Host Microbe Interactions and Mechanisms in Pathogenesis Vocabulary

Normal Flora

• Population of microbes on skin and mucosal surfaces.

• Primarily bacteria; no known normal viral flora.

• Two classes:

  • Resident flora: Fixed, variable by location/age, rapidly replenished.

  • Transient flora: Temporary, from environment, may include pathogens, little impact if normal flora intact.

• Role:

  • Not essential but important for health.

  • Produce vitamin K.

  • Prevent pathogen colonization (microbial antagonism).

  • Compete for nutrients.

  • Produce compounds that kill pathogens (colicins).

  • Depletion can lead to colonization by harmful microbes.

• Normal flora in improper locations can cause disease.

  • Streptococci in blood → endocarditis.

  • Bacteroides in peritoneum → sepsis.

  • E. coli in urinary tract → UTI.

  • Staphylococci in blood with implants → serious problems.

Normal Flora of the Skin

• Skin: first defense, constantly exposed.

• Transient and resident bacteria present.

• Resident bacteria vary by location.

• Variables: secretions, clothing, proximity to mucous membranes.

• Dominant resident bacteria:

  • Aerobic and anaerobic diphtheroid bacteria (corynebacterium, propionibacterium).

  • Non-hemolytic and a-hemolytic staphylococci (S. epidermidis, S. aureus).

  • Gm (+) aerobic spore forming bacteria (B. subtilis).

  • a-hemolytic streptococci (S. salivarius).

  • Gm (-) coliform bacteria, acinetobacter species.

  • Yeast and fungi in skin folds.

  • Non-pathogenic mycobacterium (M. smegmatis) in ears.

• Skin secretions fight foreign microbes:

  • Low pH from sebum (fatty acids).

  • Enzymes (lysozyme) kill pathogens.

  • Sweat washes off pathogens.

  • Resident flora adapted, sweat diminishes numbers, replenished from glands.

  • Can cause infection if beneath skin.

Normal Bacteria of Mouth and URI

• Sterile at birth, contaminated during birth.

• Streptococci dominant in nasal passages soon after birth.

• Followed by staphylococci, Gm (-) diplococci, diphtheroids, and lactobacilli.

• Tooth growth → colonization by anaerobic spirochetes, prevotella, fusobacterium, rothia, capnocytophaga, anaerobic vibrios, lactobacilli, actinomyces, and yeasts.

• Similar flora in pharynx and trachea.

• Bronchi and alveoli should be sterile.

• Infections:

  • Usually anaerobic bacteria.

  • Periodontal infections, abscesses, sinusitis, mastoiditis (Prevotella, fusobacterium, peptostreptococci).

  • Aspiration of saliva → URI, necrotizing pneumonia, lung abscess, empyema.

• Dental caries:

  • Tooth disintegration from surface inward.

  • Enamel demineralization due to acid products of bacterial fermentation.

  • Dentin damage due to bacterial breakdown of protein matrix.

  • Plaque formation: S. mutans and S. sobrinus produce gelatinous substance, a tenacious biofilm.

Normal Flora of the Intestinal Tract

• Sterile at birth, colonized by breast milk or formula.

• Esophagus flora resembles mouth.

• Stomach: light microbial flora (10^3-10^5/gram), due to gastric acid.

• Intestinal flora abundant in alkaline pH.

  • Duodenum: 10^3-10^6/gram.

  • Ileum: 10^5-10^8/gram.

  • Large intestine: 10^8-10^10/gram.

• Fecal matter: 10-30% bacteria (10^11/gram).

• Types of bacteria:

  • 96-99% anaerobes: bacteroides, fusobacterium, anaerobic lactobacilli, clostridia, anaerobic Gm (+) cocci.

  • 1-4% facultative anaerobes: Gm (-) coliforms, enterococci, proteus, pseudomonal, lactobacilli, candida.

• Diverse population, over 100 species.

• Metabolic functions: produce vitamin K, metabolize bile, assist nutrient absorption, antagonize pathogens.

• Antimicrobial drugs:

  • Deplete sensitive organisms.

  • May lead to colonization with antibiotic-resistant bacteria (staphylococci, enterobacter, proteus, pseudomonas, clostridium, yeasts).

  • Anaerobic bacteria (B fragilis, clostridia, peptostreptococci) responsible for infectious peritonitis.

Normal Flora of Genitourinary Tracts

• Few bacteria in urethra (flushing and harsh environment).

• 10^2 -10^4 urease producing organism per ml.

• Lactobacilli colonize birth canal.

• Acidic pH due to carbohydrate (glycogen) fermentation.

• Depletion of lactobacilli linked to yeast and bacterial infections.

• Cervical mucous contains lysozyme.

• Other flora: group B hemolytic streptococci, prevotella, clostridia, listeria, and mobiluncus.

Normal Flora of the Eye (Conjunctiva)

• Few organisms due to tears (contain lysozyme).

• Small numbers of diphtheroids (Corynebacterium xerosis), S. epidermidis, nonhemolytic streptococci, neisseriae, Gm (-) bacilli, Morexalla catarrhalis.

Pathogenesis

• Infection followed by tissue/organ damage.

• Stages:

  • Colonization: attachment to host

  • Evasion of immune system: adhesins, capsules, cellular invasion, survival post-phagocytosis.

  • Damage to host: toxins, cellular damage, induction of host damage.

• Severity depends on host immune system, virulence, infectious dose.

Etiology of Disease

• Determination of cause (infectious agent).

• Koch’s Postulates:

  • Isolate and identify causative agent from afflicted organism.

  • Introduce causative agent to healthy organism.

  • Determine if healthy organism exhibits symptoms.

  • Isolate causative agent from test organism.

  • If identical to organism originally isolated, is causative agent.

  • Often requires modification

• Exceptions:

  • Some organisms cannot be cultivated in vitro (Treponema pallidum, Mycobacterium leprae) - animal models used to confirm.

  • Many cannot be cultivated outside human host (HIV, Neisseria gonorrhoeae) - human challenge models exist for non-fatal disease.

  • Some obligate human pathogens confirmed with in vitro tissue culture systems (ETEC, EHEC, EPEC).

• Modern Techniques:

  • PCR allows detection of organisms that cannot be cultivated in vitro (Hanta virus).

Classification of Bacteria in Pathogenesis

• PATHOGEN: presence abnormal, always causes disease (Yersinia pestis, Mycobacterium tuberculosis).

• OPPORTUNISTIC PATHOGEN:

  • Normally present, cause disease if improper localization or immuno-compromised (Escherichia coli, Streptococcus pneumoniae).

  • Not normally present, cause disease only if immuno-compromised (Pseudomonas aeruginosa).

• NONPATHOGEN: incapable of causing disease.

Transmission of Disease

• Requires a RESERVOIR: source of organism.

  • Living reservoir: animals (Salmonella typhimurium, Clostridium botulinum- asymptomatic).

  • Environmental reservoir: water (Vibrio cholera, E. coli), soil (Bacillus anthracis, Clostridium botulinum, Clostridium perfringens).

• Transmission from reservoir to host:

  • VECTOR: living organisms.

  • FOMITE: inanimate objects.

• PORTAL OF ENTRY: preferred route (S. typhimurium ingestion).

The Infectious Proces s

• Attach (colonize).

• Disseminate to suitable tissue/organ.

• Multiply, damage host, cause symptoms.

• Site of multiplication varies (S. pneumoniae in lungs, V. cholera in GI tract ).

Clonal Nature of Bacterial Pathogens

• Haploid organisms, limited genetic acquisition.

• Genes for virulence acquired (plasmid, transposon, transduction).

• Diseases caused by a few clonal types.

• Useful for outbreak determination, clones specific to locations.

Regulation of Expression of Virulence Factors

• Pathogens cycle between reservoirs and hosts.

• Virulence determinants needed only in host.

• Mechanisms induce expression in host:

  • Iron limitation (Corynebacterium diphtheria toxin).

  • Temperature shift (Bordetella pertussis toxin).

  • pH shift (Vibrio cholera toxin).

  • Calcium limitation (Yersinia pestis).

Virulence Factors - Adhesins

• Attachment is first stage.

• Electrostatic repulsion is a problem.

• Many species evolved specific adhesins:

  • Fimbriae: short appendages attach to surface carbohydrates on target cell.

  • Pili: longer appendages attach to extracellular matrix proteins of host (fibronectin).

  • Adhesins attractive candidates for vaccines.

Virulence Factors - Invasion

• Entering host cells:

  • May invade phagocytic cells (MF): Requires bacterial gene products for survival

  • May invade non-phagocytic cells: Bacteria induce cells to physically engulf.

• Fusion of phagosome with lysosome destroys most organism, two known mechanisms permit growth within MF

  • Prevents fusion between phagosome and lysosome (Mycobacterium tuberculosis)

    • Organism proliferates within phagosome

    • Eventually exits cell to spread infection

  • Organisms escapes from phagosome before fusion with lysosome (Listeria monocytogenes)

    • Induces rupture of phagosome prior to lysis

    • Proliferates in cytoplasm of host cell

• Best characterized in Shigella dysenteriae and Yersinia enterocolitica

  • Both involve attachment to target cell followed by Type III secretion of effectors

  • Secreted products induce actin polymerization at site of attachment

  • Forces cell to generate pseudopods and consume organism

  • Usually escape from vacuole following invasion

  • Proliferate within cyotoplasm of host cell

  • Often escape cell by lysis to infect other cells, sometimes responsible for symptoms (Shigellosis bloody diarrhea)

Virulence Determinants - Anti-Phagocytic Factors

• Many organisms resist phagocytosis by PMNs and MF.

• Mechanisms:

  • IgG Fc binding proteins on surface, prevents opsonization.

  • Polysaccharide capsules, prevents phagocytic cell from attaching.

  • Adhesins, allow avid attachment to surfaces, prevents phagocytosis.

Virulence Determinants - Enzymes

• Secreted enzymes important to virulence.

• Some involved in dissemination
  * Collagenase - degrades collagen, ECM protein, allows penetration into tissues
  * Hyaluronidase - digests hyaluronic acid, major component of connective tissue, allows penetration into tissues
  * Fibrinolysin (streptokinase) - degrades fibrin, breaks clots, allows dissemination

• Some involved in isolation of organisms from host defense
  * Coagulase - promotes formation of fibrin clots, limits immune system access to pathogen

• May be involved in nutrient acquisition - hemolysin - lyses red blood cells, important for acquisition of iron and cofactors for enzymes

• Often involved in attack of immune system - leukocidins - lyse white blood cells (leukocytes), exemplified by (S. aureus \alpha-toxin)

• May be involved in resisting assault of immune system - superoxide dismutase - scavenges (H2O2 ) generated by PMNs and MF

• May be involved in evasion of immune system - IgA1 protease - degrades IgA1 at hinge region of antibody, prevents neutralization/coagulation of bacteria

Virulence Determinants - Exotoxins

• Produced by Gm (+) and Gm (-), secreted, variable size (10-900 kDa).

• Proteinaceous, temperature sensitive (inactivated at 60°C).

• Targets for vaccine development (toxoids).

• Not pyogenic.

• Affect cells by surface receptor binding.

• Highly potent.

• Encoded by extrachromosomal genetic elements (prophage, plasmids)

Examples of Exotoxins

• Diphtheria toxin: *Encoded by prophage, many cell types affected.

  • Two subunits - A (active), B (binding).

    • Catalyzes ADP ribosylation of elongation factor 2.

• Tetanus toxin:

  • Specifically affects nerve cells.

  • Inhibits muscle contraction.

  • spastic paralysis.

  • 50 ng lethal dose.

• Clostridium botulinum (botutoxin):

  • Targets motor neurons.

    • Prevents release of acetylcholine.

  • Motor neurons cannot fire, induces paralysis.

  • 10 ng lethal dose.

• Staphylococcus aureus (toxic shock syndrome toxin-1):

  • Super antigen - stimulates cytokine production (IL-1 and TNF\alpha).

  • Immune system responsible for symptoms.

• Vibrio cholera:
  *Epithelial cells of intestine
  *AB family of toxins, induces production of cAMP
  *Causes secretion of fluid and electrolytes into intestinal lumen
  *Fatal dehydration (20-30 L/day)

• (S. aureus): enterotoxin, neural receptors in gut
  *Transmits signal to brain, stimulates vomiting center
  *Typical signal - projectile vomiting
  *Rare symptom - diarrhea

Virulence Determinants - Endotoxins

• Component of Gm (-) cell wall.

• Lippopolysaccharide complex, lipid portion toxic.

• Heat stable.

• Poor immunogenicity.

• Not viable as vaccine candidates.

• Potent pyogens - induce fever (IL-1 production).

• Synthetic genes encoded chromosomally.

• Liberated following cell lysis.

• Pathophysiology:

  • Similar symptoms regardless of source.

  • LPS binds carrier molecules, binds toll-like receptors on MF, triggers IL-1 and TNF\alpha production.

  • May activate complement cascade and coagulation.

  • Peptidoglycan cell wall of Gm (+) organisms may produce similar symptoms, not a true endotoxin.

Virulence Determinants - Intracellular Pathogenicity and Antigenic Variation

• Some pathogens thrive within phagocytic cells.

• Many pathogens exhibit extreme heterogeneity in surface antigens.

• Iron:

  • Host is extremely iron limiting environment.

  • Many pathogens evolved mechanisms for iron acquisition
    *Many use siderophores - bind iron with high affinity
    *Others use iron contained in hemin (RBC)
    *Some intracellular pathogens use intracellular stores of iron

• Iron limitation usually increases expression of virulence determinants, dramatically decreases LD50/ID50