Lecture Notes for Exam 3, Biology 250

Host-Pathogen Relationships: Host Defenses

Host-Pathogen Relationships: Passive Host Defenses
1. Host barriers to invasion by microorganisms (first line of defense, passive defense, nonspecific except for IgA)
  1. Skin (mechanical and chemical factors)
  2. Mucous membranes
    1. Located in respiratory tract, genital tract, intestinal tract
    2. Stickiness of mucus, IgA, lysozyme, flow of mucus due to ciliated epithelial cells
  3. Saliva (enzymes, IgA, flow of saliva)
  4. Stomach (acid pH)
  5. Intestines (motility, competition by normal flora)
  6. Temperature and oxygen level
Host-Pathogen Relationships: Active Host Defenses--Inflammation
1. Inflammation and phagocytosis (second line of defense, active, nonspecific response by host)
  1. Nonspecific response to tissue injury
  2. Purpose: elimination of cause of injury (by bringing phagocytes to site of injury) and repair of damaged area
  3. Classic symptoms: heat, redness, swelling, pain
  4. Sequence of events
    1. Tissue injury (mechanical, chemical, microbial, etc.)
    2. Mast cell degranulation, release of vasoactive substances (histamine, etc.)
    3. Vasoconstriction, blood pooling
    4. Vasodilation
    5. Increased blood flow
    6. Increased capillary permeability
    7. Walls of capillaries become sticky, white blood cells (WBC) stick to walls (pavementing or margination)
    8. White blood cells pass through walls of capillaries into adjacent tissue (diapedesis)
    9. WBC (neutrophils, later macrophages) migrate to site of injury following increased concentrations of chemoattractant (chemotaxis)
    10. Phagocytosis of microorganisms, tissue debris, red blood cells
      1. First type of phagocytes to arrive: neutrophils (better at killing ingested bacteria, have some degradative enzymes)
      2. Second type of phagocytes to arrive: macrophages (more degradative enzymes, not quite so good at killing bacteria, unless activated)
    11. Tissue repair
      1. Fibroblasts synthesize collagen, which forms scar tissue
      2. Restores dead or damaged tissue to normal function
  5. Consequences: phagocytes brought into contact with particles to be phagocytized (tissue debris, bacteria, blood cells)
Host-Pathogen Relationships: Active Host Defenses--Immune System (third line of defense, active, specific response by host)
1. Characteristics of immune response
  1. Specific for particular substance (antigen) (due to activities of lymphocytes and/or specific proteins called antibodies)
  2. Usually directed toward foreign substances (antigens)
  3. Time required for cells to cooperate and communicate, to synthesize molecules, to divide, to differentiate
  4. Memory-like effect
    1. Immune system reacts more vigorously when it has encountered an antigen before
    2. Secondary response more effective than primary response (secondary is faster, reaches higher peak, declines more slowly)
  5. Immune system consists of organs, cells, and molecules
    1. Cells (lymphocytes and macrophages) originate in bone marrow (central lymphoid tissue)
    2. Some lymphocytes mature further in thymus (central lymphoid tissue) after leaving bone marrow (T lymphocytes)
    3. Mature B and T lymphocytes migrate to lymph nodes and spleen (peripheral lymphoid tissues), where immune responses begin (also blood, certain tissues)
  6. Before IR mature at about six months, infants have passive immunity acquired from their mother (some of her antibodies can cross placenta before baby's birth)
2. Cells important in immune responses
  1. B lymphocytes (divide, differentiate, produce small amounts of antibody specific for antigens)
  2. Plasma cells (differentiate from B lymphocytes, produce large amounts of ag-specific antibody)
  3. T lymhocytes
    1. Helper T lymphocytes--ag specific, help B cells and other cells respond to antigen (up-regulation)
    2. Cytotoxic T lymphocytes--specific for ag on cell surfaces, attack tumor cells and virus-infected cells
    3. Suppressor T lymphocytes--regulate immune responses (down-regulation)
    4. T_D lymphocytes--responsible for T cell-mediated allergic responses
  4. Macrophages (process and present antigen on their surface for recognition by lymphocytes)
3. Consequences of cell interactions in immune response
  1. B lymphocytes
    1. Some divide to produce pool of cells specific for this antigen
    2. Some differentiate to produce plasma cells that secrete antibody in large quantity
    3. Antibody produced is specific for bacterial antigen that provoked the response
    4. Antibody binds to bacterial antigen, lableling the bacteria as targets for destruction
  2. T lymphocytes
    1. Some T-helper cells divide to produce pool of cells specific for this antigen
    2. Some T-helper cells send signals to activate cytotoxic T cells
    3. Cytotoxic T cells kill target cells expressing foreign antigen
4. Antibodies
  1. Specific proteins with multiple antigen-binding sites
  2. Bind to antigen that stimulated the immune response that resulted in production of the ab
  3. Made in large amounts by plasma cells derived from B lymphocytes
  4. Basic subunit is Y-shaped molecule with two binding sites for antigen
  5. Antibody classes
    1. IgG (monomer, 2 ag binding sites, predominant ab in blood, predominant ab in secondary response, some IgG can cross placenta, binds complement)
    2. IgM (pentamer, 10 ag binding sites, first ab class to appear in primary response, binds complement well, found in substantial quantity in blood
    3. IgA (monomer in blood, dimer with secretory piece in secretions, protected from degradation by proteases in secretions such as saliva, tears, colostrum, mucous secretions)
    4. IgE (monomer, attaches to mast cells by its tail, binds to allergens, binding changes mast cell membrane to cause degranulation, leading to release of vasoactive substances, inflammation, and symptoms of certain allergies such as hay fevers, many food allergies, drug and venom allergies, systemic anaphylaxis)
    5. Note: not all allergies are IgE-mediated; some are T-lymphocytes mediated (delayed reaction, no ab, no B ly), such as a poison ivy rash, or a postive TB test
  6. Antibody responses
    1. Primary (made after first exposure to antigen, IgM first, then IgG)
    2. Secondary (or memory) (faster, higher peak, longer lasting, predominantly IgG, vaccination, booster shots, etc., larger pool of ag-specific cells
Selected Pathogens
1. Suggested procedure for study of this section: For each disease/pathogen discussed in class, make sure you know the following: general characteristics of pathogen, how it invades and colonizes the host, how it is transferred from one host to another, what virulence factors are important to its pathogenicity, what pathological effects (disease characteristics) it has on the host, and any other miscellaneous details concerning treatment, immunization, etc. You might want to summarize these in a giant table with these characteristics across the top and the organism/diseases down one side (or v.v.).
  1. Current events pathogen: Bacillus anthracis (not discussed in class spring, 2003)
    1. Transmission
      1. Not tranferred from person to person
      2. Acquired from soil or animal products contaminated with endospores (see below)
      3. Endospores may be inhaled, ingested, or may enter the skin or mucous membrances through an abrasion
    2. Forms of disease
      1. Cutaneous anthrax (90% of cases): endospores enter through abrasion in skin or mucous membranes, germinate, produce virulence factors leading to serious swelling and large necrotic lesions, readily treated with antibiotics, becomes more serious if bacteria enter blood
      2. Ingestion anthrax (rare): endospores are ingested in contaminated meat, germinate, and produced virulence factors
      3. Inhalation anthrax (rare): endospores are inhaled (in aerosols with particles 1 to 10 microns in diameter) and reach the lower respiratory tract; after 1 to 6 d incubation, symptoms include fever, dry cough, chest discomfort, which may seem to improve; early treatment with antibiotics usually effective; in several hours to 2 or 3 days, sudden onset of difficulty breathing, profuse sweating, cyanosis, shock, death in 24 to 36 hours; treatment with antibiotics sometimes effective.
    3. Virulence factors
      1. Endospore formation: protected form of cell, resistant to heat, radiation, chemicals, may survive in soil for decades
      2. Exotoxins: LF (toxic, lethal, causes necrosis), EF (toxic, causes swelling), PA (not toxic, but essential for binding of toxins to cells)
      3. Capsule: protein, prevents phagocytosis
      4. Toxins may destroy neutrophils, and cells are not easily killed by surviving neutrophils even if ingested
    4. Vegetative cells easily destroyed with 0.05% hypochlorite (1 T. bleach in one gallon water); endospores require burning or steam sterilization to destroy
    5. Treatment:
      1. Early treatment: antibiotics such as ciprofloxacin which has been FDA-approved for humans (penicillin derivatives would also be effective)
      2. Later treatment: antibiotics, but less effective because the toxins have already been produced and eliminating the bacteria does not eliminate the toxins
    6. Vaccine: 6 doses followed by annual boosters, ideally need first dose 4 weeks before exposure for adequate protection, effective for cutaneous and probably for inhaled anthrax, contain cell-free filtrate with PA
    7. Anthrax is a zoonosis, a disease of animals such as sheep and cattle, usually transmitted to humans by contact with animal products.
  2. Respiratory Tract Pathogens
    1. Upper respiratory tract
      1. Many normal flora, including Streptococcus pneumoniae and other streptococci, also corynebacteria, micrococci
      2. Consists of mouth, tongue, teeth, nose, sinuses, trachea
    2. Lower respiratory tract
      1. No normal flora
      2. Consists of bronchi, lungs
      3. Defenses include ciliated epithelial cells, mucous secretions, alveolar lung macrophages
      4. Pneumonia may be caused by Mycoplasma, Streptococcus pneumoniae, Klebsiella pneumoniae, influenza and other viruses
    3. Streptococcus pyogenes (URT)
      1. Gram positive cocci, often in pairs or chains
      2. Group A streptococci, by Lancefield grouping according to surface carbohydrate
      3. Transferred by products of the respiratory tract, direct contact (skin), and rarely by ingestion (to throat)
      4. Nontoxic virulence factors
        
        M protein (surface antigen)--helps resist phagocytosis, may crossreact with heart muscle proteins
        Streptokinase--dissolves fibrin clots, allowing bacteria to spread through host tissues (used to dissolve clots in heart attack patients)
        DNase--breaks down DNA released from host cells, promotes spread through host tissues
        Hemolysin SLO (streptolysin O)--oxygen labile, cause of subsurface hemolysis on blood agar plate, damage to rbc membranes
        Hemolysin SLS (streptolysin S)--oxygen stable, surface lysis, damage to rbc membranes
        Anti-SLO antibody measured to determine history of group A streptococcal disease--higher ab level indicates greater likelihood of past disease
      5. Toxic virulence factors
        
        Erythrogenic toxin--produced by some strains of Streptococcus pyogenes, causes symptoms of scarlet fever, including rash of petechial hemorrhages from damaged capillaries, fever, production requires infection with virus
      6. Suppurative, acute streptococcal disease
        
        Associated with production of exudate (host cells, bacteria, fluid)
        Streptococcal sore throat, impetigo, scarlet fever
      7. Nonsuppurative post-streptococcal complications
        
        No bacteria, no exudate
        Do have anti-SLO ab, anti-M protein ab--immune response to streptococci causes damage
        Rheumatic fever--(after pharyngitis) heart damage due to anti-M protein ab, which binds to heart muscle, causing inflammatory damage to heart (antibiotics decrease duration of infection, decreasing amount of anti-M protein ab made, and decreasing the chance of damage)
        Acute glomerulonephritis--damage to kidney glomeruli due to anti-SLO ab, which causes inflammation leading to kidney damage
        Arthritis--Ag-ab complexes deposited in joints cause inflammation and joint damage
    4. Corynebacterium diphtheriae (URT)
      1. Gram positive bacilli, fencelike groups
      2. Colonizes pharynx, epiglottis
      3. Toxic virulence factor: diphtheria toxin
        
        Only produced by strains infected with a bacterial virus and only in low iron concentration
        Inhibits protein synthesis in host cells, killing them
      4. Pseudomembrane composed of dead host cells, bacteria, protein, etc., forms leatherlike covering over epiglottis, blocking trachea, leading to asphyxiation
      5. Vaccine: Toxoid prepared from exotoxin, leads to production of preventive anti-toxin antibody that can inactivate the toxin
      6. Much harder to treat than to prevent (serious outbreak in England when children not vaccinated)
      7. Antibiotics of limited usefulness; tracheotomy often required
    5. Bordetella pertussis (LRT) (not discussed in class spring, 2003)
      1. Whooping cough--serious tracheobronchitis and CNS complications
      2. Gram negative bacilli
      3. Toxic virulence factors--endotoxin, exotoxin
      4. Nontoxic virulence factors--pili that attach bacteria to ciliated epithelial cells in bronchi, permitting colonization of bronchi
      5. Vaccine--bacterial fragments--newer vaccine has fewer complications
      6. Disease of children, spread by adult carriers
      7. Fewer cases seen in highly immunized population
    6. Streptococcus pneumoniae (LRT, etc.)
      1. Gram positive coccus, grouped in pairs
      2. Present as URT normal flora in 50%-70% of population
      3. Pneumococcal pneumonia, also otitis media, meningitis
      4. Tranferred in aerosols containing droplets inhaled by new host
      5. Nontoxic virulence factor: polysaccharide capsule--enables bacteria to grow in lungs
        
        Bacteria with capsule are not effectively phagocytized by lung macrophages
        Bacteria without capsule are effectively phagocytized
        Bacteria with capsule that have bound host anti-capsule ab (opsonization) are effectively phagocytized
        Capsule can also be shed from bacteria--polysaccharide can reacti with host ab to prevent opsonization
      6. Toxic virulence factors: NONE
      7. Treatment: antibiotics, usually penicillin or its derivatives, but 70% of healthy adults recover without antibiotics
      8. Especially vulnerable: the very young, the very old, those with impaired defenses, such as alcoholics, heavy smokers, malnutrition,
      9. Little long term damage (unlike other organisms that cause pneumonia)
      10. Vaccine: of the over 23 different types of capsular polysaccharide, only 8 types cause most human disease, vaccine offers partial protection
    7. Mycobacterium tuberculosis (LRT)
      1. Acid fast bacilli
      2. Aerobes
      3. Organisms grow slowly w/generation time of 12-20 hours, requiring longer treatment (9 to 12 months)
      4. Cell wall contains 40% lipids (waxes)
        
        Make cell wall relatively impenetrable
        Lipids are glycolipids called mycosides, which act as antigens
      5. Cells resist staining and decolorizing even with acid alcohol, disinfectants, antiseptics, antibiotics, drugs, dehydration
      6. Organism produces no toxins, no endospores, no capsule, no pili
      7. Tuberculin protein (acts as antigen)--basis of TB skin test, which measures T-cell mediated immune response to tuberculin
      8. Very readily phagocytized by macrophages, but not killed
      9. Intracellular pathogen: can live inside macrophages, and multiply inside macrophages
      10. Primary infection
        
        Inhaled in aerosol--reaches LRT where it forms small lesions
        Usually these lesions heal to form small invisible tubercles containing live bacteria
        Rarely, bacteria multiply to form larger lesion, with necrosis (often cheesy consistency--"caseation necrosis") and calcification
        
        These may eventually heal to form fibrous, calcified tubercle, visible in Xrays
        In unusual cases, the lesion may grow acroos a vein, allowing bacteria to enter blood, and disease becomes systemic
        
        Early in course of primary infection, there is little host resistance, little immune response, mycobacteria are intracellular in macrophages, inflammation may promote tissue damage
        Later, lesion becomes productive, begins to release mycobacteria extracellularly, T-cell mediated immune response begins
        Macrophages activated by T-cell IR can now kill mycobacteria
        Person may become positive for TB skin test
        Tubercles now form calcified lesions with mycobacteria, host cells, fibrous materials, calcification
        As IR continues and lesions grow, response itself may contribute to destruction of lung tissue
      11. Reactivation disease
        
        More than 2/3 of all cases are reactivation of primary infections
        Usually positive for TB skin test by this time
      12. Diagnosis
        
        Look for AFB in sputum sample with microscope
        Can culture sputum, but AFB grow slowly (weeks)
        Xray to look for calcified tubercles
        TB skin test, PPD test (can have false positives or negatives, BCG vaccination will make person TB+)
      13. Treatment
        
        Preventive treatment: INH (isoniazid)--6 to 12 months for TB test positives with no sign of disease
        Therapeutic treatment of disease: combination of rifampin, INH, ethambutol for 9 to 24 months
        Long treatment required because organisms grow slowly, resist penetration, and are protected inside macrophages or tubercle
        Treatment complicated by increasing incidence of drug-resistant strains, especially in AIDS patients
  3. Gastrointestinal Tract Pathogens
    1. Usually tranferred by products of the gastrointestinal tract, such as by fecal contamination of food, milk, water that is ingested by host
    2. Food poisoning due to bacterial products (Clostridium botulinum, Bacillus, Staphylococcus, Escherichia coli
    3. Food poisoning due to live bacteria cells that colonize host (Salmonella, Shigella, Escherichia)
    4. Food poisoning example: Salmonella typhimurium (salmonellosis)
      1. Gram negative bacilli, lactose negative enteric bacteria
      2. Distinguished by surface antigen typing
        
        O antigen--polysaccharide of endotoxin
        H antigen--flagellar protein
        Combination of both gives specific type of Salmonella--O21H17, O9H23
      3. Cause gastroenteritis
      4. Ingested in contaminated foods such as eggs, milk, uncooked meat)
      5. Incubation period: about eight hours
      6. Organisms colonize large intestine, cause endotoxin-like effects: fever, diarrhea, abdominal pain
      7. Lasts 2-5 days
      8. Not life-threatening, except in very young or very old patients
      9. Not necessarily treated with antibiotics, which may prolong a carrier state in which host still infected but displays no symptoms, sheds bacteria in feces
    5. Food poisoning example: Clostridium botulinum (botulism)
      1. Gram positive bacillus, anaerobe, forms endospores
      2. No infection no colonization of host necessary--rather, this is an intoxication
      3. Scenario
        
        Person cans green beans from garden at too low temperature, too low pressure, or for too short a time
        Green beans are contaminated with garden soil containing endospores of Clostridium botulinum
        Endospores survive, germinate in carbohydrate rich, mosist, anaerobic environment
        Vegetative cells produce exotoxin (potent neurotoxin)
        If beans are cooked before serving (10 min, 100^oC), toxin is denatured and no disease results
        If beans are used uncooked (in salad?), the toxin remains intact and is ingested, enters GI tract, and is absorbed into blood
      4. Neurotoxin blocks transmission of nerve signals to muscle cells, leading to flaccid paralysis
      5. One in four people die of respiratory paralysis
      6. Treatment: respiratory support, antitoxin (antibiotics not useful)
    6. Major GI tract pathogen: Salmonella typhi (typhoid fever)
      1. Gram negative enteric bacillus, lactose negative
      2. Transferred by ingestion of fecally contaminated water or milk, or from contact with carrier
      3. Nontoxic virulence factors
        
        Adhesins--help bacteria stick to lining of intestine
        Intracellular growth in macrophages
      4. Toxic virulence factor: endotoxin
        
        Causes rash of petechial hemorrhages of small blood vessels called "rose spots"
        Causes vascular effects in liver, spleen, bone marrow
      5. After ingestion, organism colonizes small intestine during a 7-14 day incubation period
      6. First week: organism penetrates wall of intestine, enter lymph vessels, moves to lymph tissue (mesenteric lymph nodes, others), begins to grow intracellularly
      7. First week symptoms: flu-like, fever, aching, constipation
      8. Second week: organism moves into bloodstream (bacteremia), and colonizes gall bladder
      9. Second week symptoms: fever over 104^o, rose spots, tender abdomen, diarrhea
      10. Third week: patient exhausted if alive, begins to recover, lower fever, immune response (T cell mediated) enables activated macrophages to kill intracellular bacteria
      11. Immune response is specific for typhoidin (bacterial protein)
      12. Antibiotics may be helpful
      13. Vaccine: contains weakened strain of pathogen
      14. About 3% of patients become carriers
      15. Organism persists in gall bladder, surviving bacteria shed in feces
      16. Prevention: good sanitation, adequate sewage treatment and disposal, no carriers as food handlers, avoid fecal water pollution, pasteurization of milk
    7. Major GI tract pathogen: Shigella dysenteriae (bacillary dysentery)
      1. Gram negative bacilli, nonmotile, enteric bacteria
      2. Serology: no H ag (no flagella), O ag (LPS), K ag (outside of cell)
      3. Transferred by ingestion of contaminated food, rarely person-to-person
      4. Toxic virulence factors
        
        Endotoxin
        Shiga neurotoxin: paralysis, diarrhea, death
        enterotoxin (may be same molecule as neurotoxin): acts on GI tract, diarrhea, decreased absorption of fluid by intestinal lining
      5. Course of disease
        
        After ingestion of as few as ten cells, there is an incubation period of 1-4 days, during which the bacteria colonize the colon and terminal ileum
        The bacteria penetrate the epithelial cells of the intestinal lining and grow intracellularly in the epithelial cells
        This results in damage to the epithelial cells and inflammatory damage to the intestinal lining.
        Symptoms: abdominal cramps, diarrhea containing mucus and blood, pain, fever, possibly dehydration
        The lesion (damaged area) of the intestinal wall may become necrotic, but there is rarely perforation of the intestinal wall
        For four weeks after symptoms disappear, the patient may shed bacteria and antibodies to bacteria in feces
      6. Treatment: antibiotics (but resistant strains may be a problem), fluid and electrolyte replacement
      7. Immune response: Before bacteria become intracellular, antibodies may be protective.
      8. Comparison with typhoid fever: Salmonella grows in macrophages, is more invasive, main defense is T cell-mediated; Shigella grows in epithelial cells, is less invasive, main defense is antibody until bacteria are intracellular
    8. Major GI tract pathogen: Vibrio cholerae (cholera)
      1. Gram negative curved bacilli, not in same family as other enteric bacteria, tolerates more basic pH (9.0-9.6) than most bacteria
      2. Toxic virulence factor: choleragen
        
        Exotoxin, enterotoxin
        Protein that acts on epithelial cells that line GI tract
        Activates adenylate cyclase, which increases concentration of cAMP, increases chloride secretion, decreases sodium absorption
        Results in massive water loss (16-20 l/day), watery diarrhea (rice water stools)
        Fluid loss causes decrease in plasma volume, loss of electrolytes, decrease in blood pH (more acid)
        Untreated, people are likely to go into shock and die
      3. Nontoxic virulence factor: adhesins
      4. Course of disease
        
        After ingestion, there is an incubation period of 2-5 days, during which bacteria colonize small intestine and attach to surface of intestinal lining with help of adhesins
        The bacteria remain on the surface, producing the enterotoxin which causes the symptoms (no invasion of intestinal wall or movement to other tissues)
        Symptoms: nausea, vomitting, abdominal cramps, diarrhea (IgA ab may be present in stools)
      5. Treatment:
        
        Fluid and electrolyte replacement (oral)
        Antibiotic: tetracycline
        Untreated, 50% die; treated, 1-2% die
    9. Unusual GI tract pathogen: enteropathic, enterotoxigenic Escherichia coli
      1. Gram negative bacillus, enteric bacterium, lactose positive, may be normal flora if nontenterotoxigenic
      2. Serology: O ag, H ag
      3. Nontoxic virulence factors: CFA-I, CFA-II (adhesins which assist colonization and penetration of intestinal lining, in 5-7 hours)
      4. Toxic virulence factors
        
        endotoxin, ST toxin
        LT toxin (very similar to choleragen) in size, subunits (one part toxic, one part binds to receptors on epithelial cells), LT toxin gene located on plasmid (choleragen is part of chromosome)
        10%-20% of E. coli are ET/EP--they have the LT plasmid, other strains don't
      5. Course of disease
        
        Transferred by ingestion of contaminated food, very short incubation period (hours)
        Symptoms: sudden onset, severe watery diarrhea, dehydration, shock, 16% mortality (especially severe in newborns)
      6. Treatment: fluid and electrolyte replacement, antibiotic
    10. Comparison of enteric pathogens
      1. Fluid loss: Vc > Ec > Sd > St
      2. Antibiotic usefulness: St > Ec, Sd > Vc
      3. Stays in intestine: St--no; Sd, Vc, Ec--yes