25. Human-Pathogen Interactions, Virulence, and Bacterial Toxins

Human-Pathogen Interactions

Pathogen and Opportunistic Pathogen

Understanding the types of pathogens and their interactions with hosts is fundamental in microbiology and infectious disease.

• Pathogen: A microbe that causes disease in its host. Pathogens include bacteria, viruses, fungi, and protozoa.

• Opportunistic pathogen: Microorganisms that normally do not cause disease but can become pathogenic in hosts with compromised immune systems (e.g., HIV/AIDS, cancer, immunosuppressive therapy).

Example: Pseudomonas aeruginosa is an opportunistic pathogen that can cause infections in immunocompromised patients.

Infection vs. Disease

It is important to distinguish between infection and disease in clinical microbiology.

• Infection: The growth and establishment of a pathogen in a host, which may or may not result in disease.

• Disease: The damage caused to host tissues by the pathogen, resulting in clinical symptoms.

Example: HIV infection may not immediately cause disease, but progression to AIDS results in disease symptoms.

Microbial Adherence

Mechanisms of Adherence

Adherence is the enhanced ability of microbes to attach to host cells or surfaces, a critical first step in pathogenesis.

• Adherence often occurs at mucosal surfaces (e.g., respiratory tract, gastrointestinal tract).

• Pathogens bind to tissues via receptor molecules on host cell surfaces.

• Adhesins: Glycoproteins or lipoproteins on the pathogen surface that enable binding to host cells.

Example: Staphylococcus aureus uses the SdrG adhesin to bind to fibrinogen on host tissues.

Adherence Structures

Microorganisms utilize various structures to facilitate adherence.

• Pili, fimbriae, capsules, and flagella are common adherence structures.

• Capsules can enhance adherence and protect against host defenses.

Example: Bacillus anthracis capsule aids in adherence and evasion of phagocytosis.

Colonization and Invasion

Colonization

Colonization refers to the growth of microorganisms after they have gained access to host tissues.

• Typically begins at mucous membranes, which are coated in mucus (a thick secretion of proteins and glycoproteins).

• Biofilm formation can occur during colonization, enhancing microbial persistence.

Invasion

Invasion is the ability of a pathogen to enter host cells or tissues, spread, and cause disease.

• Pathogens may grow locally or spread systemically.

• Bacteremia: Presence of bacteria in the bloodstream.

• Septicemia: Bacteria multiply in the bloodstream, leading to systemic infection, inflammation, septic shock, and potentially death.

Virulence and Virulence Factors

Definition and Strategies

Virulence is the relative ability of a pathogen to cause disease. Pathogens employ various strategies to establish virulence.

• Virulence factors: Substances or strategies that facilitate and promote infection (e.g., toxins, enzymes, adhesins).

Measuring Virulence

Virulence can be quantified experimentally using the LD50 (lethal dose 50).

• LD50: The amount of an agent that kills 50% of the animals in a test group.

$LD_{50} = \text{Dose at which 50\% of test animals die}$

Example: Streptococcus pneumoniae is highly virulent, requiring fewer cells to reach LD50 than Salmonella enterica.

Attenuation

Attenuation is the decrease or loss of virulence of a pathogen, often observed when pathogens are grown in laboratory culture.

• Attenuated strains are important in vaccine development (e.g., live attenuated vaccines).

Salmonella Virulence Factors

Genetic Elements

Salmonella species encode numerous virulence factors.

• Pathogenicity islands: Clusters of genes on the chromosome that encode virulence traits.

• Virulence plasmids: Extrachromosomal DNA elements carrying additional virulence genes.

Specific Factors

• Type I fimbriae (adherence)

• Endotoxin in LPS layer (fever)

• Injectisome (protein secretion system)

• Siderophores (iron uptake)

• Anti-phagocytic proteins

• Cytotoxins (inhibit host cell protein synthesis)

• Flagellum (motility)

Compromised Host

Definition and Examples

A compromised host has one or more inactive resistance mechanisms, increasing the probability of infection.

• Factors include immunosuppression, cancer, treatment with immunosuppressive drugs, poor diet, stress, and alcohol use.

• Opportunistic pathogens cause disease only in the absence of normal host resistance.

Enzymes as Virulence Factors

Tissue-Destroying Enzymes

Certain enzymes produced by pathogens facilitate invasion by breaking down host tissues.

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

• Collagenase: Degrades collagen, allowing deeper tissue invasion.

Coagulase and Streptokinase

• Coagulase: Forms clots, protecting pathogens from immune cells.

• Streptokinase: Breaks down clots, facilitating pathogen spread.

Exotoxins

Definition and Categories

Exotoxins are proteins released from the pathogen cell as it grows, causing toxicity by inhibiting host cell function or killing host cells.

• AB toxins: Consist of two subunits (A and B); B binds to host cell receptor, A exerts toxic effect.

• Cytolytic toxins: Degrade cytoplasmic membrane integrity, causing cell lysis and death.

• Superantigen toxins: Overstimulate the immune system, leading to toxic shock syndrome (TSS).

AB Toxins

• Examples include diphtheria toxin, botulinum toxin, and tetanus toxin.

• Botulinum toxins are among the most potent biological toxins known.

• Tetanus toxin is an AB protein neurotoxin affecting neurotransmitter release.

Cytolytic Toxins

• Hemolysins: Toxins that lyse red blood cells.

• Staphylococcal α-toxin: Kills nucleated cells and lyses erythrocytes.

Superantigens

• Cause excessive activation of the immune system, leading to shock and death.

• Staphylococcus aureus TSS is usually localized; Streptococcus pyogenes TSS is typically systemic.

Endotoxins

Definition and Detection

Endotoxins are the lipopolysaccharide (LPS) portion of the cell envelope of certain gram-negative Bacteria; they are toxic when solubilized.

• Generally less toxic than exotoxins.

• Detected by the Limulus amoebocyte lysate (LAL) assay.

Comparison of Exotoxins and Endotoxins

Properties Table

The following table summarizes the key differences between exotoxins and endotoxins:

Property	Exotoxins	Endotoxins
Chemistry	Proteins (often enzymes)	Lipopolysaccharide (LPS) complex
Mode of Action; Symptoms	Specific targets; varied symptoms (e.g., paralysis, cell lysis)	General effects; fever, inflammation, shock
Toxicity	High (potent toxins)	Low to moderate
Immune Response	Strong, can induce antitoxin antibodies	Weak, poor immunogenicity
Toxoid Potential	Can be inactivated to form toxoids (used in vaccines)	Cannot be converted to toxoids
Fever Potential	Usually none	High (pyrogenic)
Genetic Origin	Plasmid or chromosomal genes	Chromosomal (LPS genes)

Review Questions

• Why do gram-positive bacteria not produce endotoxins?

• What key features are shared by all AB exotoxins?

• What circumstances can contribute to attenuation of a pathogen?

Additional info: This study guide covers topics from Ch. 24+25 (Microbial Symbioses with Humans | Microbial Infection and Pathogenesis) and Ch. 28 (Immune Disorders and Antimicrobial Therapy) of a college-level microbiology course.