Microbial Mechanisms of Pathogenicity: Study Notes

Microbial Mechanisms of Pathogenicity

Introduction

This chapter explores how microorganisms cause disease, focusing on the mechanisms by which pathogens invade hosts, evade defenses, and damage host cells. Understanding these processes is essential for microbiology students to grasp the basis of infectious diseases and host-pathogen interactions.

Pathogenicity and Virulence

Definitions and Concepts

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

• Virulence: The degree of pathogenicity, indicating how severe the disease caused by the microorganism can be.

Portals of Entry

How Microorganisms Enter a Host

Pathogens must enter the host through specific routes called portals of entry. Most pathogens have a preferred portal of entry that is critical for their ability to cause disease.

• Mucous membranes: Respiratory, gastrointestinal, and genitourinary tracts.

• Skin: Usually through cuts, abrasions, or hair follicles.

• Parenteral route: Direct deposition into tissues when barriers are penetrated (e.g., punctures, injections, bites).

Numbers of Invading Microbes

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

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

Portal of Entry	ID50
Skin	10–50 endospores
Inhalation	10,000–20,000 endospores
Ingestion	250,000–1,000,000 endospores

Toxins	LD50
Botulinum	0.03 ng/kg
Shiga toxin	250 ng/kg
Staphylococcal enterotoxin	1350 ng/kg

Adherence to Host Tissues

Mechanisms of Adherence

Adherence is a critical step in pathogenesis, allowing pathogens to attach to host cells using specific molecules.

• Adhesins (ligands): Surface molecules on pathogens that bind to complementary receptors on host cells (e.g., glycocalyx, fimbriae).

• Biofilms: Communities of microbes that share nutrients and are often more resistant to host defenses.

Penetration or Evasion of Host Defenses

Capsules and Cell Wall Components

• Capsules: Glycocalyx layers that impair phagocytosis (e.g., Streptococcus pneumoniae, Bacillus anthracis).

• M protein: Resists phagocytosis (Streptococcus pyogenes).

• Opa protein: Aids attachment to host cells (Neisseria gonorrhoeae).

• Mycolic acid: Waxy lipid that resists digestion (Mycobacterium tuberculosis).

Enzymes Produced by Pathogens

• Coagulases: Coagulate fibrinogen to form clots.

• Kinases: Digest fibrin clots.

• Hyaluronidase: Digests polysaccharides that hold cells together.

• Collagenase: Breaks down collagen.

• IgA proteases: Destroy IgA antibodies.

Antigenic Variation

• Pathogens alter their surface antigens to evade immune detection, rendering antibodies ineffective.

Penetration into Host Cells

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

• Some bacteria use actin to move from one cell to another.

• Survival inside phagocytes by escaping phagosomes, preventing lysosomal fusion, or surviving in low pH.

Biofilms and Evasion of Phagocytosis

• Biofilm bacteria are shielded by extracellular polymeric substances (EPS), making them more resistant to phagocytosis.

How Bacterial Pathogens Damage Host Cells

Using the Host’s Nutrients: Siderophores

• Siderophores: Proteins secreted by pathogens to bind iron more tightly than host cells, facilitating iron uptake essential for bacterial growth.

Direct Damage

• Pathogens disrupt host cell function, use host nutrients, produce waste products, and may cause cell rupture by multiplying inside host cells.

Production of Toxins

• Toxins: Poisonous substances produced by microorganisms that can cause fever, cardiovascular problems, diarrhea, and shock.

• Toxigenicity: The ability to produce toxins.

• Toxemia: Presence of toxins in the host’s blood.

• Intoxications: Disease caused by toxins without microbial growth in the host.

Exotoxins and Endotoxins

Exotoxins

• Proteins produced and secreted by bacteria, usually gram-positive, that destroy host cells or inhibit metabolic functions.

• Antitoxins: Antibodies against specific exotoxins.

• Toxoids: Inactivated exotoxins used in vaccines.

Types of Exotoxins

• A-B toxins: Contain an enzyme (A part) and a binding component (B part). Example: Diphtheria toxin.

• Genotoxins: Damage DNA, causing mutations and possibly cancer.

• Membrane-disrupting toxins: Lyse host cells by disrupting plasma membranes (e.g., leukocidins, hemolysins, streptolysins).

• Superantigens: Cause intense immune responses by stimulating cytokine release, leading to fever, shock, and death.

Representative Diseases Caused by Exotoxins

Disease	Bacterium	Type of Exotoxin	Mechanism
Botulism	Clostridium botulinum	A-B	Neurotoxin prevents nerve impulse transmission; flaccid paralysis.
Tetanus	Clostridium tetani	A-B	Neurotoxin blocks muscle relaxation; uncontrollable contractions.
Diphtheria	Corynebacterium diphtheriae	A-B	Cytotoxin inhibits protein synthesis.
Cholera	Vibrio cholerae	A-B	Enterotoxin causes fluid/electrolyte secretion; diarrhea.
Toxic shock syndrome	Staphylococcus aureus	Superantigen	Causes fluid/electrolyte loss; shock.

Endotoxins

• Lipid A portion of lipopolysaccharides (LPS) in the outer membrane of gram-negative bacteria.

• Released during bacterial multiplication and cell death.

• Stimulate macrophages to release cytokines, causing fever and potentially disseminated intravascular coagulation.

Comparison of Exotoxins and Endotoxins

Property	Exotoxins	Endotoxins
Bacterial Source	Gram-positive and gram-negative	Gram-negative
Chemistry	Proteins, usually with two parts (A-B)	Lipid portion of LPS
Heat Stability	Unstable (destroyed at 60–80°C)	Stable (withstands autoclaving)
Toxicity	High	Low
Immunity	Can be converted to toxoids for vaccines	Not easily immunized against
Representative Diseases	Gas gangrene, tetanus, diphtheria, cholera	Typhoid fever, urinary tract infections, meningococcal meningitis

Detection of Endotoxins

• Limulus amebocyte lysate (LAL) assay: Uses horseshoe crab blood cells that clot in the presence of endotoxin, used to test for endotoxin contamination.

Plasmids, Lysogeny, and Pathogenicity

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

• Lysogenic conversion: Incorporation of a prophage can change a microbe’s characteristics, sometimes making it pathogenic.

Pathogenic Properties of Viruses

Cytopathic Effects (CPE)

• Visible effects of viral infection on host cells, including:

  • Stopping cell synthesis

  • Causing lysosomal enzyme release

  • Creating inclusion bodies

  • Fusing cells to form syncytia

  • Inducing chromosomal changes

  • Loss of contact inhibition (may lead to cancer)

• Alpha and beta interferons: Produced by virally infected cells to protect neighboring cells by inhibiting viral protein synthesis and inducing apoptosis in infected cells.

Pathogenic Properties of Fungi, Protozoa, Helminths, and Algae

Fungi

• Produce toxic metabolic products and provoke allergic responses.

• Trichothecene toxins inhibit protein synthesis.

• Proteases modify host cell membranes.

• Capsules prevent phagocytosis.

• Some produce alkaloid toxins (ergot), carcinogenic toxins (aflatoxin), and neurotoxins (mycotoxins).

Protozoa

• Cause symptoms by their presence and waste products.

• Avoid host defenses by digesting cells, growing in phagocytes, and antigenic variation.

Helminths

• Use host tissue for growth, produce large masses, and cause cellular damage and symptoms via waste products.

Algae

• Some produce neurotoxins, such as saxitoxin, causing paralytic shellfish poisoning.

Portals of Exit

How Pathogens Leave the Host

Portals of exit are generally the same as portals of entry and are critical for the spread of infection.

• Respiratory tract: Coughing and sneezing

• Gastrointestinal tract: Feces and saliva

• Genitourinary tract: Urine and genital secretions

• Skin

• Blood: Via arthropod bites, needles, or syringes

Additional info: This guide expands on the original slides by providing definitions, mechanisms, and examples for each concept, as well as tables and images that clarify key points. It is suitable for exam preparation and review of microbial pathogenicity mechanisms.