Microbe-Host Interactions Notes

A Glimpse of History

• Ancients believed diseases were divine punishment.
• Leeuwenhoek's discovery of microorganisms in the 17th century suggested they might cause disease.
• Robert Koch (1876) provided evidence for the germ theory of disease.
  • Showed Bacillus anthracis causes anthrax.
  • Later work on tuberculosis.
  • Formalized Koch's postulates for establishing the cause of disease.

Bacteria Are Ubiquitous

• We encounter numerous microorganisms daily through:
  • Breathing.
  • Ingestion with food and drink.
  • Contact with skin.
• Most microorganisms do not harm us.
• Some colonize body surfaces, while others are shed with dead epithelial cells.
• Most swallowed microorganisms are killed in the stomach or eliminated in feces.
• Relatively few microorganisms are pathogens that cause damage.
• Distinct characteristics allow pathogens to avoid some body defenses.

Microbes, Health, and Disease

• Most microbes are harmless.
• Many microbes are beneficial.
• Normal microbiota (normal flora) are organisms that routinely reside on body surfaces.
• The relationship between microbes and the host is a delicate balance.
• Some microbes can cause disease if there is an opportunity.
• Weaknesses or defects in innate or adaptive defenses can leave individuals vulnerable to invasion.
  • Individuals are said to be immunocompromised.
  • Factors include malnutrition, cancer, AIDS or other diseases, surgery, wounds, genetic defects, alcohol or drug abuse, and immunosuppressive therapy.

16.1. The Anatomical Barriers as Ecosystems

• Skin and mucous membranes are barriers and host complex ecosystems of microorganisms.
• Symbiosis: "living together."
  • Mutualism: both partners benefit.
    • Example: In the large intestine, some bacteria synthesize vitamin K and B vitamins, which the host can absorb. The bacteria are supplied with warmth and energy sources.
  • Commensalism: one partner benefits, and the other is unharmed.
    • Many microbes living on the skin are neither harmful nor helpful but obtain food and necessities from the host.
  • Parasitism: one organism benefits at the expense of the other.
    • All pathogens are parasites, but medical microbiologists often reserve the term for eukaryotic pathogens (e.g., protozoa, helminths).

16.2. The Normal Microbiota

• Normal microbiota
  • Resident microbiota inhabit sites for extended periods.
  • Transient microbiota inhabit temporarily.
• Important to human health.
• Relatively little is known about the normal microbiota.
• The Human Microbiome Project studies microbiota using metagenomics, which involves the analysis of DNA without culturing.

16.2. The Normal Microbiota - Composition

• Colonization at birth
• Contribution from breastfeeding
• Composition differs among individuals and over time
• Changes with the physiological state and lifestyle of the host.
• Example: More Firmicutes in obese people, more Bacteroidetes in thin people.
• Weight loss changes microbiota to resemble that of lean people.

16.2. The Normal Microbiota - Protective Role

• Significant contribution to protection against pathogens.
  • Covering of binding sites prevents attachment.
  • Consumption of available nutrients.
  • Production of compounds toxic to other bacteria.
• When normal microbiota is killed or suppressed (e.g., during antibiotic treatment), pathogens may colonize and cause disease.
  • Some antibiotics inhibit Lactobacillus (predominates in the vagina of mature females and suppresses the growth of Candida albicans), resulting in vulvovaginal candidiasis.
  • Oral antibiotics can inhibit intestinal microbiota, allowing overgrowth of toxin-producing Clostridium difficile.

16.2. The Normal Microbiota - Further Protective Roles

• Stimulation of the adaptive immune system.
  • Antibodies against normal microbiota also bind to pathogens.
• Mice in microbe-free environments have underdeveloped mucosal-associated lymphoid tissue (MALT).
• Important in the development of oral tolerance.
  • The immune system lessens the response to many microbes in the gut and food.
  • Basis of the hygiene hypothesis: insufficient exposure to microbes leads to allergies.
• Aid in digestion: breakdown of fibers.

16.3. Principles of Infectious Disease

• Colonization refers to a microbe establishing itself and multiplying.
• The term infection can be used to refer to a pathogen.
  • Can be subclinical: no or mild symptoms.
• Infectious disease yields noticeable impairment.
  • Symptoms are subjective effects experienced by the patient (pain, nausea).
  • Signs are objective evidence (rash, pus formation, swelling).
• The initial infection is a primary infection.
  • Damage can predispose an individual to developing a secondary infection (respiratory illness impairing the mucociliary escalator).

16.3. Principles of Infectious Disease - Pathogenicity

• Primary pathogen: a microbe or virus that causes disease in an otherwise healthy individual.
  • Diseases include plague, malaria, measles, influenza, diphtheria, tetanus, and tuberculosis.
• Opportunistic pathogen (opportunist): Causes disease only when the body’s innate or adaptive defenses are compromised or when introduced into an unusual location.
  • Can be members of the normal microbiota or common in the environment (Pseudomonas).
• Virulence refers to the degree of pathogenicity.
• Virulence factors allow a microorganism to cause disease.

16.3. Principles of Infectious Disease - Characteristics

• Communicable or contagious diseases are easily spread.
• Infectious dose: the number of microbes necessary to establish an infection.
• $ID_{50}$ is the number of cells that infects 50% of the population.
• Shigellosis results from ~10–100 ingested Shigella.
• Salmonellosis results from as many as $10^6$ ingested Salmonella enterica serotype Enteritidis.
  • The difference partially reflects the ability to survive stomach acid.

16.3. Principles of Infectious Disease - Disease Course

• Incubation period: time between infection and onset.
  • Varies considerably: a few days for the common cold to years for leprosy.
  • Depends on growth rate, host’s condition, and infectious dose.
• Illness: signs and symptoms of disease.
  • May be preceded by a prodromal phase (vague symptoms).
• Convalescence: recuperation, recovery from disease.
• Carriers may harbor and spread an infectious agent for long periods of time in the absence of signs or symptoms.

16.3. Principles of Infectious Disease - Symptom Duration

• Acute infections: symptoms develop quickly and last a short time (strep throat).
• Chronic infections: develop slowly and last for months or years (tuberculosis).
• Latent infections: never completely eliminated; the microbe exists in host tissues without causing symptoms.
  • A decrease in immunity may allow reactivation.
  • Chickenpox (acute illness) results from the varicella-zoster virus; the immune response stops, but the virus hides in sensory nerves and can later produce viral particles resulting in shingles.
  • Tuberculosis, cold sores, and genital herpes are also examples.

16.3. Principles of Infectious Disease - Pathogen Distribution

• Localized infection: the microbe is limited to a small area (boil caused by Staphylococcus aureus).
• Systemic infection: the agent spreads throughout the body (measles).
• Suffix -emia means “in the blood.”
  • Bacteremia: bacteria circulating in the blood.
    • Not necessarily a disease state (can occur transiently following vigorous tooth brushing).
  • Toxemia: toxins circulating in the bloodstream.
  • Viremia: viruses circulating in the bloodstream.
• Sepsis: acute, life-threatening inflammation caused by infectious agents or products in the bloodstream.

16.4. Establishing the Cause of Infectious Disease - Koch's Postulates

• Criteria Robert Koch used to establish that Bacillus anthracis causes anthrax:
  • The microorganism must be present in every case of the disease.
  • The organism must be grown in pure culture from a diseased host.
  • The same disease must be produced when the pure culture is introduced into susceptible hosts.
  • Organisms must be recovered from experimentally infected hosts.

16.4. Establishing the Cause of Infectious Disease - Limitations & Molecular Koch's Postulates

• Some limitations of Koch’s Postulates:
  • Some organisms cannot be grown in laboratory medium (the causative agent of syphilis).
  • Infected individuals do not always have symptoms (cholera, polio).
  • Some diseases are polymicrobial (periodontal).
  • Suitable animal hosts are not always available for testing.
• Molecular Koch’s Postulates:
  • A virulence factor gene or product is found in pathogenic strains of the organism.
  • Mutating the gene to disrupt its function should reduce virulence.
  • Reversion or replacement of the gene should restore virulence.

Mechanisms of Pathogenesis

• Several general patterns:
  • Produce toxins that are ingested (Clostridium botulinum, Staphylococcus aureus).
  • Colonize mucous membranes and produce toxins (Vibrio cholerae, E. coli O157:H7, Corynebacterium diphtheriae).
  • Invade host tissues and avoid defenses (Mycobacterium tuberculosis, Yersinia pestis, Salmonella enterica).
  • Invade host tissues and produce toxins (Shigella dysenteriae, Clostridium tetani).
• Pathogens and hosts generally evolve toward balanced pathogenicity (myxoma virus and rabbits).

16.5. Establishing Infection - Adherence & Colonization

• Adherence:
  • Adhesins attach to the host cell receptor.
  • Often located at the tips of fimbriae.
  • Can be a component of capsules or various cell wall proteins.
  • Binding is highly specific and exploits the host cell receptor.
• Colonization:
  • Growth in biofilms.
  • Siderophores bind iron.
  • Avoidance of secretory IgA:
    • Rapid pili turnover, antigenic variations, IgA proteases.
  • Compete with normal microbiota and tolerate toxins.

16.5. Establishing Infection - Effector Proteins

• Delivering Effector Proteins to Host Cells
  • Secretion systems in Gram-negatives.
    • Several types discovered; some can inject molecules other than proteins.
  • Type III secretion system (injectisome).
    • Effector proteins induce changes (altering the cell’s cytoskeleton structure).
    • Can induce uptake of bacterial cells.

16.6. Invasion—Breaching the Anatomical Barriers - Skin & Mucous Membranes

• Penetrating the Skin
  • A difficult barrier to penetrate; bacteria rely on injuries.
  • Staphylococcus aureus enters via a cut or wound; Yersinia pestis is injected by fleas.
• Penetrating Mucous Membranes
  • The entry point for most pathogens.
• Directed Uptake by Cells
  • Pathogen induces cells to engulf via endocytosis.
    • Salmonella uses type III secretion; actin molecules rearrange, causing ruffling of the membrane and uptake of bacteria.

16.6. Invasion—Breaching the Anatomical Barriers - Antigen Sampling

• Exploiting Antigen-Sampling Processes
  • Mucosal-associated lymphoid tissue (MALT) samples.
  • Some pathogens use M cells to cross the intestinal barrier.
  • Shigella survives phagocytosis by macrophages, induces apoptosis, binds to the base of mucosal epithelial cells, and induces uptake.
  • Some invade by alveolar macrophages (Mycobacterium tuberculosis produces surface proteins, directs uptake, and avoids macrophage activation).

16.7. Avoiding the Host Defenses - Hiding Within Host & Phagocyte Encounters

• Hiding Within a Host Cell
  • Allows avoidance of complement proteins, phagocytes, and antibodies.
  • Shigella directs transfer from intestinal epithelial cell to adjacent cells by causing host cell actin polymerization.
  • Listeria monocytogenes does the same.
• Avoiding Destruction by Phagocytes
  • Preventing Encounters with Phagocytes
    • C5a peptidase: degrades chemoattractant C5a (Streptococcus pyogenes).
    • Membrane-damaging toxins: kill phagocytes and other cells (S. pyogenes makes streptolysin O).

16.7. Avoiding the Host Defenses - Phagocyte Attachment

• Avoiding Recognition and Attachment
  • Capsules: interfere with opsonization; some bind the host’s regulatory proteins that inactivate C3b (Streptococcus pneumoniae).
  • M protein: the cell wall of Streptococcus pyogenes binds a regulatory protein that inactivates C3b.
  • Fc receptors: bind the Fc region of antibodies (Staphylococcus aureus, Streptococcus pyogenes).

16.7. Avoiding the Host Defenses - Survival Within & Serum Resistance

• Avoiding Destruction by Phagocytes (continued…)
  • Surviving Within Phagocytes
    • Escape from phagosome: prior to lysis with lysosomes.
      • Listeria monocytogenes produces a molecule that forms pores in the membrane; Shigella species lyse the phagosome.
    • Prevent phagosome-lysosome fusion: avoid destruction.
      • Salmonella senses ingestion by a macrophage and produces a protein that blocks the fusion process.
    • Survive within phagolysosome: few can survive the destructive environment.
      • Coxiella burnetii (Q fever) can withstand and delays fusion, allowing time to equip itself to survive.
  • Serum resistant bacteria
    • Neisseria gonorrhoeae hijacks the host system, binds complement regulatory proteins to avoid the membrane attack complex.

16.7. Avoiding the Host Defenses - Avoiding Antibodies

• Avoiding Antibodies
  • IgA protease: cleaves IgA, found in mucus and secretions.
    • Produced by Neisseria gonorrhoeae and others
  • Antigenic variation: alter the structure of surface antigens, stay ahead of antibody production.
    • Neisseria gonorrhoeae varies the antigenic structure of pili.
  • Mimicking host molecules: cover the surface with molecules similar to those found in the host cell, appear to be “self.”
    • Streptococcus pyogenes forms a capsule from hyaluronic acid, a polysaccharide found in tissues.

16.8. Damage to the Host - Direct and Indirect Effects

• Direct or indirect effects
  • Direct: toxins produced
  • Indirect: the immune response
• Damage may help the pathogen to exit and spread.
  • Vibrio cholerae induces watery diarrhea, up to 20 liters/day, which contaminates water supplies.
  • Bordetella pertussis triggers severe coughing, and pathogens are released into the air.

16.8. Damage to the Host - Exotoxins

• Exotoxins: proteins with specific damaging effects.
  • Secreted or leak into tissues following bacterial lysis.
  • Foodborne intoxication results from consumption.
  • Destroyed by heating; most exotoxins are heat-sensitive.
  • Can act locally or systemically.
  • Proteins; the immune system can generate antibodies.
  • Many are fatal before an immune response occurs.
  • Vaccines are critical: toxoids are inactivated toxins.
  • Antitoxin is a suspension of neutralizing antibodies.
• Neurotoxins damage the nervous system.
• Enterotoxins cause intestinal disturbance.
• Cytotoxins damage a variety of cell types.

16.8. Damage to the Host - Exotoxins Produced by Various Primary Pathogens (Table 16.1)

• Refer to Table 16.1 in the transcript for specific exotoxins, their mechanisms, and associated diseases.
• A-B Toxins
  • Composed of two subunits, A and B. The A subunit is the toxic, or active, part; the B subunit binds to the target cell.
    • Neurotoxins
      • Clostridium botulinum: Botulism; botulinum toxin - Flaccid paralysis - Blocks transmission of nerve signals to the muscles by preventing the release of acetylcholine.
      • Clostridium tetani: Tetanus; tetanospasmin - Spastic paralysis - Blocks the action of inhibitory neurons by preventing the release of neurotransmitters.
    • Enterotoxins
      • Enterotoxigenic E. coli: Traveler's diarrhea; heat-labile enterotoxin (cholera-like toxin) - Severe watery diarrhea - Modifies a regulatory protein in intestinal cells, causing those cells to continuously secrete electrolytes and water.
      • Vibrio cholerae: Cholera; cholera toxin - Severe watery diarrhea - Modifies a regulatory protein in intestinal cells, causing those cells to continuously secrete electrolytes and water.
    • Cytotoxins
      • Bacillus anthracis: Anthrax; edema factor, lethal factor - Inhaled form-septic shock; cutaneous form-skin lesions - Edema factor modifies a regulatory protein in cells, causing accumulation of fluids. Lethal factor inactivates proteins involved in cell signaling functions.
      • Bordetella pertussis: Pertussis (whooping cough); pertussis toxin - Sudden bouts of violent coughing - Modifies a regulatory protein in respiratory cells, causing accumulation of respiratory secretions and mucus. Other factors also contribute to the symptoms.
      • Corynebacterium diphtheriae: Diphtheria; diphtheria toxin - Pseudomembrane in the throat; heart, nervous system, kidney damage - Inhibits protein synthesis by inactivating an elongation factor of eukaryotic cells. Kills local cells (in the throat) and is carried in the bloodstream to various organs.
      • Shigella dysenteriae: Dysentery, hemolytic uremic syndrome; shiga toxin - Diarrhea that contains blood, pus, and mucus; kidney damage - Inactivates the 60S subunit of eukaryotic ribosomes, stopping protein synthesis.
      • E. coli O157:H7: Bloody diarrhea, hemolytic uremic syndrome; shiga toxin - Diarrhea that may be bloody; kidney damage - Inactivates the 60S subunit of eukaryotic ribosomes, stopping protein synthesis.
• Membrane-Damaging Toxins (cytotoxins)
  • Disrupt plasma membranes, causing leakiness that results in cell lysis.
    • Clostridium perfringens: Gas gangrene; α-toxin - Extensive tissue damage - Removes the polar head group on the phospholipids in the membrane, damaging membrane structure.
    • Staphylococcus aureus: Wound and other infections; leukocidin - Accumulation of pus - Inserts into membranes, forming pores that allow fluids to enter the cells.
    • Streptococcus pyogenes: Pharyngitis and other infections; streptolysin O - Accumulation of pus - Inserts into membranes, forming pores that allow fluids to enter the cells.

16.8. Damage to the Host - Superantigens & Other Toxic Proteins (Table 16.1 Continued)

• Refer to Table 16.1 in the transcript for specific exotoxins, their mechanisms, and associated diseases.
  • Superantigens
    • Override the specificity of the T-cell response.
      • Staphylococcus aureus (certain strains): Foodborne intoxication; staphylococcal enterotoxins - Nausea and vomiting - Not well understood with respect to how the ingested toxins lead to the characteristic symptoms of foodborne intoxication.
      • Staphylococcus aureus (certain strains): Staphylococcal toxic shock; toxic shock syndrome toxin (TSST) - Fever, vomiting, diarrhea, muscle aches, rash, low blood pressure - Systemic toxic effects due to the resulting massive release of cytokines.
      • Streptococcus pyogenes (certain strains): Streptococcal toxic shock; streptococcal pyrogenic exotoxins (SPE) - Fever, vomiting, diarrhea, muscle aches, rash, low blood pressure - Systemic toxic effects due to the resulting massive release of cytokines.
  • Other Toxic Proteins
    • Staphylococcus aureus: Scalded-skin syndrome; exfoliatin - Separation of the outer layer of the skin - Destroys the material that holds the layers of skin together.
    • Various organisms: Various diseases; proteases, lipases, and other hydrolases - Tissue damage - Degrades proteins, lipids, and other compounds that make up tissues.

16.8. Damage to the Host - Exotoxins cont.

• A-B toxins have two parts:
  • The A (active) subunit is toxic, usually an enzyme.
  • The B subunit binds to the cell, determines the cell type to be infected.
  • The structure allows novel approaches for vaccines and therapies; can use the B subunit to deliver medically useful compounds to a specific cell type.
• Membrane-Damaging Toxins
  • Cytotoxins that disrupt plasma membranes and lyse cells.
  • Hemolysins lyse red blood cells.
    • Some insert into membranes and form pores (streptolysin O from Streptococcus pyogenes).
    • Phospholipases hydrolyze phospholipids of the membrane (α-toxin of Clostridium perfringens, gas gangrene).

16.8. Damage to the Host - Superantigens & Other Toxic Proteins cont.

• Superantigens: simultaneously bind MHC class II and T-cell receptor.
  • The T-cell interprets this as antigen recognition.
  • The toxic effect is from massive cytokine release from $T_H$ cells.
  • Include toxic shock syndrome toxin (TSST) and several others from Staphylococcus aureus and Streptococcus pyogenes.
• Other Toxic Proteins
  • Some damaging proteins are not A-B toxins, membrane-damaging toxins, or superantigens.
    • Exfoliatin from Staphylococcus aureus causes scalded skin syndrome.
      • Destroys the material that binds together skin layers.
      • Bacteria may be growing in a small lesion, but the toxin spreads systemically.
    • Various hydrolytic enzymes, including proteases, lipases, and collagenases, break down connective tissue.
      • Destroy tissues; some help bacteria spread.

16.8. Damage to the Host - Endotoxin & Cell Wall Components

• Endotoxin: lipid A of LPS (lipopolysaccharide).
  • Lipid A triggers an inflammatory response.
    • When localized, the response helps clear the infection.
    • When systemic, it causes a widespread response: septic shock or endotoxic shock.
  • Lipid A is released following cell lysis.
  • Activates innate and adaptive defenses.
    • Toll-like receptors induce cytokine production; T-independent antigen response of B-cells at high concentrations.
  • Heat-stable; autoclaving does not destroy it.
  • Causes fever and disseminated intravascular coagulation.
  • Peptidoglycans and other components also trigger fever, sepsis, and septic shock.

16.8. Damage to the Host - Exotoxin vs. Endotoxin

• Comparison of Exotoxins and Endotoxin
  • Exotoxins from Gram-positives and Gram-negatives
    • Protein; potent; usually heat-inactivated
  • Endotoxins only from Gram-negatives
    • Lipid A component of LPS; small localized amounts yield an appropriate response, but systemic distribution can be deadly; heat-stable

Bacterial Infections: 3 Take-Away Points

• Tissue Damage: When some bacteria adhere to/invade tissue cells, they produce toxins and disrupt normal tissue activity.
• Blood Clots: When some bacteria secrete toxins that cause blood to clot, it causes a blockage in blood vessels, causing those tissues to die.
• Fluid Leakage from Blood Vessels: Some bacterial toxins can cause massive inflammation, resulting in a severe drop in blood volume and blood pressure; hence, inadequate blood flow to the brain and other vital organs.