbacteriology. net 30

Streptococcus Diverse group of Gram positive, facultatively anaerobic cocci arranged in pairs or chains. Group A Bacterial pharyngitis, scarlet & rheumatic fever b-hemolysis Utilize several virulence factors 31

M proteins Attachment & interferes with host immune response Hyaluronic acid capsule Camouflages the bacterium Streptokinases Dissolves blood clots Peptidases Degrades proteins involved in immune response Pyrogenic toxins Stimulate fever, rash & shock Streptolysins Lyse erythrocytes, leukocytes & platelets 32

Neisseria gonorrhoeae A Gram-negative coccus Adhere to columnar epithelial cells Fimbriae antigenic variation textbookofbacteriology. net 33

Types of Infection 1. Localized Infection 2. Systemic Infection 3. Focal Infection 4. Mixed Infection (Polymicrobial Disease) 5. Primary and Secondary Infections 6. Acute Infections 7. Chronic Infections 8. Asymptomatic (Subclinical) Infections 34

Types of Infection (continued) 9. Communicable 10. Contagious – Highly communicable 11. Non-communicable 35

Process of Clinical Infection 4 General Stages 1. Incubation period 2. Prodromal Stage 3. Period of Invasion 4. Convalescent Period 36

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! 37

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) 38

How Microbes are Transferred (More terms) Vehicle: inanimaterial 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 39

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 40

Nosocomial Infections (continued) Genitourinary tract- 39% E. coli, Klebsiella, Pseudomonas Respiratory – 18% Surgical Infections- 17% Skin – 8% Septicemia – 6% Other – gastroenteritis, meningitis, etc. 12% 41

Nosocomial Infections (continued) Organisms Involved- Mainly opportunists! Bacteria: Gram-negatives include E. coli, Klebsiella, Pseudomonas Gram-positives include Staphylococcus and Streptococcus Fungi: Candida albicans 42