Microbial Mechanisms of Pathogenicity: Study Notes

Microbial Mechanisms of Pathogenicity

Pathogenicity

Pathogenicity refers to the ability of a microorganism to cause disease in a host organism. The degree of pathogenicity is termed virulence, which varies among different microbes.

• Pathogen: An organism that causes disease.

• Virulence: The extent of pathogenicity.

• Infection: The invasion and multiplication of pathogens in the body.

• Disease: The result of infection, leading to impaired body function.

• Example: Staphylococcus aureus can cause skin infections due to its virulence factors.

Portals of Entry

Microorganisms must enter the host through specific portals to initiate infection. The main portals of entry include:

• Mucous membranes: Respiratory tract, gastrointestinal tract, genitourinary tract, conjunctiva.

• Skin: Usually impenetrable, but some microbes enter through cuts or bites.

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

• Example: Influenza virus enters via the respiratory mucosa.

Numbers of Invading Microbes

The likelihood of disease increases with the number of invading microbes. The infectious dose (ID50) and lethal dose (LD50) are quantitative measures of pathogenicity.

• ID50: Dose required to infect 50% of a population.

• LD50: Dose required to kill 50% of a population.

Microbe	Portal of Entry	ID50
Bacillus anthracis	Skin	10-50 endospores
Bacillus anthracis	Inhalation	10,000-20,000 endospores
Bacillus anthracis	Ingestion	250,000-1,000,000 endospores
Shigella	Ingestion	250 cells
Vibrio cholerae	Ingestion	108 cells

Adherence

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

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

• Biofilms: Communities of microbes that adhere to surfaces and are protected by an extracellular matrix.

• Example: Streptococcus mutans uses adhesins to attach to tooth surfaces, forming dental plaque.

Penetrating Host Defenses

Microbes employ various strategies to evade or overcome host defenses.

• Capsules: Glycocalyx layers that inhibit phagocytosis.

• Cell wall components: Proteins such as M protein (in Streptococcus pyogenes) resist phagocytosis.

• Enzymes: Coagulases, kinases, hyaluronidase, collagenase, and IgA proteases help microbes invade tissues and evade immune responses.

Damaging Host Cells

Pathogens damage host cells through direct mechanisms or by producing toxins.

• Direct damage: Disruption of host cell function, production of waste products, and cell lysis.

• Toxins: Poisonous substances produced by microbes that contribute to pathogenicity.

Production of Toxins

Toxins are classified as exotoxins or endotoxins, each with distinct properties and effects.

• Exotoxins: Proteins secreted by bacteria, often highly potent and specific in action.

• Endotoxins: Lipopolysaccharides (LPS) found in the outer membrane of Gram-negative bacteria, released upon cell death.

Exotoxins

Exotoxins are produced mainly by Gram-positive bacteria and are secreted into the surrounding medium.

• Characteristics: Highly toxic, specific for target cells, can be neutralized by antitoxins.

• Types: Cytotoxins, neurotoxins, enterotoxins.

• Example: Diphtheria toxin inhibits protein synthesis in host cells.

A-B Toxins

A-B toxins are a major class of exotoxins composed of two parts: A (active) and B (binding).

• A subunit: Enzymatic component that causes damage.

• B subunit: Binds to host cell receptors and facilitates entry of the A subunit.

• Example: Botulinum toxin and cholera toxin are A-B toxins.

Exotoxin Diseases

Exotoxins are responsible for a variety of diseases. The following table summarizes some key examples:

Toxin	Microbe	Effect	Disease
Diphtheria toxin	Corynebacterium diphtheriae	Inhibits protein synthesis	Diphtheria
Botulinum toxin	Clostridium botulinum	Blocks nerve impulses	Botulism
Tetanus toxin	Clostridium tetani	Blocks inhibitory nerve impulses	Tetanus
Cholera toxin	Vibrio cholerae	Causes water and electrolyte loss	Cholera

Endotoxins

Endotoxins are part of the outer membrane of Gram-negative bacteria and are released when the bacteria die and the cell wall breaks apart.

• Structure: Lipid A component of lipopolysaccharide (LPS).

• Effects: Fever, inflammation, shock.

• Example: Escherichia coli endotoxin can cause septic shock.

Plasmids, Lysogeny, and Pathogenicity

Genetic elements such as plasmids and bacteriophage lysogeny can enhance microbial pathogenicity.

• Plasmids: Carry genes for toxins, antibiotic resistance, and other virulence factors.

• Lysogeny: Integration of bacteriophage DNA can confer new properties to bacteria, including toxin production.

Pathogenic Properties of Viruses

Viruses cause disease by entering host cells and disrupting cellular processes.

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

• Inhibition of host cell functions: Viruses may block protein synthesis or induce apoptosis.

• Example: Herpesviruses cause cell lysis and inflammation.

Pathogenic Properties of Fungi

Fungi can cause disease through direct tissue invasion, toxin production, and allergic responses.

• Mycotoxins: Toxic compounds produced by fungi (e.g., aflatoxin).

• Allergic reactions: Fungal spores can trigger immune responses.

• Example: Aspergillus species produce aflatoxins.

Pathogenic Properties of Protozoa

Protozoa cause disease by destroying host tissues and evading immune responses.

• Antigenic variation: Changing surface antigens to avoid immune detection.

• Direct damage: Invasion and destruction of host cells.

• Example: Plasmodium species cause malaria by infecting red blood cells.

Pathogenic Properties of Helminths

Helminths (parasitic worms) cause disease by consuming host nutrients and producing waste products.

• Physical damage: Blockage of organs, tissue destruction.

• Immune modulation: Altering host immune responses.

• Example: Schistosoma species cause schistosomiasis.

Pathogenic Properties of Algae

Some algae produce toxins that can cause disease in humans.

• Neurotoxins: Produced by dinoflagellates, causing paralytic shellfish poisoning.

• Example: Alexandrium species produce saxitoxin.

Portals of Exit

Microbes leave the host through specific portals, facilitating transmission to new hosts.

• Respiratory tract: Coughing, sneezing.

• Gastrointestinal tract: Feces, saliva.

• Genitourinary tract: Urine, vaginal secretions.

• Skin: Direct contact, wounds.

• Blood: Biting arthropods, needles.