Microbial Mechanisms of Pathogenicity: Study Notes

Microbial Mechanisms of Pathogenicity

Pathogenicity and Virulence

Pathogenicity refers to the ability of a microorganism to cause disease, while virulence describes the degree of pathogenicity. Understanding these concepts is fundamental to microbiology and infectious disease.

• Pathogenicity: The capacity of a microbe to cause disease in a host.

• Virulence: The relative severity or harmfulness of a pathogen.

Portals of Entry

Microorganisms must enter the host through specific portals to initiate infection. Most pathogens have a preferred portal of entry.

• Mucous membranes: Includes respiratory tract (most common), digestive canal, genitourinary system, and conjunctiva.

• Skin: Usually impenetrable unless through hair follicles or sweat gland ducts.

• Parenteral route: Direct deposition into tissues via punctures, injections, bites, cuts, wounds, or surgery.

Numbers of Invading Microbes: ID50 and LD50

The number of microbes required to cause infection or death is measured by ID50 and LD50.

• ID50: Infectious dose for 50% of a sample population; measures virulence.

• LD50: Lethal dose for 50% of a sample population; measures toxin potency.

Example: Bacillus anthracis has different ID50 values depending on the portal of entry.

Adherence

Adherence is the process by which pathogens attach to host tissues, a critical step in infection.

• Adhesins (ligands): Molecules on pathogens that bind to host cell receptors.

• Examples: Glycocalyx of Streptococcus mutans (adheres to teeth), fimbriae of Actinomyces, viral spikes (e.g., SARS-CoV-2 binding to ACE2).

Penetration or Evasion of Host Defenses

Pathogens employ various strategies to evade host defenses and establish infection.

• Capsules: Glycocalyx layer impairs phagocytosis (e.g., Streptococcus pneumoniae).

• Cell wall components: M protein (resists phagocytosis), Opa protein (attachment), mycolic acid (resists digestion).

• Enzymes: Coagulases (coagulate fibrinogen), kinases (digest fibrin clots), hyaluronidase (digests hyaluronic acid), collagenase (breaks down collagen), IgA proteases (destroy IgA antibodies).

• Antigenic variation: Pathogens alter surface antigens to evade immune response (e.g., influenza virus).

• Invasins: Surface proteins that rearrange actin filaments, causing membrane ruffling and bacterial entry (e.g., Salmonella).

• Biofilms: Bacterial communities resistant to antibiotics and phagocytosis.

How Bacterial Pathogens Damage Host Cells

Bacterial pathogens damage host cells through nutrient acquisition, direct damage, and toxin production.

• Siderophores: Proteins that bind iron more tightly than host proteins, facilitating iron acquisition.

• Direct damage: Disruption of host cell function, nutrient use, waste production, and cell rupture.

• Toxins: Poisonous substances causing fever, shock, and other symptoms.

Exotoxins

Exotoxins are proteins secreted by bacteria, often highly specific and potent.

• Antitoxins: Antibodies against exotoxins.

• Toxoids: Inactivated exotoxins used in vaccines.

• A-B toxins: Consist of an active (A) and binding (B) component (e.g., diphtheria toxin).

• Membrane-disrupting toxins: Lyse host cells (e.g., hemolysins).

• Superantigens: Cause intense immune response via cytokine release.

Endotoxins

Endotoxins are lipid A components of lipopolysaccharides (LPS) in gram-negative bacteria, released upon cell death.

• Effects: Fever, shock, disseminated intravascular coagulation, weakening of blood-brain barrier.

• LAL assay: Detects endotoxins using horseshoe crab blood.

Comparison of Exotoxins and Endotoxins

Exotoxins and endotoxins differ in their source, chemistry, effects, and immunological properties.

Property	Exotoxins	Endotoxins
Bacterial Source	Gram-positive and gram-negative	Gram-negative
Chemistry	Proteins (A-B structure)	Lipid A (LPS)
Heat Stability	Unstable (destroyed at 60-80°C)	Stable (withstands autoclaving)
Toxicity	High	Low
Fever-Producing	No	Yes
Immunology	Can be neutralized by antitoxin	Not easily neutralized
Lethal Dose	Small	Large
Representative Diseases	Tetanus, botulism, diphtheria	Typhoid fever, meningitis

Plasmids, Lysogeny, and Pathogenicity

Plasmids and lysogenic conversion contribute to microbial pathogenicity by encoding virulence factors and toxins.

• Plasmids: Carry genes for toxins, antibiotic resistance, and enzymes.

• Lysogenic conversion: Incorporation of prophage genes alters microbial characteristics (e.g., diphtheria toxin).

Pathogenic Properties of Viruses

Viruses cause disease through cytopathic effects (CPE), which are visible changes in host cells.

• Cytocidal effects: Kill host cells.

• Noncytocidal effects: Cause cell damage without death.

• Examples: Inclusion bodies (Negri bodies in rabies), syncytium formation (measles), transformation (cancer).

• Interferons: Alpha and beta interferons protect neighboring cells from viral infection.

Virus (Genus)	Cytopathic Effect
Poliovirus (Enterovirus C)	Cytocidal (cell death)
Genital warts virus (Alphapapillomavirus)	Acidophilic inclusion bodies in nucleus, transformation
Adenovirus (Mastadenovirus)	Basophilic inclusion bodies in nucleus
Rabies (Lyssavirus)	Acidophilic inclusion bodies in cytoplasm
CMV (Cytomegalovirus)	Acidophilic inclusion bodies in nucleus and cytoplasm
Measles virus (Morbillivirus)	Cell fusion
HIV (Lentivirus)	Destruction of T cells
SARS-CoV-2 (Betacoronavirus)	Syncytia, cilia shrinkage, altered junctions

Pathogenic Properties of Fungi, Protozoa, Helminths, and Algae

These eukaryotic pathogens cause disease through toxins, tissue damage, and immune evasion.

• Fungi: Produce toxic metabolites, provoke allergies, inhibit protein synthesis, and prevent phagocytosis with capsules.

• Protozoa: Cause symptoms via waste products, digest host cells, grow in phagocytes, and undergo antigenic variation.

• Helminths: Use host tissue for growth, produce large masses, and release waste products causing symptoms.

• Algae: Some produce neurotoxins (e.g., saxitoxin) causing paralytic shellfish poisoning.

Portals of Exit

Microorganisms exit the host through specific portals, often the same as their entry points.

• Respiratory tract: Coughing and sneezing.

• Gastrointestinal tract: Feces and saliva.

• Genitourinary tract: Urine and genital secretions.

• Skin

• Blood: Via arthropod bites or needles.

Additional info: These notes expand on brief points with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.