Microbial Mechanisms of Pathogenicity: How Microbes Cause Disease

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Microbial Mechanisms of Pathogenicity

Introduction to Pathogenicity and Virulence

Pathogenicity refers to a microorganism's ability to cause disease by overcoming host defenses, while virulence describes the degree of pathogenicity. For a microbe to cause disease, it must enter the host, adhere to host tissues, penetrate or evade host defenses, and damage host tissues. Some microbes cause disease not by direct damage, but by the accumulation of their waste products.

Portals of Entry

Main Portals of Entry

Pathogens enter the human body through specific routes known as portals of entry. The main portals include mucous membranes, skin, and the parenteral route (direct deposition into tissues when barriers are penetrated). Most pathogens have a preferred portal of entry, which is essential for their ability to cause disease.

Mucous membranes: Respiratory, gastrointestinal, genitourinary tracts, and conjunctiva.
Skin: Usually impenetrable unless there are openings.
Parenteral route: Through punctures, injections, bites, cuts, wounds, or surgery.

Respiratory Tract

The respiratory tract is the easiest and most common portal of entry for many pathogens, which are inhaled into the nose or mouth in droplets of moisture. Diseases include the common cold, pneumonia, tuberculosis, influenza, and measles.

Diagram of the human respiratory system

Gastrointestinal Tract

Pathogens enter via food, water, or contaminated fingers. Most are destroyed by stomach acid, but those that survive can cause diseases such as polio, hepatitis A, and typhoid fever. These pathogens are often eliminated with feces and can be transmitted to others via contaminated water, food, or fingers.

Diagram of the human digestive system

Genitourinary Tract

This portal is associated with sexually transmitted infections (STIs). Some microbes penetrate unbroken mucous membranes, while others require a cut or abrasion. Examples include HIV, genital warts, chlamydia, herpesvirus-2, syphilis, and gonorrhea.

Diagram of the female genitourinary tract Diagram of the male genitourinary tract

Skin

The skin is an important defense against disease. Unbroken skin is generally impenetrable, but some microbes can enter through hair follicles, sweat gland ducts, or by boring through intact skin (e.g., hookworm larvae). Some fungi infect the skin or grow on keratin.

Diagram of skin structure

Parenteral Route

Microorganisms can be deposited directly into tissues via punctures, injections, bites, cuts, wounds, or surgery. Diseases such as HIV, hepatitis, tetanus, and gangrene can be transmitted this way.

Tick bite as a parenteral route Mosquito bite as a parenteral route

Preferred Portal of Entry

Pathogens often require a specific portal of entry to cause disease. Entry through a different portal may not result in disease.

Numbers of Invading Microbes

ID50 and LD50

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

ID50: Infectious dose for 50% of a sample population (measures virulence).
LD50: Lethal dose for 50% of a sample population (measures toxin potency).

Table showing ID50 for Bacillus anthracis by portal of entry Table showing LD50 for various toxins

Adherence to Host Tissues

Mechanisms of Adherence

Most pathogens must attach to host tissues to establish infection. This process, called adherence (or adhesion), involves adhesins (ligands) on the pathogen binding to specific receptors on host cells. Structures involved include the glycocalyx and fimbriae. Microbes may also form biofilms, which are communities that share nutrients and enhance survival.

Diagram of pathogen adherence to host cell via adhesins and receptors Biofilm formation on toothbrush bristle

Penetration or Evasion of Host Defenses

Virulence Factors

Capsules: Glycocalyx layers that impair phagocytosis.
Cell wall components: M protein (resists phagocytosis), Opa protein (attachment), mycolic acid (resists digestion).
Enzymes: Exoenzymes such as coagulases, kinases, hyaluronidase, collagenase, and IgA proteases aid in invasion and evasion.
Antigenic variation: Pathogens alter their surface antigens to evade immune detection.
Invasins: Surface proteins that rearrange host cell actin, causing membrane ruffling and facilitating entry (e.g., Shigella, Listeria).
Biofilms: Protect bacteria from phagocytosis and enhance resistance.

SEM image of Salmonella causing membrane ruffling

How Bacterial Pathogens Damage Host Cells

Mechanisms of Damage

Once inside the host, pathogens can damage cells in several ways:

Using the host’s nutrients (e.g., iron via siderophores).
Causing direct damage at the site of invasion.
Producing toxins that affect distant sites.
Inducing hypersensitivity reactions.

Using Host Nutrients: Siderophores

Iron is essential for most pathogenic bacteria. Siderophores are proteins secreted by pathogens that bind iron more tightly than host cells, allowing bacteria to acquire this vital nutrient.

Diagram of siderophore-mediated iron uptake Chemical structure of a siderophore binding Fe3+

Direct Damage

Pathogens can disrupt host cell function, use host nutrients, produce waste products, and multiply within host cells, causing cell rupture.

Production of Toxins

Toxins are poisonous substances produced by microorganisms. They can cause fever, cardiovascular problems, diarrhea, and shock. Toxigenicity is the ability to produce toxins, while toxemia is the presence of toxins in the blood. There are two main types of toxins:

Exotoxins: Proteins secreted by bacteria (mainly Gram-positive) during growth. They are highly toxic and can act repeatedly.
Endotoxins: Lipid A component of lipopolysaccharides (LPS) in the outer membrane of Gram-negative bacteria, released upon cell death.

Exotoxin production and release by bacteria Diagram of Gram-negative cell wall showing LPS and endotoxin location

Exotoxins vs. Endotoxins

Property	Exotoxins	Endotoxins
Bacterial Source	Gram-positive and Gram-negative bacteria	Gram-negative bacteria
Relation to Microorganism	Metabolic product of growing cell	Present in LPS of outer membrane; released on cell lysis
Chemistry	Protein, usually with two parts (A-B)	Lipid portion (Lipid A) of LPS
Pharmacology (Effect on Body)	Specific for a particular cell function	General, such as fever, weakness, aches, shock
Heat Stability	Unstable (destroyed at 60–80°C)	Stable (withstands autoclaving)
Toxicity	High	Low
Fever-Producing	No	Yes
Immunogenicity	Can be converted to toxoids for vaccines	Not easily neutralized by antitoxin; no effective toxoid
Representative Diseases	Gas gangrene, tetanus, botulism, diphtheria, scarlet fever	Typhoid fever, urinary tract infections, meningococcal meningitis

Diagram of endotoxin release from Gram-negative bacteria

Endotoxins and the Pyrogenic Response

Endotoxins stimulate macrophages to release cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF-α), which travel to the hypothalamus and induce fever by triggering prostaglandin production.

Diagram of the pyrogenic response to endotoxins

Portals of Exit

How Pathogens Leave the Host

Pathogens exit the host through specific portals, often the same as their entry points. These include:

Respiratory tract: Coughing and sneezing
Gastrointestinal tract: Feces and saliva
Genitourinary tract: Urine and genital secretions
Skin
Blood: Via arthropod bites or needles

Summary of portals of exit for pathogens

Key Concepts and Summary

Several factors are required for a microbe to cause disease: entry, adherence, penetration/evasion, and damage to host tissues.
Pathogens usually leave the body via specific portals of exit, which are generally the same as their portals of entry.
Understanding microbial pathogenicity is fundamental to understanding infectious disease processes.

Summary diagram of microbial pathogenicity mechanisms and portals