Microbial Mechanisms of Pathogenicity: Understanding Bacterial Virulence

Posted on Aug 22, 2024 in Biology

Microbial Mechanisms of Pathogenicity

Introduction

Pathogenicity: The ability to cause disease.

Virulence: The extent of pathogenicity.

Four Virulence Factors

• Adhesion factors
• Extracellular enzymes
• Toxins
• Antiphagocytic factors

Number of Invading Microbes

ID₅₀: Infectious Dose for 50% of the test population.

LD₅₀: Lethal Dose (of a toxin) for 50% of the test population.

Example: Bacillus anthracis (Creates Endospores)

Adherence

Adhesions/Ligands bind to receptors on host cells.

• Streptococcus mutans – Glycocalyx
• Escherichia coli – Fimbriae
• Streptococcus pyogenes – M protein
• Neisseria gonorrhoeae – Opa protein
• Treponema pallidum – Tapered end

Adhesin Proteins

Ligands (surface lipoproteins or glycoproteins) bind host cell receptors.

The ability to change/block the ligand or its receptor prevents infection.

The inability to make attachment proteins or adhesins renders microorganisms avirulent.

Exoenzymes

• Coagulase: Coagulates blood.
• Kinases: Digest fibrin clots.
• Hyaluronidase: Hydrolyzes hyaluronic acid.
• Collagenase: Hydrolyzes collagen.
• IgA Proteases: Destroy IgA antibodies.
• Siderophores: Take iron from host iron-binding proteins.
• Antigenic Variation: Alter surface proteins.

Extracellular Enzymes: Hyaluronidase & Collagenase

1. Invasive bacteria reach the epithelial surface.
2. Bacteria produce hyaluronidase and collagenase.
3. Bacteria invade deeper tissues.

Extracellular Enzymes: Coagulase & Streptokinase

1. Bacteria produce coagulase.
2. A blood clot forms.
3. Bacteria later produce kinase, dissolving the clot and releasing bacteria.

Toxins

Toxin: Substances that contribute to pathogenicity.

Toxigenicity: Ability to produce a toxin.

Toxemia: Presence of toxin in the host’s blood.

Toxoid: Inactivated toxin used in a vaccine.

Antitoxin: Antibodies against a specific toxin.

Bacteria secrete exotoxins (e.g., a cytotoxin that kills host cells).

Dead bacteria release endotoxin (Lipid A), which induces fever, inflammation, diarrhea, shock, and blood coagulation.

Exotoxins (Produced Inside During Growth & Metabolism)

Toxins secreted externally after lysis.

Type III Toxins [A-B Toxins]

1. Bacterium produces and releases exotoxin.
2. B-Component (binding) of the exotoxin binds to the host cell receptor, and the exotoxin enters the cell.
3. A-Component (active) of the exotoxin alters cell function by inhibiting protein synthesis.

Type II Toxins [Membrane-Disrupting Toxins]

Lyse host’s cells by forming protein channels in the plasma membrane and disrupting the phospholipid bilayer.

Type I Toxins [Superantigens]

Cause an intense immune response due to cytokine release from host cells.

Symptoms include fever, nausea, vomiting, diarrhea, shock, and death.

Examples of Exotoxins

• Corynebacterium diphtheria: Type III – Inhibits protein synthesis (Lysogenic Conversion)
• Streptococcus pyogenes: Type II – Erythrogenic (Lysogenic Conversion)
• Clostridium botulinum: Type III – Neurotoxin (Lysogenic Conversion)
• C. tetani: Type III – Neurotoxin
• Vibrio cholera: Type III – Enterotoxin (Lysogenic Conversion)
• Staphylococcus aureus: Type I – Enterotoxin

Endotoxins (Secreted After Bacteria Dies & Cell Wall Breaks)

Endotoxin Mechanism

1. Macrophage ingests a Gram (-) bacterium.
2. Bacterium degrades in the vacuole, releasing endotoxins that induce the macrophage to produce Interleukin-1 (IL-1).
3. IL-1 is released by the macrophage into the bloodstream and travels to the hypothalamus.
4. IL-1 induces the hypothalamus to produce prostaglandins, which reset the body’s thermostat to a higher temperature, causing fever.

Antiphagocytic Factors

Certain factors prevent phagocytosis by the host’s phagocytic cells.

Bacterial Capsule (Phagocytosis Blocked by Capsule Around Bacterium)

• Slippery – Difficult for phagocytes to engulf bacteria.
• Composed of chemicals found in the body and not recognized as foreign.

Antiphagocytic Chemicals (Bacteria Reproduced)

• Prevent fusion of lysosomes and phagocytic vesicles.
• Leukocidins directly destroy phagocytic white blood cells.

Cytopathic Effects of Viruses

Pathogenic Properties of Fungi

Chronic infections provoke an allergic response.

Mycotoxins: Neurotoxins, Aflatoxin, Ergot Toxin, Tichothecene Toxins, Proteases.

Capsule: Prevents phagocytosis.

Pathogenic Properties of Protozoa

Avoid host defenses by growing in phagocytes and antigenic variation.

Pathogenic Properties of Helminths

Use host tissue and interfere with host function (except in the gastrointestinal tract).

Pathogenic Properties of Algae

Neurotoxins produced by dinoflagellates.

Mechanisms of Pathogenicity

1. Portals of Entry: Mucous membranes, respiratory tract, gastrointestinal tract, genitourinary tract, conjunctiva, skin, parenteral route.
2. Number of Invading Microbes + Adherence
3. Penetration/Evasion of Host Defenses: Capsules, cell wall, enzymes, siderophores, antigenic variation, cytoskeleton.
4. Damage to Host Cells/Cytopathic Effects: Direct damage, toxins (exo/endo), lysogenic conversion.
5. Portals of Exit: Same as entry.

Chapter 19 – Cocci – Gram (+)

Genera of Cocci-Shaped Organisms

Staphylococcus, Streptococcus, Enterococcus

Staphylococcus (Clusters) (2 Species Related to Human Disease)

Opportunistic, facultative anaerobes.

No spores, no flagella (non-motile), rarely encapsulated.

Tolerant of salt, sebum on human skin, and desiccation (allows survival on environmental surfaces: fomites) (60°C for 60 minutes).

Live microbes, resistant to many disinfectants and antibodies, nosocomial infections.

Pyogenic Cocci: Pus-forming [1. Vesicle 2. Bulla 3. Macule 4. Pustule (Deep)]

1. Staph. aureus: Virulent strain; variety of conditions depending on the site of infection.
2. Staph. epidermidis: Normal microbiota of human skin; opportunistic infections in immunocompromised individuals.

Virulence Factors

Staph infections breach the body’s physical barriers.

ID₅₀: A few hundred bacteria can result in disease.

Pathogenicity results from three features:

1. Structures that enable it to evade phagocytosis.
2. Production of enzymes.
3. Production of toxins.

Structural Defenses Against Phagocytosis

Protein A coats the cell surface.

• Interferes with the humoral immune response by binding IgG antibodies.
• Inhibits the complement cascade.

Bound Coagulase

• Converts soluble blood protein fibrinogen into insoluble fibrin, forming blood clots.
• Fibrin clots hide bacteria from phagocytic cells.

Synthesize loosely organized polysaccharide slime layers.

• Inhibit chemotaxis of and phagocytosis by leukocytes.
• Facilitates attachment of Staph. to artificial surfaces.

Enzymes

Cell-free coagulase

• Triggers blood clotting.
• Virulence test for Staph. aureus.

Hyaluronidase

• Breaks down hyaluronic acid, leading to tissue destruction.
• Facilitates the spread of bacteria.

Staphylokinase

• Dissolves fibrin threads in blood clots, allowing S. aureus to escape from clots.

Lipases (Digest lipids)

• Allows Staph. to grow on the skin’s surface and in cutaneous oil glands.

β-lactamase (penicillinase)

• Breaks down and inactivates penicillin.
• Allows bacteria to survive treatment with β-lactam antimicrobial drugs.

Toxins

S. aureus produces toxins more frequently than S. epidermidis.

Hemolysins: Rupture red blood cells.

Leukocidins: Lyse white blood cells.

Enterotoxins: Stimulate intestinal muscle contractions, nausea, and intense vomiting (heat-stable at 100°C for 30 minutes) (Staph. food poisoning).

Exfoliative Toxins (epithelial toxin): Cause skin cells to separate from each other and slough off, leading to skin shedding.

Toxic-shock-syndrome Toxin: Causes toxic shock syndrome.

Staphylococcal Diseases

Noninvasive Disease: Food poisoning – Ingestion of enterotoxin.

• Deli Foods: Not cooked or refrigerated properly; high tolerance to salt preservatives.
• Enterotoxin: Onset 2-6 hours later, resolved within 24 hours, no intervention required.

Localized Diseases – Cutaneous Disease (Localized Skin Infections)

• Impetigo or nasal infections.
• Scalded skin syndrome: Bright red flush and blistering (umbilical cord infections) (Exfoliating toxin).
• Folliculitis: Infection of the hair follicle.
• Furuncles: Small skin infections.
• Carbuncles: Large skin infections.

Systemic Diseases

• Toxic Shock Syndrome: TSS toxin absorbed into the blood, causing shock, fever, vomiting, and rash – Renal and liver involvement (can be fatal).
• Bacteremia: Presence of bacteria in the blood.
• Endocarditis: Occurs when bacteria attack the lining of the heart.
• Pneumonia: Inflammation of the lungs in which the alveoli and bronchioles become filled with fluid.
• Osteomyelitis: Inflammation of the bone marrow and the surrounding bone.

Systemic Disease Spread From

Cutaneous Infections

• Nose: Pneumonia
• Carbuncles: Bacteremia or osteomyelitis

Nosocomial Secondary Infections

• Kidney, liver, spleen: Abscesses
• Endocarditis, meningitis

Diagnosis

• Detect Gram-positive bacteria in grape-like arrangements isolated from pus, blood, or other fluids.
• Grown on blood agar plates or mannitol salt agar (MSA) plates.
• Catalase test (Staph. positive & Strep. negative) and coagulase test.

Treatment

• Methicillin: Treats Staph. infections (semisynthetic form of penicillin not activated by β-lactamase).
• Abscesses: Must be perforated or antibiotics cannot penetrate.
• Clean the wound and then use topical antiseptics.
• S. aureus is resistant to penicillin, erythromycin, ampicillin, tetracycline, and methicillin (MRSA).
• Vancomycin: Last line of defense.

Prevention

• Human reservoir – Cannot be eliminated.
• Hygiene, proper cleansing, aseptic techniques, proper disposal.

Streptococcus (Arranged in Pairs or Chains)

Facultative anaerobes.

More fragile than Staph. and cannot survive on skin.

Normal flora of the mouth and pharynx.

No spores, no flagella (non-motile), encapsulated (virulent!).

Catalase (-) and prefer enriched media.

Often categorized based on the Lancefield classification (Rebecca Lancefield).

• Divides the streptococci into serotype groups based on the bacteria’s surface antigens.
• Lancefield groups A and B include the significant streptococcal pathogens of humans (Group A and B are β-hemolytic).

Humans are the only reservoir for Streptococci (~5% of the population carries β-hemolytic Strep.).

Group A Streptococcus: Streptococcus Pyogenes

Group A – 80% of strep throat (Streptococcus pyogenes).

β-hemolytic.

Pathogenic strains often form a capsule.

Group A Streptococci Causes Disease When…

1. Normal microbiota are depleted.
2. A large inoculum enables the streptococci to establish themselves before antibodies are formed against them.
3. Specific immunity is impaired.

Virulence Factors

Structural Components

• Lipoteichoic acid: In the cell wall; increases adherence to epithelial cells.
• Protein M and fimbriae: Interfere with opsonization and complement binding.
• Hyaluronic Acid Capsule: Camouflages bacteria from white blood cells.

Enzymes

• Streptokinases: Digest blood clots.
• Hyaluronidases: Break down connective tissue.
• Deoxynucleases: Depolymerizes DNA.
• C5a peptidase: Cleaves and inactivates C5a (chemotaxin for white blood cells).

Toxins

• Streptolysins (Hemolysins): Rupture red blood cells, white blood cells, platelets, liver cells, and heart muscle cells.
• Pyrogenic Toxins (Erythrogenic toxin): Stimulate macrophages and T helper cells to release cytokines.
  • Fever, rash, shock, strep throat, scarlet fever, puerperal fever (childbirth fever).
  • Toxin genes carried on lysogenic bacteriophage.

Group A Streptococcal Diseases

Localized Infections

• Pyoderma (erysipelas): Confined, pus-producing lesion that occurs on the face, arms, or legs.
• Pharyngitis (strep throat): Inflammation of the pharynx.
• Tonsillitis: Inflammation of the tonsils or pus-forming lesions.
• Scarlet fever: Starts as pharyngitis; lysogenic bacteriophage (rash begins on the chest and spreads across the body).

Systemic Infections – Results of Toxin Production

• Streptococcal Toxic Shock Syndrome: Bacteremia and severe multisystem infections (40% fatal).
• Rheumatic Fever: Inflammation damages heart valves.
• Glomerulonephritis: Inflammation of the glomeruli and nephrons of the kidneys, which obstructs blood flow through the kidneys.
• Necrotizing fasciitis: Destroys muscle and fat tissue (flesh-eating bacteria, 6 hours, fascia – soft tissue between muscle and bone – dissolves) (Treatment: Debridement or amputation).

Diagnosis

• Observe Gram-positive bacteria in short chains or pairs.
• Immunological tests identify the presence of group A streptococcal antigens (antibodies specific to Group A antigens).

Treatment

• Penicillin is effective, or derivatives.
• Penicillin allergies: Cephalosporin.

Transmission

• Direct contact, droplets, fomites (food).
• 5-15 year olds most affected.
• Invasion changes with season: Summer – Skin & Winter – Pharynx.

Prevention

Long-term protection – Antibodies against M protein, but only if the same strain.

Group B Streptococcus: Streptococcus Agalactiae

Divide to form chains.

Smaller zone of beta-hemolysis on blood agar plates.

Resistant to bacitracin.

Found in cow udders and passed to neonates in cow milk.

Normally colonizes the lower gastrointestinal, genital, and urinary tracts.

Diseases

• Neonatal bacteremia, meningitis, and pneumonia.
• Immunocompromised older patients – Group B infections.

Enterococcus

Group D Streptococci & form short chains and pairs & NO CAPSULE.

Found in the human colon (normal flora) (rarely pathogenic at this site).

Urinary tract infections or bloodstream infections.

Nosocomial infections.

Difficult treatment: Enterococci are resistant to antimicrobials (VRE – Vancomycin-resistant enterococci – Bacteriophage with resistance gene).

Difficult prevention: Weakened immune systems.

Streptococcus Pneumoniae

Not in Lancefield classifications.

Commonly forms pairs but also forms chains.

Blood agar: Forms unpigmented, alpha-hemolytic colonies (anaerobic incubation produces beta-hemolytic colonies).

Normally colonizes the mouth and pharynx but can cause disease if it travels to the lungs.

Disease is highest in children and the elderly.

Pathogenicity

• Phosphorylcholine: Chemical in the cell wall (stimulates cells to phagocytize the bacteria).
• Polysaccharide Capsule: Protects bacteria from digestion after phagocytosis.
• Protein Adhesin: Mediates binding of the cells to epithelial cells of the pharynx.
• Secretory IgA protease: Destroys IgA.