Microbial Pathogenesis: Mechanisms of Infection and Host Damage

Microbial Pathogenesis

Overview of Pathogenesis

Microbial pathogenesis refers to the process by which microorganisms cause disease in a host. To establish infection, pathogens must enter the host, adhere to tissues, evade or penetrate host defenses, and inflict damage.

• Portals of Entry: Mucous membranes, skin, and parenteral routes are the main entry points for pathogens.

• Key Steps: Entry, adherence, invasion, evasion of host defenses, and host damage.

Portals of Entry

Mucous Membranes

Mucous membranes line the respiratory, gastrointestinal (GI), and genitourinary tracts, as well as the conjunctiva. They are the most common portals of entry for pathogens.

• Respiratory Tract: Easiest and most frequently used portal. Pathogens are inhaled via droplets or dust. Examples: Common cold, pneumonia, tuberculosis, influenza, measles.

• Gastrointestinal Tract: Pathogens enter via contaminated food, water, or fingers. Most are destroyed by stomach acid, but some survive and cause disease. Examples: Poliomyelitis, hepatitis A, typhoid fever, amebic dysentery, giardiasis, shigellosis, cholera.

• Genitourinary Tract: Portal for sexually transmitted diseases (STDs). Examples: HIV, genital warts, chlamydia, herpes, syphilis, gonorrhea.

• Conjunctiva: Lines eyelids and covers the white of the eyes. Examples: Conjunctivitis, trachoma, ophthalmia neonatorum.

Skin

The skin is a major barrier to infection. Most microorganisms cannot penetrate unbroken skin, but some can enter through hair follicles, sweat gland ducts, or by boring through intact skin (e.g., hookworm larvae).

• Diseases: Conjunctivitis, trachoma, ophthalmia neonatorum.

Parenteral Route

Pathogens can be deposited directly into tissues beneath the skin or mucous membranes via punctures, injections, bites, cuts, wounds, or surgery.

• Examples: Infections following injuries, surgery, or insect bites.

Preferred Portal of Entry

Many pathogens must enter through a specific portal to cause disease. Entry through a non-preferred portal may not result in infection.

Numbers of Invading Microbes

The likelihood of disease increases with the number of invading pathogens. Two important measures are:

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

• LD50: Lethal dose for 50% of a sample population.

Adherence

Mechanisms of Adherence

Adherence is the process by which pathogens attach to host tissues, a critical step in infection. This is mediated by surface molecules called adhesins or ligands that bind to specific receptors on host cells.

• Adhesins: Often glycoproteins or lipoproteins found on glycocalyx, pili, fimbriae, or flagella.

• Host Receptors: Typically sugars such as mannose.

• Prevention: Altering adhesins or receptors can prevent infection.

Biofilms

Microbes can form biofilms—complex communities of microorganisms adhering to surfaces and encased in an exopolymer matrix. Biofilms are resistant to disinfectants and antibiotics and are implicated in many human infections.

• Examples: Dental plaque, algae on swimming pool walls, shower door scum.

Penetration of Host Defenses

Capsules

Some bacteria produce a glycocalyx that forms a capsule around the cell wall, increasing virulence by resisting phagocytosis.

• Examples: Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Bacillus anthracis.

Cell Wall Components

Certain cell wall components enhance virulence:

• M protein: Found in Streptococcus pyogenes; resists phagocytosis and aids attachment.

• Opa protein: In Neisseria gonorrhoeae; aids attachment and invasion of host cells.

• Mycolic acid: Waxy lipid in Mycobacterium tuberculosis; resists digestion by phagocytes.

Enzymes (Exoenzymes)

Some bacteria secrete enzymes that aid in invasion and evasion of host defenses:

• Coagulases: Clot fibrinogen in blood, protecting bacteria from phagocytosis.

• Kinases: Digest fibrin clots, allowing spread of infection.

• Hyaluronidase: Hydrolyzes hyaluronic acid in connective tissue, aiding spread.

• Collagenase: Breaks down collagen, facilitating tissue invasion (e.g., gas gangrene).

• IgA proteases: Destroy IgA antibodies, aiding evasion of immune response.

Antigenic Variation

Some pathogens can alter their surface antigens, evading recognition by host antibodies. This allows persistent or recurrent infections.

• Examples: Neisseria gonorrhoeae, influenza virus.

Penetration into Host Cell Cytoskeleton

Certain bacteria (e.g., Salmonella, E. coli) produce invasins that rearrange actin filaments, causing membrane ruffling and engulfment of the microbe by the host cell.

How Bacterial Pathogens Damage Host Cells

Mechanisms of Host Damage

• Using the host's nutrients (e.g., iron via siderophores)

• Causing direct damage at the site of infection

• Producing toxins that affect distant sites

• Inducing hypersensitivity reactions

Using Host Nutrients: Siderophores

Pathogens secrete siderophores to scavenge iron from host iron-binding proteins (e.g., lactoferrin, transferrin, ferritin, hemoglobin), which is essential for bacterial growth.

Direct Damage

Pathogens can damage host cells by using them for nutrients and producing waste products, often leading to cell rupture.

Toxins

Toxins are poisonous substances produced by certain microbes, often the primary factor in disease. They can be classified as exotoxins or endotoxins.

• Toxigenicity: The ability to produce toxins.

• Toxemia: Presence of toxins in the blood.

Exotoxins

Exotoxins are proteins secreted by bacteria (both Gram-positive and Gram-negative) that are highly specific and potent. They are often encoded by plasmids or phages and can be neutralized by antitoxins.

• Types: A-B toxins, membrane-disrupting toxins, superantigens.

• Toxoids: Inactivated exotoxins used in vaccines (e.g., diphtheria, tetanus).

Types of Exotoxins

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

• Membrane-Disrupting Toxins: Cause cell lysis by disrupting plasma membranes.

  • Leukocidins: Kill phagocytic leukocytes (produced by staphylococci, streptococci).

  • Hemolysins: Destroy erythrocytes.

• Superantigens: Bacterial proteins that provoke intense immune responses by nonspecifically activating T cells.

Specific Exotoxins

• Diphtheria Toxin: Inhibits protein synthesis in eukaryotic cells (Corynebacterium diphtheriae).

• Erythrogenic Toxins: Damage capillaries, causing red skin rash (scarlet fever, Streptococcus pyogenes).

• Botulinum Toxin: Blocks nerve impulses, causing flaccid paralysis (Clostridium botulinum).

• Tetanus Toxin: Blocks relaxation pathway, causing muscle spasms (Clostridium tetani).

• Vibrio Enterotoxin: Causes secretion of fluids and electrolytes, leading to diarrhea (Vibrio cholerae).

• Staphylococcal Enterotoxin: Superantigen affecting intestines (Staphylococcus aureus).

Endotoxins

Endotoxins are part of the outer membrane of Gram-negative bacteria (specifically, Lipid A of lipopolysaccharide, LPS). They are released upon bacterial cell death and lysis, causing generalized symptoms such as fever, shock, and sometimes death.

• Pyrogenic Response: Endotoxins stimulate macrophages to release cytokines (e.g., IL-1, TNF-α), which induce fever by acting on the hypothalamus.

• Detection: Limulus amebocyte lysate (LAL) assay is used to detect endotoxins in medical materials.

Comparison of Exotoxins and Endotoxins

	Exotoxin	Endotoxin
Species	Some Gram-positive and Gram-negative	Most Gram-negative, some Listeria
Protein Location	Secreted from cell	Part of cell wall (LPS)
Gene Location	Plasmid or bacteriophage	Bacterial chromosome
Toxicity	High	Low
Antigenicity	Highly antigenic	Poorly antigenic
Vaccine	Available (toxoids)	Not available
Heat Stability	Heat labile	Heat stable
Example	Cholera, tetanus, botulism	Meningococcemia, sepsis

Plasmids, Lysogeny, and Pathogenicity

Plasmids and bacteriophages can carry genes for virulence factors, antibiotic resistance, and toxins. Lysogenic conversion can result in new pathogenic properties in bacteria.

• Examples: Tetanus neurotoxin, heat-labile enterotoxin, staphylococcal enterotoxin, diphtheria toxin, erythrogenic toxins, botulinum neurotoxin.

Pathogenic Properties of Viruses

Viruses cause disease by entering host cells, evading immune responses, and damaging or killing host cells during replication.

• Mechanisms: Inhibition of host macromolecular synthesis, lysosomal enzyme release, formation of inclusion bodies, syncytium formation, antigenic changes, chromosomal changes, and transformation leading to cancer.

• Cytopathic Effects (CPE): Visible effects of viral infection, used in diagnosis.

Pathogenic Properties of Fungi, Protozoa, Helminths, and Algae

• Fungi: Some produce toxic metabolic products.

• Protozoa: Cause disease via presence and waste products.

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

• Algae: Some species produce neurotoxins.

Portals of Exit

Pathogens exit the host via specific routes, often the same as their portal of entry. This facilitates transmission to new hosts.

• Respiratory Tract: Coughing, sneezing.

• Gastrointestinal Tract: Feces, saliva.

• Genitourinary Tract: Secretions, urine.

• Skin and Wounds: Drainage, contact with fomites.

• Blood: Biting insects, contaminated needles.