Infectious process and infectious disease - molecular mechanisms of the infectious process (entry and colonization, microbial adhesion and tissue tropism, colonization, dissemination).
- 1 Entry and Initial Colonization
The first step in infection — pathogen enters and begins to establish itself on or in host tissues.
Routes of entry (with examples)
- Respiratory tract: Influenza virus, Mycobacterium tuberculosis.
- Gastrointestinal tract: Salmonella, Vibrio cholerae.
- Urogenital tract: Neisseria gonorrhoeae, HIV.
- Skin (injuries, bites): Clostridium tetani, rabies virus.
Mechanisms
- Overcoming physical barriers (e.g., mucociliary clearance, stomach acid).
- Surviving initial innate immune responses.
- 2 Microbial Adhesion and Tissue Tropism
Microbial adhesion
Definition: Ability of pathogens to attach to host cells —essential for colonization.
Mechanisms
- Adhesins: Microbial surface structures that bind to host receptors.
- Examples:
- Pili (fimbriae) in E. coli —attach to urinary tract epithelium.
- Protein F in Streptococcus pyogenes —binds to fibronectin.
Tissue tropism
Definition: Preference of certain pathogens for specific tissues.
Determinants
- Presence of specific host receptors.
- Local environment (pH, temperature, nutrients).
Examples
- Hepatitis viruses → hepatocytes (liver).
- Rabies virus → neurons.
- H. pylori → gastric mucosa.
- 3 Colonization
Definition
Multiplication of the pathogen at the site of entry.
Mechanisms
- Evasion of local immunity:
- IgA proteases in Neisseria gonorrhoeae, H. influenzae → degrade secretory IgA.
- Biofilm formation:
- Staphylococcus epidermidis on catheters.
- Pseudomonas aeruginosa in cystic fibrosis lungs
Result
- Establishment of a stable microbial population.
- Localized or progressing infection depending on pathogen virulence and host defense.
- 4 Dissemination (Spread)
Spread of pathogens from initial site to other body sites.
Routes
- Local spread: Direct extension into adjacent tissues (e.g., cellulitis from skin infection).
- Lymphatic spread: Entry into lymphatics → regional lymph nodes.
- Hematogenous spread: Entry into bloodstream → distant organs (e.g., bacteremia → sepsis).
- Nerve spread: Along nerves (e.g., rabies, herpes simplex).
Mechanisms facilitating dissemination
- Enzymes degrading tissue barriers (e.g., hyaluronidase in Streptococcus ).
- Resistance to phagocytosis and complement-mediated killing.
- Survival inside phagocytes (e.g., Listeria monocytogenes, Mycobacterium tuberculosis ).
- tuberculosis ).