Microbial Mechanisms of Pathogenicity

Microbial Mechanisms of Pathogenicity

Introduction to Pathogenicity and Virulence

Pathogenicity refers to the ability of a microorganism to cause disease, while virulence describes the degree or extent of pathogenicity. Understanding these concepts is fundamental to microbiology, as they explain how and why certain microbes cause disease in hosts.

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

• Virulence: The relative ability of a pathogen to infect or damage a host.

Portals of Entry

Major Routes of Entry

Microorganisms can enter the human body through several portals, each providing a unique environment for infection. The preferred portal of entry often determines the outcome of infection.

• Mucous membranes: Lining the respiratory, gastrointestinal, and genitourinary tracts.

• Skin: Usually impenetrable, but some microbes can enter through cuts or hair follicles.

• Parenteral route: Direct deposition beneath the skin or mucous membranes (e.g., punctures, injections).

• Preferred portal of entry: Some pathogens cause disease only if they enter through a specific route.

Numbers of Invading Microbes

The likelihood of disease depends on the number of invading microbes. Two important measures are ID50 and LD50:

• ID50 (Infectious Dose 50): The number of microbes required to infect 50% of a test population.

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

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

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

Adherence to Host Tissues

Mechanisms of Adherence

For infection to occur, pathogens must adhere to host tissues. This is mediated by surface molecules called adhesins or ligands, which bind specifically to complementary receptors on host cells.

• Adhesins/ligands: Surface molecules on pathogens that bind to host cell receptors.

• Examples:

  • Glycocalyx (e.g., Streptococcus mutans)

  • Fimbriae (e.g., Escherichia coli)

  • M protein (e.g., Streptococcus pyogenes)

• Biofilms: Communities of microbes that adhere to surfaces and each other (e.g., dental plaque, algae on swimming pools).

Penetration of Host Defenses

Capsules

Some bacteria produce capsules that help them evade the host immune system by preventing phagocytosis.

• Examples: Streptococcus pneumoniae, Haemophilus influenzae, Bacillus anthracis

• Mechanism: Capsules impair phagocytosis by host immune cells.

Cell Wall Components

• M protein: Resists phagocytosis (Streptococcus pyogenes).

• Opa protein: Inhibits T helper cells (Neisseria gonorrhoeae).

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

Enzymes

Pathogenic bacteria may secrete enzymes that aid in spreading infection and evading host defenses.

• Coagulase: Coagulates fibrinogen, forming clots.

• Kinases: Digest fibrin clots (e.g., streptokinase).

• Hyaluronidase: Hydrolyzes hyaluronic acid, aiding tissue penetration.

• Collagenase: Hydrolyzes collagen, facilitating spread through connective tissue.

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

Penetration into the Host Cell Cytoskeleton

Certain pathogens manipulate the host cell cytoskeleton to enter cells or move between them.

• Invasins: Proteins that induce host cell membrane ruffling, allowing entry (e.g., Salmonella).

• Actin-based motility: Some bacteria use host actin to move from cell to cell (e.g., Listeria).

Antigenic Variation

Some pathogens evade the immune system by altering their surface antigens, a process known as antigenic variation.

• Example: Trypanosoma brucei (sleeping sickness), Trypanosoma cruzi (Chagas disease).

• Mechanism: The pathogen periodically changes its surface proteins, preventing effective immune response.

Toxins and Their Effects

Direct Damage and Toxin Production

Pathogens can cause disease by directly damaging host cells or by producing toxins. Toxins are substances that contribute to pathogenicity.

• Toxigenicity: The ability to produce toxins.

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

• Toxoid: Inactivated toxin used in vaccines.

• Antitoxin: Antibodies against a specific toxin.

Exotoxins vs. Endotoxins

There are two main types of bacterial toxins: exotoxins and endotoxins. They differ in source, chemistry, and effects.

Property	Exotoxins	Endotoxins
Source	Mostly Gram-positive bacteria	Gram-negative bacteria
Relation to Microbe	By-products of growing cell	Outer membrane
Chemistry	Protein	Lipid A (part of LPS)
Fever?	No	Yes
Neutralized by Antitoxin?	Yes	No
LD50	Small	Relatively large

Mechanism of A-B Exotoxins

A-B exotoxins are composed of two parts: the A (active) component and the B (binding) component. The B component binds to the host cell, and the A component exerts the toxic effect, often by inhibiting protein synthesis.

Membrane-Disrupting Toxins

These toxins lyse host cells by disrupting the plasma membrane, either by forming protein channels or by disrupting the phospholipid bilayer.

• Leukocidins: Destroy white blood cells.

• Hemolysins: Destroy red blood cells.

• Streptolysins: Produced by streptococci, lyse various cells.

Superantigens

Superantigens cause an intense immune response by stimulating excessive cytokine release from host cells, leading to symptoms such as fever, nausea, vomiting, diarrhea, shock, and death.

Exotoxins and Lysogenic Conversion

Some exotoxins are encoded by genes acquired through lysogenic conversion (integration of bacteriophage DNA into the bacterial genome).

• Corynebacterium diphtheriae: A-B toxin (diphtheria toxin)

• Streptococcus pyogenes: Membrane-disrupting erythrogenic toxin

• Clostridium botulinum: A-B neurotoxin

• Vibrio cholerae: A-B enterotoxin

• Staphylococcus aureus: Superantigen

Endotoxins and the Pyrogenic Response

Endotoxins, specifically lipid A from Gram-negative bacteria, can trigger fever (pyrogenic response) by inducing macrophages to release cytokines such as IL-1 and TNF-α, which act on the hypothalamus to raise body temperature.

Portals of Exit

Major Routes of Exit

Pathogens leave the host through specific portals of exit, which often correspond to the portals of entry. This is essential for the transmission of infectious diseases.

• Respiratory tract: Coughing and sneezing

• Gastrointestinal tract: Feces and saliva

• Genitourinary tract: Urine and vaginal secretions

• Skin

• Blood: Via arthropod bites, needles, or syringes