Mechanisms of Pathogenicity: Microbial Strategies and Host Interactions

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Mechanisms of Pathogenicity

Definitions: Pathogenicity and Virulence

Pathogenicity refers to the ability of a microorganism to cause disease in a host. Virulence is the degree of pathogenicity, indicating how severe the disease caused by the microorganism can be.

Pathogenicity: Qualitative ability to cause disease.
Virulence: Quantitative measure of disease severity; often assessed by parameters such as ID50 and LD50.

Parameters for Microorganisms to Cause Disease

For a microorganism to cause disease, several factors must be met:

Entry into the host via a portal of entry
Adherence to host tissues
Penetration or evasion of host defenses
Damage to host tissues

Primary Portals of Entry

Microorganisms enter the host through specific portals:

Mucous membranes: Respiratory tract (e.g., influenza virus), gastrointestinal tract (e.g., Salmonella), genitourinary tract.
Skin: Usually through breaks or cuts; e.g., Staphylococcus aureus.
Parenteral route: Direct deposition into tissues via punctures, bites, injections; e.g., Bacillus anthracis via insect bite.

ID50 and LD50

ID50 (Infectious Dose 50) is the number of microorganisms required to cause infection in 50% of a test population. LD50 (Lethal Dose 50) is the dose required to kill 50% of a test population.

ID50 Example: Bacillus anthracis has different ID50 values depending on the portal of entry.
LD50 Example: Botulinum toxin has a very low LD50, indicating high virulence.

Formula:

Microbial Adherence to Host Cells

Adherence is a critical step in pathogenesis. Microbes use surface molecules called adhesins to attach to host cells.

Adhesins: Proteins or glycoproteins found on microbial surfaces; commonly located on fimbriae, pili, or flagella.
Examples: Neisseria gonorrhoeae uses Opa protein; Escherichia coli uses fimbriae.

Host Cell Penetration and Evasion of Defenses

Microbes employ various strategies to penetrate host cells and evade immune responses.

Capsules: Polysaccharide layers that protect microbes from phagocytosis, increasing virulence. Examples: Streptococcus pneumoniae, Bacillus anthracis.
Cell Wall Components:
- M protein: Resists phagocytosis (Streptococcus pyogenes).
- Opa protein: Aids in adherence (Neisseria gonorrhoeae).
- Mycolic acid: Waxy lipid in cell wall of Mycobacterium tuberculosis, resists digestion.
Exoenzymes: Enzymes that facilitate invasion and spread.
- Coagulase: Clots fibrin in blood, protecting bacteria.
- Kinase (e.g., Streptokinase): Dissolves clots, allowing spread.
- Hyaluronidase: Breaks down connective tissue.
- Collagenase: Breaks down collagen.
- IgA protease: Destroys IgA antibodies.
Antigenic Variation: Microbes alter surface antigens to evade immune detection. Examples: Influenza virus, Trypanosoma brucei.
Invasins: Proteins that induce membrane ruffling, facilitating entry into host cells.

Mechanisms of Bacterial Damage to Host Cells

Bacteria cause damage through four main mechanisms:

Using Host Cell Nutrients: Bacteria secrete siderophores to capture iron from host proteins.
Direct Damage: Bacteria invade and lyse host cells.
Production of Toxins:
- Toxins: Poisonous substances produced by microbes.
- Toxigenicity: Ability to produce toxins.
- Toxemia: Presence of toxins in blood.
- Intoxication: Disease caused by ingestion of toxins.
- Exotoxins: Secreted proteins, often enzymes; highly specific and potent.
  - A-B toxins: Two-part toxins; A (active) and B (binding) components.
  - Membrane-disrupting toxins: Cause cell lysis; examples include leukocidins (destroy white blood cells), hemolysins (destroy red blood cells), and streptolysins (produced by Streptococcus).
  - Antitoxin: Antibody against a toxin.
  - Toxoid: Inactivated toxin used in vaccines.
  - Superantigens: Cause excessive immune response; e.g., toxic shock syndrome.
  - Genotoxins: Damage host DNA.
- Endotoxins: Lipid A component of lipopolysaccharide (LPS) in gram-negative bacteria; released upon cell death.
  - Cause fever and other symptoms.
  - Detected by limulus amebocyte lysate assay.

Role of Plasmids and Lysogeny in Pathogenicity

Plasmids are extrachromosomal DNA that can carry genes for toxins, resistance, or other virulence factors. Lysogeny refers to the integration of bacteriophage DNA into the bacterial genome, which can confer new pathogenic traits.

Pathogenic Properties of Viruses

Viruses cause disease by damaging host cells and inducing cytopathic effects.

Inclusion bodies: Aggregates of viral particles (e.g., Negri bodies in rabies).
Syncytium: Fusion of cells into multinucleated giant cells.
Antigenic changes: Can trigger autoimmunity.
Loss of contact inhibition: Leads to uncontrolled cell growth.

Pathogenic Effects of Fungi

Fungi produce toxins and enzymes that damage host tissues.

Trichothecene toxin: Inhibits protein synthesis; found on grains and wall boards.
Proteases: Break down host proteins.
Ergot toxin: Causes hallucinations; natural LSD.
Mycotoxins: Toxic compounds; e.g., Amanita phalloides produces amanitin (death cap mushroom).

Pathogenic Effects of Protozoa

Protozoa cause disease by digesting host cells, growing within phagocytes, and evading immune responses via antigenic variation.

Antigenic variation: Example: Trypanosoma brucei.

Pathogenic Effects of Helminths

Helminths use host tissues for growth, produce masses, and release toxic waste products.

Masses: Can cause conditions like elephantiasis.

Pathogenic Effects of Algae

Some algae produce toxins such as saxitoxin, which can cause paralytic shellfish poisoning and are associated with red tides.

Portals of Exit

Microorganisms exit the host via specific portals to spread to new hosts. Examples include:

Respiratory tract: Coughing, sneezing
Gastrointestinal tract: Feces, saliva
Genitourinary tract: Urine, vaginal secretions
Skin: Lesions
Blood: Arthropod bites, needles

Organisms use specific portals to maximize transmission efficiency.