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17: Pathogenicity and Virulence Factors

Last updated

Mar 16, 2025
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- 16.E: Microbial Mechanisms of Pathogenicity (Exercises)
- 17.1: Stages of Pathogenesis

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Pathogenicity refers to a microorganism’s ability to cause disease, while virulence describes the severity of disease it causes. Bacterial pathogens use adhesion factors (fimbriae, adhesins, biofilms) to attach to host cells, and they evade immune defenses using capsules, antigenic variation, and intracellular survival. Many bacteria produce toxins, such as exotoxins (e.g., cholera toxin, botulinum toxin) that directly harm cells, and endotoxins (lipopolysaccharides) that trigger inflammatory responses. Enzymes like hyaluronidase and coagulase help bacteria invade tissues. True pathogens (e.g., Mycobacterium tuberculosis) cause disease in healthy individuals, while opportunistic pathogens (e.g., Pseudomonas aeruginosa) require a weakened immune system to establish infection.

Viruses, unlike bacteria, rely on host cells for replication and use viral attachment proteins (e.g., HIV gp120, SARS-CoV-2 spike protein) to enter cells. To evade immunity, they undergo antigenic variation (influenza virus), establish latency (herpesvirus), or inhibit immune signaling (Epstein-Barr virus). Some viruses cause cytopathic effects (CPEs) such as apoptosis, syncytia formation, or immune-mediated damage. Unlike bacteria, viruses do not produce toxins, but they manipulate the host’s immune system to enhance their survival. The severity of viral disease depends on both viral virulence factors and host defenses, such as genetic resistance (e.g., CCR5 mutation protects against HIV) and microbiome interactions.

Both bacterial and viral infections depend on a complex interplay between microbial virulence factors and host susceptibility. Bacterial pathogens can be treated with antibiotics (if susceptible), while viral infections often require antivirals or vaccines for prevention. Host immune status, genetic predisposition, and microbiome balance play critical roles in determining infection outcomes. Understanding the mechanisms of pathogenicity and virulence helps in developing therapies, vaccines, and public health strategies to combat infectious diseases.

Figure 15-17 capsule.jpg — Figure $\PageIndex{1}$ : (a) A micrograph of capsules around bacterial cells. (b) Antibodies normally function by binding to antigens, molecules on the surface of pathogenic bacteria. Phagocytes then bind to the antibody, initiating phagocytosis. (c) Some bacteria also produce proteases, virulence factors that break down host antibodies to evade phagocytosis. (credit a: modification of work by Centers for Disease Control and Prevention)

17.1: Stages of Pathogenesis
Koch’s postulates are used to determine whether a particular microorganism is a pathogen. Molecular Koch’s postulates are used to determine what genes contribute to a pathogen’s ability to cause disease. Virulence, the degree to which a pathogen can cause disease, can be quantified by calculating either the ID50 or LD50 of a pathogen on a given population. Primary pathogens are capable of causing pathological changes associated with disease in a healthy individual.
17.2: Bacterial Virulence Factors- Adhesins and Exoenzymes
Virulence factors contribute to a pathogen’s ability to cause disease. Exoenzymes and toxins allow pathogens to invade host tissue and cause tissue damage. Exoenzymes are classified according to the macromolecule they target and exotoxins are classified based on their mechanism of action. Bacterial toxins include endotoxin and exotoxins. Endotoxin is the lipid A component of the LPS of the gram-negative cell envelope. Exotoxins are proteins secreted mainly by gram-positive bacteria.
17.3: Bacterial Virulence Factors- Toxins
This page discusses bacterial toxins, detailing endotoxins and exotoxins. Endotoxins, from gram-negative bacteria, induce inflammatory responses, while exotoxins, from gram-positive bacteria, are lethal in small doses and categorized into types like A-B toxins. Specific examples include cholera toxin, botulinum toxin, and tetanus toxin, each affecting bodily functions in distinct ways.
17.4: Immune Evasion and Viral Virulence Factors
This page examines bacterial and viral mechanisms for evading the immune system, detailing virulence factors such as capsules, proteases, and antigenic variation. Bacteria use exotoxins and endotoxins, while viruses rely on adhesins and undergo antigenic drift and shift. Examples include Streptococcus pneumoniae and influenza, emphasizing the need for vaccines due to the emergence of new strains.
17.E: Pathogenicity and Virulence Factors (Exercises)
These are exercises for Chapter 15 "Microbial Mechanisms of Pathogenicity" in OpenStax's Microbiology Textmap.

Thumbnail: Ulcer-causing bacterium (H. Pylori) crossing mucus layer of stomach. (Public Domain/modified from original; National Science Foundation via Wikimedia Commons).

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