Microbiology Revision Part 1: Pathogenesis and Virulence

Vocabulary flashcards summarising essential terms related to microbial transmission, portals of entry, pathogenesis steps, and major virulence factors discussed in the lecture.

Pathogen

Any organism (bacterial, viral, fungal, protozoan, metazoan) capable of causing disease in a susceptible host.

Pathogenicity

The ability of an organism to cause disease.

Pathogenesis

The step-by-step process by which a microbe produces disease in a host.

Virulence

The degree of severity or harmfulness of a disease caused by a pathogen.

Virulence Factors

Microbial components (e.g., toxins, enzymes, capsules) that enable a pathogen to cause disease.

Infection

Growth and multiplication of a microorganism within a host.

Transmission

Spread of a microorganism from one host to another.

True Pathogen

Microbe able to overcome normal host defenses and cause disease even in healthy hosts (e.g., Vibrio cholerae, Yersinia pestis).

Opportunistic Pathogen

Normally commensal organism that causes disease only when host defenses are compromised (e.g., Staphylococcus spp., Candida albicans).

Aerosol Transmission

Spread via tiny respiratory droplets or particles suspended in air.

Contact Transmission

Spread through direct physical contact or indirect contact via contaminated surfaces.

Fomite

Inanimate object that passively carries pathogens between hosts.

Faecal-Oral Transmission

Ingestion of pathogens shed in feces, often via contaminated food or water.

Vector-Borne Transmission

Spread of pathogens through living carriers such as mosquitoes or ticks.

Zoonotic Transmission

Passage of a pathogen from animals to humans.

Mucosal Site

Moist epithelial surface such as oral, nasal, respiratory, intestinal, vaginal, or penile mucosa that serves as a portal of entry.

Polysaccharide Capsule

Sticky outer layer that shields bacteria from phagocytosis and enhances virulence.

Mycolic Acid

Waxy cell-wall lipid in Mycobacterium spp. that resists phagocytosis and drug penetration.

Hyaluronidase

Enzyme that degrades hyaluronic acid in connective tissue, promoting microbial spread.

Collagenase

Clostridial enzyme that breaks down collagen, facilitating tissue invasion (e.g., gas gangrene).

Streptokinase

Streptococcal enzyme that dissolves fibrin clots, aiding bacterial dissemination.

Coagulase

Staphylococcus aureus enzyme that induces clotting to wall off and protect bacteria.

Proteases, Nucleases, Lipases

Extracellular enzymes that depolymerize host proteins, nucleic acids, and lipids to aid colonization.

Cytolytic Toxin (Haemolysin)

Exotoxin that lyses host cells by attacking membranes; produces clear zones on blood agar.

A-B Toxin

Two-subunit exotoxin where the B subunit binds the cell and the A subunit exerts toxic activity (e.g., diphtheria toxin).

Superantigen Toxin

Exotoxin that non-specifically activates many T-cells, triggering massive inflammatory responses.

Enterotoxin

Exotoxin that targets the small intestine, causing fluid secretion, vomiting, and diarrhoea (e.g., cholera toxin).

Endotoxin

Lipopolysaccharide (LPS) of Gram-negative outer membrane released on cell lysis, inducing fever and systemic inflammation.

Lipopolysaccharide (LPS)

Gram-negative outer membrane molecule whose lipid A portion is responsible for endotoxin activity.

Leukocidin

Cytolytic toxin that specifically lyses white blood cells, weakening host immunity.

α-Toxin of Clostridium perfringens

Phospholipase (lecithinase) exotoxin that destroys membranes and causes gas gangrene.

Streptolysin O

Oxygen-labile haemolysin of streptococci that targets membrane sterols.

Diphtheria Toxin

Potent A-B exotoxin that inactivates elongation factor-2, halting host protein synthesis.

α-Haemolysis

Partial (green) haemolysis produced by organisms like Streptococcus pneumoniae.

β-Haemolysis

Complete haemolysis producing clear zones, characteristic of Streptococcus pyogenes.

γ-Haemolysis

No haemolysis on blood agar, seen with Enterococcus faecalis.

Phagocytosis

Host cell process of engulfing and destroying microbes; inhibited by capsules or mycolic acid.

Steps of Pathogenesis

Transmission → entry → adherence/invasion → colonisation → immune evasion → tissue damage/disease.

Factors Affecting Pathogenesis

Combination of pathogen traits (virulence factors), host status (age, immunity, stress), and environment (temperature, UV, nutrients).

Acute-Phase Cytokines

Host mediators (e.g., IL-1, TNF-α) released in response to endotoxin, causing fever and inflammation.

Bacteremia

Presence of bacteria in the bloodstream; high endotoxin levels can lead to septic shock.

Infecting Dose

Quantity of organisms required to establish infection; often higher for low-virulence parasites.

Intracellular Parasite

Pathogen, typically a virus, that replicates only inside host cells.

Chronic Infection

Long-lasting infection, often associated with parasites and some viruses, persisting for months or years.

Host Factors

Patient characteristics (e.g., malnutrition, chemotherapy, genetic defects, stress) that influence susceptibility.

Environmental Factors

External conditions (e.g., UV exposure, temperature, chemical presence) that impact pathogen survival and host defense.

Passive Defense Mechanism

Non-enzymatic structural feature, such as capsule or cell-wall lipid, that protects bacteria from host attack.