Microbiology Revision Part 1: Pathogenesis and Virulence

Learning Outcomes

• Define and differentiate the terms: pathogen, pathogenicity, infection, transmission, virulence and virulence factors
• Recognise principal routes of transmission & main portals of entry
• Describe the sequential steps in pathogenesis and where virulence factors act
• Distinguish exotoxins from endotoxin – structure, mechanism & clinical effects

Key Terminology

• Pathogen – any organism (bacterial, viral, fungal, protozoan, metazoan) that produces an adverse effect (disease) in a susceptible host
• Pathogenicity – ability of an organism to cause disease
• Pathogenesis – the mechanistic process by which the organism produces disease
• Virulence – quantitative/qualitative measure of disease severity
• Virulence factor – discrete microbial component (gene product or structural element) contributing to pathogenicity
• Infection – growth and multiplication of a microorganism within a host
• Transmission – movement of a microorganism from one host to another

Categories of Pathogens

• True (primary) pathogens – can readily overcome intact host defences
– Bacteria: Clostridium botulinum, Vibrio cholerae, Yersinia pestis, Mycobacterium tuberculosis
– Viruses: HIV, Hepatitis viruses
• Opportunistic pathogens – normally commensal, cause disease in hosts with compromised defences
– Bacteria: Staphylococcus spp., Pseudomonas aeruginosa
– Yeast: Candida albicans
• Host conditions favouring opportunists: malnutrition, recurrent infections, chemotherapy/antibiotic therapy, invasive medical procedures, genetic defects, chronic illnesses (HIV, diabetes, TB), psychological or substance-related stressors (depression, alcoholism)

• Pathogens exist in every microbial domain/kingdom:
– Viruses (obligate intracellular) e.g. HSV-1
– Bacteria (surface or intracellular) e.g. E. coli
– Fungi (free-living forms and spores) e.g. Candida albicans
– Protozoa/Metazoa (often low virulence; disease linked to infecting dose; typically chronic) e.g. Trypanosoma cruzi, Plasmodium spp.

Transmission Routes

• Aerosol / airborne
• Contact
– Direct: physical contact between infected & susceptible hosts
– Indirect: via intermediary object (fomite)
• Faecal–oral
• Sexual
• Vector-borne (arthropods, etc.)
• NOTE: Many organisms exploit multiple routes (e.g. SARS-CoV-2 – droplets, fomites, perinatal, zoonotic, lab exposure, etc.)

Sites & Portals of Entry

• Mucosal: oral, nasal, respiratory, intestinal, vaginal, penile
• Conjunctiva
• Endometrium
• Tongue
• Anal canal
• Bloodstream / parenteral (needles, transfusions, bites)

Typical portal/pathogen pairings:
• Respiratory – Streptococcus spp., Mycobacterium tuberculosis, influenza virus, measles virus
• Gastrointestinal – Shigella, Vibrio cholerae, Salmonella enterica, Hepatitis A, mumps virus
• Genitourinary – Neisseria gonorrhoeae, Treponema pallidum, HIV, HSV
• Skin/parenteral – Clostridium perfringens, C. tetani, Rickettsia (R. rickettsii), rabies virus, Plasmodium spp.

Sequential Steps in Pathogenesis

1. Transmission to new host

2. Entry via specific portal

3. Adherence & invasion of host tissue

4. Colonisation & growth (nutrient acquisition, multiplication)

5. Avoidance / evasion / compromise of host defences

6. Induction of disease via tissue damage &/or toxicity

Factors Affecting Pathogenesis

• Pathogen factors: intrinsic virulence determinants, specialised adaptations
• Host factors: age, genetic background (immune genes, mutations), physiological stress
• Environmental factors: temperature, UV, nutrient availability, inorganic/organic chemicals

Virulence Factors – Overview

• Surface structures providing passive defence (capsule, cell-wall lipids)
• Secreted enzymes that facilitate spread or nutrient release
• Toxins (exotoxins, enterotoxins, endotoxin) causing direct or mediated tissue damage

Passive Defence Mechanisms

• Polysaccharide capsule
– Antiphagocytic barrier; hides surface antigens
– Example: Streptococcus pneumoniae – <10 encapsulated organisms (i.p.) lethal to mice vs 10\,000 non-capsulated (hyaluronidase-treated) required
– Capsule also confers mucoid colony morphology (Bacillus anthracis)
• Mycolic acid (waxy lipid) in cell wall of Mycobacterium spp.
– Reduces phagocytosis & antibiotic penetration

Active Enzymatic Virulence Factors (Extracellular Proteins)

• Hyaluronidase (streptococci, staphylococci, clostridia) – hydrolyses host hyaluronic acid → enhanced tissue spread
• Proteases, nucleases, lipases – degrade host macromolecules for nutrients & invasion
• Collagenase (κ-toxin) of gas-gangrene Clostridium – cleaves collagen scaffolding
• Streptokinase (S. pyogenes) – dissolves fibrin clots, counteracting host containment
• Coagulase (S. aureus) – induces fibrin clot around bacteria, shielding from phagocytes

Toxins – General Framework

• Toxin = microbial product that damages host either directly (lys- or apoptosis) or indirectly (triggering harmful host responses)

Exotoxins (secreted proteins; both Gram + and −)

1. Cytolytic (haemolysins)

• Enzymatically disrupt cell membranes
• Phospholipases / lecithinases: e.g. α-toxin of Clostridium perfringens
• Sterol-targeting: Streptolysin O (Streptococcus spp.)
• Leukocidins – selectively lyse WBCs → immune suppression
• Phenotypic detection on blood agar:
– \beta-haemolysis (complete; e.g. S. pyogenes)
– \alpha-haemolysis (partial “green”; S. pneumoniae)
– \gamma-haemolysis (none; Enterococcus faecalis)

2. A-B Toxins

• Two-subunit architecture: B (binding) + A (active)
• Mechanism: B attaches to receptor → endocytosis/transport → A translocates to cytosol → targets vital function
• Example: Diphtheria toxin (Corynebacterium diphtheriae, phage-encoded tox gene)

1. Secreted as single AB polypeptide

2. B binds host receptor

3. Proteolytic cleavage separates A & B

4. A ADP-ribosylates elongation factor-2 → halts protein synthesis

5. One toxin molecule sufficient to kill a susceptible eukaryotic cell

3. Superantigen Toxins

• Non-specific activation of T-cells (bind TCR \beta-chain & MHC-II outside antigen groove)
• Massive cytokine release → systemic inflammation, fever, shock
• Classical examples: TSST-1 of S. aureus, SpeA of S. pyogenes

Enterotoxins (subset of exotoxins)

• Act specifically on small intestinal epithelium
• Produce profuse fluid loss → vomiting, diarrhoea
• Often pore-forming; sometimes A-B (e.g. cholera toxin)
• Produced by food poisoning and enteric agents: S. aureus, C. perfringens, B. cereus, Vibrio cholerae, enterotoxigenic E. coli, Salmonella enteritidis

Endotoxin (LPS – Lipid A component of Gram − outer membrane)

• Released on cell lysis; not actively secreted
• Prototypical organisms: Escherichia, Shigella, Salmonella
• Mechanism:
– Binds CD14/TLR4 on macrophages, B-cells, etc.
– Induces acute-phase cytokines: IL-1, TNF-α, IL-6, prostaglandins
– Stimulates B-cell proliferation
• Dose-dependent outcomes:
– Low: fever, vasodilatation, local inflammation
– High (bacteremia): high fever, systemic inflammatory response syndrome (SIRS), diarrhoea, hypotension, lympho-/leukopenia, thrombocytopenia, haemorrhagic shock, necrosis → possible death

Exotoxin vs Endotoxin – Comparative Snapshot

• Chemical nature: protein vs lipid-A LPS
• Secretion: actively secreted vs released on lysis
• Heat stability: labile vs stable
• Toxicity: often highly potent (ng range) vs lower potency but massive amounts during bacteremia
• Antigenicity: strong → toxoid vaccines possible (e.g. tetanus, diphtheria) vs weak antigen
• Clinical: focal tissue damage vs systemic inflammatory effects

Clinical & Practical Implications

• Capsule & enzyme-based virulence guide vaccine design (e.g. pneumococcal polysaccharide conjugates)
• Understanding transmission routes informs infection control (PPE for aerosols, fomite decontamination, safe food practices, vector control)
• Endotoxin detection crucial in parenteral drug manufacture (Limulus amoebocyte lysate test)
• Superantigen-mediated diseases (toxic shock) require rapid supportive therapy and elimination of focus

Revision Tips

• Master terminology first – exams often begin with definitions
• Link each virulence factor to the step of pathogenesis it facilitates
• Memorise exemplar organisms; many MCQs phrase questions around them
• Use portals-of-entry table to practise matching diseases with routes
• Work through clinical scenarios: predict likely virulence factors given presentation (e.g. rapid hypotension + Gram − rods → endotoxin release)