Microbial Mechanisms of Pathogenicity: Study Notes

Pathogenicity and Virulence

Definitions and Concepts

Pathogenicity and virulence are central concepts in understanding how microorganisms cause disease. These terms describe both the ability of a microbe to cause disease and the severity of the disease produced.

• Pathogenicity: The ability of a microorganism to cause disease in a host organism.

• Virulence: The degree or intensity of pathogenicity; a measure of the severity of the disease caused.

• Most pathogens have a preferred portal of entry that determines their ability to initiate infection.

Portals of Entry

Major Entry Routes for Pathogens

Microorganisms must enter the host through specific portals to establish infection. The effectiveness of these routes influences the likelihood of disease.

• Mucous membranes: Include the respiratory, digestive, urinary, and reproductive tracts. The respiratory tract is the most common portal of entry.

• Skin: Unbroken skin is generally impenetrable to most microbes.

• Parenteral route: Involves direct deposition of microbes into tissues, often through cuts, bites, or injections.

Infectious Dose and Lethal Dose

Quantifying Infectivity and Lethality

These measures help determine the risk and severity of infection by a particular pathogen.

• ID50 (Infectious Dose 50): The number of microbes required to cause infection in 50% of a test population. Indicates infectivity.

• LD50 (Lethal Dose 50): The number of microbes (or amount of toxin) required to kill 50% of a test population. Indicates lethality.

Adherence

Attachment to Host Cells

Adherence is a critical step in the establishment of infection, allowing pathogens to attach to host tissues.

• Adhesins: Surface molecules on pathogens that bind to specific receptors on host cells.

• Fimbriae: Hair-like appendages that help bacteria attach to host cells.

• Viral spikes: Glycoprotein projections on viruses that facilitate attachment to host cells.

Capsules and Cell Wall Components

Structures that Enhance Virulence

Certain bacterial structures help evade host defenses and increase pathogenicity.

• Capsules: Polysaccharide layers that impair phagocytosis by host immune cells.

• M protein: Found in Streptococcus pyogenes, helps resist phagocytosis.

• Mycolic acid: Waxy lipid in Mycobacterium cell walls, resists digestion by lysosomal enzymes.

• Example: Streptococcus pneumoniae uses its capsule to cause pneumonia.

Enzymes and Virulence Factors

Enzymatic Mechanisms of Pathogenicity

Pathogens produce enzymes that facilitate invasion and damage to host tissues.

• Coagulases: Promote clot formation, protecting bacteria from immune cells.

• Kinases: Break down clots, allowing spread of infection.

• Hyaluronidase: Digests hyaluronic acid, aiding tissue penetration.

• Collagenase: Breaks down collagen, facilitating spread through connective tissue.

• IgA proteases: Destroy IgA antibodies, weakening mucosal immunity.

Antigenic Variation

Evading the Immune System

Some pathogens alter their surface proteins to avoid detection by the host immune system.

• Antigenic variation: Changes in surface antigens to evade immune response.

• Examples: Influenza viruses and Neisseria gonorrhoeae use antigenic variation.

Penetration into the Host

Mechanisms of Invasion

Pathogens use various strategies to penetrate and survive within host cells.

• Invasins: Bacterial proteins that rearrange actin filaments, facilitating entry.

• Membrane ruffling: Host cell membrane changes that engulf bacteria.

• Some pathogens can survive and multiply within phagocytes.

Biofilms

Community-Based Resistance

Biofilms are structured communities of microbes that provide protection from host defenses and antimicrobial agents.

• Biofilms: Aggregates of microorganisms encased in an extracellular polymeric substance (EPS).

• Resist antibiotics and disinfectants; involved in about 65% of infections.

• EPS protects bacteria from immune responses and chemical agents.

Direct Damage to the Host

Cellular and Tissue Injury

Pathogens can directly damage host cells through various mechanisms.

• Disruption of host cell membranes.

• Utilization of host cell nutrients.

• Multiplication within cells may cause lysis and cell death.

Toxins

Microbial Poisons

Toxins are potent substances produced by microbes that can cause significant damage to the host.

• Toxins: Poisonous substances produced by microorganisms.

• Toxemia: Presence of toxins in the blood.

• Intoxication: Disease caused by toxins without microbial infection.

Exotoxins

Protein Toxins with Specific Targets

Exotoxins are secreted proteins with high specificity and potency.

• Produced mainly by gram-positive bacteria.

• Highly specific for their targets.

• Antitoxins: Antibodies that neutralize specific toxins.

• Toxoids: Inactivated exotoxins used in vaccines.

Types of Exotoxins

• A-B toxins: Consist of an active (A) component and a binding (B) component. Example: Diphtheria toxin.

• Genotoxins: Damage host DNA.

• Membrane-disrupting toxins: Cause lysis of host cells.

• Leukocidins: Destroy phagocytic white blood cells.

• Hemolysins: Destroy red blood cells.

• Superantigens: Trigger excessive cytokine release, leading to severe immune reactions.

Diseases Caused by Exotoxins

• Botulism: Causes flaccid paralysis.

• Tetanus: Causes uncontrollable muscle spasms.

• Cholera: Toxin causes massive fluid loss.

• Clostridioides difficile: May cause antibiotic-associated diarrhea.

Endotoxins

Lipid-Based Toxins from Gram-Negative Bacteria

Endotoxins are components of the outer membrane of gram-negative bacteria, released upon cell death.

• Endotoxins: Lipid A portion of lipopolysaccharide (LPS) in the outer membrane.

• Released when bacteria die and cell walls break apart.

• Stimulate macrophages to release cytokines, leading to fever and inflammation.

• Endotoxic shock: Severe drop in blood pressure due to widespread endotoxin release.

Exotoxins vs Endotoxins

Property	Exotoxins	Endotoxins
Chemical Nature	Proteins	Lipid A (part of LPS)
Source	Mostly Gram-positive bacteria	Gram-negative bacteria
Heat Stability	Heat labile	Heat stable
Conversion to Toxoid	Possible	Not possible
Specificity	Highly specific	General effects (fever, shock)

Plasmids and Lysogeny

Genetic Elements Influencing Virulence

Mobile genetic elements can carry genes that enhance microbial pathogenicity.

• R plasmids: Carry genes for antibiotic resistance.

• Virulence plasmids: Carry genes that increase a microbe's ability to cause disease.

• Lysogenic conversion: Occurs when a bacteriophage integrates into the bacterial genome, potentially introducing new virulence factors.

Pathogenic Properties of Viruses

Viral Strategies for Infection and Evasion

Viruses employ several mechanisms to evade host defenses and cause disease.

• Evade immunity by living inside host cells.

• Use antigenic variation to avoid immune detection.

• Cytopathic effects: Visible changes in host cells due to viral infection.

• Cytocidal effects: Viral effects that kill host cells.

Portals of Exit

How Pathogens Leave the Host

Pathogens exit the host through specific routes, facilitating transmission to new hosts.

• Respiratory tract: Coughing and sneezing expel pathogens.

• Gastrointestinal tract: Pathogens leave via feces.

• Blood: Pathogens may exit through needles, insect bites, or wounds.