Medical Microbiology: How Bacteria Cause Disease

Core Biological Strategy of Microbes

• Fundamental Survival Requisites: To ensure survival and propagation, microbes must perform three primary tasks:     * Identify and locate a suitable habitat.     * Successfully exploit the resources within that habitat.     * Disperse to a new, suitable habitat once the current one is depleted or to expand population range.

• Conflict of Interests: Bacterial species living in communities face constant competition for limited nutrients and space.

• Evolutionary Strategies for Survival: Different organisms have evolved distinct strategies to maximize survival chances:     * Directly damaging the host organism.     * Damaging or destroying competing bacterial species.

• Pathogen Survival Requirements: For a pathogen to persist in a population, it requires:     * A large pool of susceptible host individuals.     * A high rate of transmission between infected and uninfected hosts.

• Disease Manifestations: Many symptoms and clinical manifestations of infectious diseases are evolutionary adaptations that facilitate the spread of the infectious agents.

Koch’s Postulates and Molecular Virulence

• Robert Koch’s Classic Postulates: These four criteria are used to establish a causative relationship between a microbe and a disease:     1. The causative organism must be isolated from every individual suffering from the disease.     2. The causative organism must be cultivated artificially in a pure culture.     3. When the causative organism is inoculated from the pure culture into a healthy host, the typical symptoms of the infection must result.     4. The causative organism must be recoverable from the individuals who were infected experimentally.

• Virulence Gene ‘Postulates’ (Molecular Koch’s Postulates): These refine the original postulates to focus on specific genetic traits:     * The gene encoding the trait of interest should be present and transcribed/translated in a virulent strain.     * The gene should NOT be present or should be silent in a strain that does not cause disease.     * Disruption of the gene in a virulent strain should result in a strain incapable of causing disease.     * Introduction of the gene into a previously non-pathogenic strain should transform it into a pathogen.

• Pathogenicity Islands (PAIs): Large genomic regions containing one or more virulence genes, often acquired via horizontal gene transfer.     * Salmonella spp.: Example includes the $SPI-1$ and $SPI-2$ (Salmonella Pathogenicity Islands $1$ and $2$).     * Escherichia coli Pathotypes: Acquisition of different pathogenicity islands leads to distinct disease-causing variants:         * EHEC: Enterohaemorrhagic E. coli.         * EPEC: Enteropathogenic E. coli.         * UPEC: Uropathogenic E. coli.

The Cycle of Infection

• Conceptual Framework: The process of bacterial pathogenesis follows a cyclical progression:     * Encounter $\rightarrow$ Entry $\rightarrow$ Spread $\rightarrow$ Evade Defences $\rightarrow$ Multiply & Damage $\rightarrow$ Disperse.

Phase 1: Encounter

• Endogenous Infections: Caused by commensal microbiota (organisms already present on/in the host) relocating to an inappropriate body site.     * Oral streptococci: Can enter the bloodstream and cause Endocarditis (infection of the aortic and mitral valves).     * Honeymoon cystitis: Urinary tract infection often related to sexual activity.     * Pneumococcal pneumonia: Caused by Streptococcus pneumoniae.     * Meningococcal meningitis: Caused by Neisseria meningitidis.

• Exogenous Infections: Caused by microbes encountered from external sources.     * Common Routes and Examples:         * Respiratory: e.g., Tuberculosis ($TB$).         * Faecal-oral: e.g., Shigellosis.         * Venereal spread: e.g., Syphilis.         * Vectors: Via inanimate objects (fomites) or animate vectors.

• Transmission Patterns:     * Horizontal spread: Transmission between members of the same generation (e.g., person-to-person).     * Vertical spread: Transmission from parent to offspring (e.g., in utero, during birth).

Phase 2: Entry

• Mechanisms of Entry:     * Ingress: Entry into body cavities (e.g., inhalation, ingestion) without crossing epithelial barriers initially.     * Penetration: Direct entry into tissues crossing the epithelia via:         * Scratches/Injuries.         * Arthropod vectors: e.g., Tsetse fly ($ext{African Sleeping Sickness/Trypanosoma bruceii}$), Fleas ($ext{Plague/Yersinia pestis}$).     * Zoonoses: Diseases transmitted from animals to humans (e.g., Cattle and Anthrax caused by Bacillus anthracis).     * Iatrogenic Entry: Resulting from medical procedures.         * Blood transfusions: Can transmit Syphilis or Brucellosis.         * Contaminated medical devices: e.g., Artificial heart valves and coagulase-negative staphylococci.

• Bacterial Adhesion and Tissue Tropism: Adhesins on the microbe bind to specific receptors on the host cell, determining which tissues are infected.

  Microbe	Adhesin	Receptor
  Staphylococcus aureus	Clumping factor A	Fibrinogen
  Staphylococcus spp.	MSCRAMM	Extracellular matrix components
  Streptococcus, group A	LTA-M protein complex / F protein / MSCRAMM	Extracellular matrix (fibronectin, laminin, collagen)
  Streptococcus pneumoniae	Adhesins and other proteins	N-Acetylhexosamine-galactose
  Escherichia coli	Type $1$ fimbriae	D-Mannose
  E. coli	Colonization factor antigen fimbriae	GM ganglioside $1$
  E. coli	P fimbriae	P blood group glycolipid
  Neisseria gonorrhoeae	Fimbriae	GD, ganglioside
  Treponema pallidum	P$1$, P$2$, P$3$	Fibronectin
  Chlamydia trachomatis	Cell surface lectin	N-Acetylglucosamine
  Mycoplasma pneumoniae	Protein P$1$	Sialic acid
  Vibrio cholerae	Type $4$ pili	Fucose and mannose
  Type 3 Secretion Systems (T3SS): "Molecular syringes" used by bacteria (like Salmonella) to inject effector proteins into host cells.
  * SPI-1: Involved in invasion and enteropathogenesis; targets M-cell adhesion and translocation/direct invasion of epithelial cells.
  * SPI-2: Critical for intracellular pathogenesis; allows survival and growth within macrophages by manipulating vesicles through effectors like $sifA$, $sseJ$, and $sspH2$.
  * Salmonella Entry Process:
  1. M-cell adhesion and translocation.
  2. Luminal capture by dendritic cells.
  3. Direct invasion of epithelial cells.
  4. Survival in macrophages, causing apoptosis and inflammation via $IL-1\beta$.
  5. Potential systemic dissemination through the basolateral side of the epithelial cell.
  Phase 3: Spread and Evasion of Defences

  • Strategies for Evasion:     * Inhibit Opsonization: e.g., Staphylococcus aureus Protein A binds the Fc region of antibodies.     * Inhibit Chemotaxis: Prevent recruitment of immune cells.     * Kill Phagocytes: Secretion of toxins like leukocidins.     * Inhibit Phagocytosis: e.g., Streptococcus pneumoniae polysaccharide capsule.     * Intracellular Survival:         * Inhibiting lysosomal fusion (e.g., Mycobacterium tuberculosis).         * Escaping the lysosome to grow in the cytoplasm.         * Resisting antibacterial lysosomal action (e.g., Mycobacterium leprae, Salmonella spp., S. aureus).         * Blocking activation by Interferon-$\gamma$ ($ext{IFN-}\gamma$).

  • Specific Example: Streptococcus pyogenes:     * Capsule: Composed of Hyaluronic acid, mimicking host connective tissue to avoid detection.     * Leukocidins: Streptolysin O/S and NADase to destroy immune cells.     * Degradative Enzymes:         * Hyaluronidase: Breaks down connective tissue.         * Streptokinase: Dissolves blood clots.         * Streptodornase: Degrades DNA.         * Proteases: Degrades proteins.

  Phase 4: Damage

  • Intoxication Illnesses: Damage caused by pre-formed bacterial products (e.g., S. aureus, Bacillus cereus, Clostridium botulinum).

  • AB Toxins: Composed of a Binding (B) component and an Active (A) component.     * Protein Synthesis Inhibition: Corynebacterium diphtheriae (Diphtheria toxin) inactivates elongation factor-$2$ ($EF-2$), leading to cell death.     * Hyperactivation: Vibrio cholerae (Cholera toxin) binds to ganglioside receptors, increasing adenylate cyclase activity and $cAMP$ levels. This causes a loss of cell nutrients ($Na^+$, $H_2O$, $Cl^-$, $K^+$, $HCO_3^-$) resulting in severe diarrhea.     * Nerve-Muscle Transmission: Clostridium tetani (Tetanospasmin) blocks the release of inhibitory transmitters, leading to continuous stimulation by excitatory transmitters.

  • Immunopathogenesis: Damage caused by the host's own immune response.     * Superantigens: Hyper-stimulate $CD4^+$ T-cells.     * Endotoxins (LPS): Trigger macrophages to release $TNF$ and $IL-1$, causing fever and potentially leading to hypotension, shock, and Disseminated Intravascular Coagulation ($DIC$).

  Host Factors Affecting Susceptibility

  • Sex: Anatomical/hormonal differences (e.g., increased UTI risk in females).

  • Stress: Glucocorticoids can reactivate latent infections like tuberculosis.

  • Age:     * Neonates: Immature immunity (high susceptibility to Bordetella pertussis).     * Elderly: Declining defences (risk of TB reactivation).

  • Nutrition: Protein deficiencies impair cell-mediated immunity.

  • Overcrowding: Increases carriage and transmission rates of pathogens like Neisseria meningitidis.

  • Tissue Damage: Violent exercise or physical trauma can predispose an individual to conditions like gas gangrene.