Microbial Virulence & Pathogenesis Quiz

Microbial Virulence & Pathogenesis

• Microbial virulence and pathogenesis refer to the ability of microorganisms to cause disease. This involves the mechanisms by which pathogens initiate an infectious process and cause symptoms.
• Definitions are crucial to understanding pathogenesis.
  • Pathogen: An organism capable of causing disease.
  • Pathogenesis: The process of disease development, including the initiation of the infectious process and the mechanisms leading to symptoms.
  • Pathogenicity: The ability of a microorganism to cause a disease, dependent on virulence factors.
  • Virulence: The quantitative ability of a microorganism to cause disease.
  • Toxigenicity: The ability of a microorganism to produce toxins.
  • Incubation period: The time period between infection and the appearance of symptoms.
  • Carrier: A person who harbors a pathogen without showing symptoms of the disease.

Morphology of Bacteria

• Coccus: Spherical bacteria
• Diplococcus: Pairs of cocci
• Tetracoccus: Groups of four cocci
• Streptococcus: Chains of cocci
• Staphylococcus: Clusters of cocci
• Bacillus: Rod-shaped bacteria
• Diplobacillus: Pairs of bacilli
• Spirillum: Spiral-shaped bacteria

Study of Bacteria (Microscopy)

• Wet Preparations (Wet Mount): Used to examine motility, molds, ova, and cysts.
• Stained Preparations:
  • Simple Stains: Use a single dye to visualize bacteria, showing size, shape, and arrangement.
  • Negative Stains: Stain the background, leaving the bacteria uncolored, useful for observing bacterial capsules. Examples: India ink stain.
  • Differential Stains: Use multiple dyes to distinguish between different types of bacteria. Examples: Gram stain, Ziehl-Neelsen stain, Albert stain (various applications).

Gram Stain

• Principle: Differentiates bacteria based on cell wall structure (peptidoglycan).
• Gram-positive bacteria have thick peptidoglycan layers; they retain the primary stain (crystal violet-iodine complex), appearing purple.
• Gram-negative bacteria have thin peptidoglycan layers; they do not retain the primary stain and are counterstained (safranin), appearing pink or red.

Importance of Gram Stain

• Guiding clinicians in selecting the appropriate antibiotic when culture and sensitivity results are pending.
• Aiding in the selection of culture media, particularly when a mixed flora is present.
• Evaluating the quality of specimens (e.g., sputum).
• Different bacterial species have different cell wall compositions which affect their stain uptake

Special Stains

• Techniques used to stain specific bacterial structures (endospores, capsules).

Structure of Bacteria

• The structure of bacteria includes cell membrane, cell wall, capsule, pili (attachment, sex pili).
• These structural differences contribute to their virulence.

Cell Wall

• A complex, semi-rigid structure.
• Thickness varies among bacterial species.
• Gram stain differentiates bacteria based on the cell wall structure.

Gram-positive Cell Wall

• Components: peptidoglycan, teichoic acid, lipoteichoic acid.
  • Peptidoglycan provides structural support.
  • Teichoic acids are associated with the cell wall structure
  • Lipoteichoic acids are components of the cell wall.

Gram-Negative Cell Wall

• Components: peptidoglycan layer, lipoprotein layer, outer membrane.
• Outer membrane contains lipopolysaccharide (LPS/endotoxin).
• Peptidoglycan layer exists in a much smaller amount in Gram-negative.

Lipopolysaccharide (LPS)/Endotoxin

• Component of the outer membrane of Gram-negative bacteria.
• Releases during bacterial lysis.
• Responsible for toxic properties of Gram-negative bacteria, primarily through the lipid A region of the LPS.

Exotoxin

• Proteins produced and secreted by live bacteria.
• Heat-labile
• Can be inactivated, forming toxoids (used as vaccines).
• Different types of exotoxins include cytotoxins, neurotoxins, and enterotoxins.

Adhesion Factors/Adhesins

• Adhere bacteria to host cells.
• Types:
  • Fimbrial adhesins (e.g., pili).
  • Non-fimbrial adhesins (e.g., protein A).

Capsule

• Covering external to the cell wall.
• Prevents phagocytosis by innate immune system cells, increasing the bacteria's survival rate
• Mediates attachment to host tissues.

Flagella

• Whip-like appendages
• Enable bacterial motility (movement).
• Exhibit chemotaxis

Enzymes

• Promote the spread of the pathogen.
• Collagenase degrades collagen, allowing spread through tissues.

Horizontal Gene Transfer

• Transduction, conjugation, transformation are primary mechanisms for the interchange of genetic material between bacteria.

Determinants of Bacterial Pathogenesis

• Exposure or transmission.
• Adherence to host cells
• Invasion, Inflammation and Intracellular Survival
• Toxin Production
• Immunopathogenesis

Mode of Transmission

• Human-to-human (direct contact, blood, transplacental).
• Non-human-to-human (soil, water, animals, fomites).
• Respiratory tract, gastrointestinal tract, genital tract, breached skin are primary portals for entry.

Invasion

• Bacterial invasion of host cells depends on interactions of bacterial proteins with specific receptors on host cells.

Intracellular Survival

• Enhances ability to cause disease.
• Includes species like Mycobacterium, Legionella, and Listeria.

Toxin Production

• Endotoxins (Gram-negative bacteria).
• Exotoxins (various types produced by many bacterial species).

Immunopathogenesis

• Symptoms of diseases are not directly caused by the bacteria, but rather by the immune response to the organism.

Biofilm

• An aggregate of interactive bacteria.
• Protects bacteria from antibiotics and host immune defenses.
• Found on many surfaces.

Shigella species

• Causes bacillary dysentery (Shigellosis).
• Low Infectious dose (10-100 organisms).
• Fecal-oral route of transmission.
• Factors in transmission: fingers, flies, food, feces.
• Virulence factors include endotoxin, shiga toxin, and intestinal adherence factor.
• Key aspect of pathogenesis relates to invasion of cells and causing bloody diarrhea.

Laboratory Diagnosis

• Stool routine/microscopy: observes WBCs and RBCs.
• Stool culture on MacConkey agar: identifies non-lactose fermenting colonies (Gram-negative bacilli).
• Biochemical tests: determine specific characteristics of the organism.
• Agglutination tests: confirms the species.

Virus Virulence

• Adhesion to host cells (surface proteins).
• Cell entry mechanisms (endocytosis/fusion).
• Immune evasion (inhibiting antigen presentation).
• Modulation of host cellular machinery (favouring replication).
• Immunomodulatory proteins (inhibiting interferon, disrupting antiviral pathways).

Fungi Virulence

• Factors:
  • Cell wall glycoproteins (adhesion to host cells).
  • Capsule (antiphagocytic, preventing phagocytosis).
  • Exoenzymes (tissue invasion).
  • Toxins (tissue necrosis).