Microbial Mechanisms of Pathogenicity: Study Notes

Microbial Mechanisms of Pathogenicity

Introduction

This chapter explores how microorganisms cause disease, focusing on the mechanisms by which pathogens invade hosts, evade defenses, and damage host cells. Understanding these processes is fundamental to microbiology and infectious disease control.

How Microorganisms Enter a Host

Portals of Entry

Pathogens must enter the host through specific sites known as portals of entry. The main portals include:

• Mucous membranes: Lining the respiratory, gastrointestinal, and genitourinary tracts.

• Skin: Usually impenetrable, but some pathogens enter through cuts or hair follicles.

• Parenteral route: Direct deposition into tissues via punctures, bites, or injections.

Most pathogens have a preferred portal of entry that is critical for their ability to cause disease.

Pathogenicity and Virulence

• Pathogenicity: The ability of a microorganism to cause disease.

• Virulence: The degree of pathogenicity, often measured by infectious or lethal dose.

Numbers of Invading Microbes

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

• LD50: Lethal dose for 50% of a sample population; measures toxin potency.

Adherence to Host Tissues

Pathogens attach to host cells using adhesins (ligands) that bind to specific receptors on host cell surfaces. Structures involved include glycocalyx and fimbriae. Microbes may also form biofilms, which are communities that share nutrients and resist immune responses.

How Pathogens Penetrate Host Defenses

Capsules and Cell Wall Components

• Capsules: Glycocalyx layers that impair phagocytosis (e.g., Streptococcus pneumoniae).

• M protein: Resists phagocytosis (Streptococcus pyogenes).

• Opa protein: Aids attachment (Neisseria gonorrhoeae).

• Mycolic acid: Waxy lipid resists digestion (Mycobacterium tuberculosis).

Enzymes as Virulence Factors

• Coagulases: Coagulate fibrinogen to protect bacteria from immune cells.

• Kinases: Digest fibrin clots to spread infection.

• Hyaluronidase: Digests polysaccharides holding cells together.

• Collagenase: Breaks down collagen in connective tissue.

• IgA proteases: Destroy IgA antibodies.

Antigenic Variation

Some pathogens alter their surface antigens to evade immune detection, rendering antibodies ineffective.

Penetration into Host Cells

• Invasins: Bacterial proteins that rearrange host actin, causing membrane ruffling and entry (e.g., Shigella, Listeria).

• Some bacteria survive inside phagocytes by escaping the phagosome, preventing lysosome fusion, or tolerating low pH.

Biofilms

Biofilms protect bacteria from phagocytosis and immune responses due to their extracellular polymeric substance (EPS).

How Bacterial Pathogens Damage Host Cells

Using the Host’s Nutrients: Siderophores

Pathogens secrete siderophores to bind iron more tightly than host proteins, facilitating bacterial growth.

Direct Damage

• Pathogens disrupt host cell function, use nutrients, produce waste, and may cause cell rupture by multiplying inside cells.

Production of Toxins

• Toxins: Poisonous substances produced by microbes, causing fever, shock, and tissue damage.

• Toxigenicity: Ability to produce toxins.

• Toxemia: Presence of toxins in the blood.

• Intoxications: Disease caused by toxins without microbial growth.

Exotoxins

• Proteins secreted by bacteria, usually affecting specific cell functions.

• Types include A-B toxins, membrane-disrupting toxins, superantigens, and genotoxins.

• Antitoxins are antibodies against exotoxins; toxoids are inactivated toxins used in vaccines.

A-B Toxins

• Consist of an active (A) and binding (B) component. The B part binds to host cells, and the A part exerts toxic effects (e.g., diphtheria toxin).

Membrane-Disrupting Toxins

• Lyse host cells by disrupting plasma membranes (e.g., hemolysins, leukocidins).

Superantigens

• Cause intense immune responses by stimulating excessive cytokine release, leading to fever, shock, and sometimes death.

Genotoxins

• Damage DNA, causing mutations and potentially cancer.

Endotoxins

• Lipid A portion of lipopolysaccharides (LPS) in the outer membrane of gram-negative bacteria.

• Released upon bacterial death, causing fever, inflammation, and shock.

Comparison of Exotoxins and Endotoxins

Property	Exotoxins	Endotoxins
Bacterial Source	Gram-positive and gram-negative	Gram-negative
Relation to Microorganism	Metabolic product of growing cell	Part of LPS, released on cell death
Chemistry	Proteins (often A-B)	Lipid A of LPS
Effect on Body	Specific (cell functions, nerves, GI tract)	General (fever, shock, aches)
Heat Stability	Unstable (destroyed at 60–80°C)	Stable (withstands autoclaving)
Toxicity	High	Low
Fever-Producing	No	Yes
Immunology	Can be neutralized by antitoxin	Not easily neutralized
Lethal Dose	Small	Large
Representative Diseases	Gas gangrene, tetanus, diphtheria	Typhoid fever, meningococcal meningitis

Detection of Endotoxins

• Limulus amebocyte lysate (LAL) assay: Uses horseshoe crab blood to detect endotoxins by clot formation.

Plasmids, Lysogeny, and Pathogenicity

• Plasmids: May carry genes for toxins, antibiotic resistance, and enzymes.

• Lysogenic conversion: Incorporation of a prophage can turn nonpathogenic bacteria into pathogens.

Pathogenic Properties of Viruses

Cytopathic Effects (CPE)

Viruses cause visible changes in infected cells, including:

• Stopping cell synthesis

• Causing lysosome release

• Forming inclusion bodies

• Fusing cells into syncytia

• Inducing chromosomal changes

• Loss of contact inhibition (cancer)

Interferons

• Alpha and beta interferons are produced by infected cells to protect neighboring cells by inhibiting viral protein synthesis and inducing apoptosis.

Pathogenic Properties of Fungi, Protozoa, Helminths, and Algae

Fungi

• Produce toxic metabolic products and provoke allergic responses.

• Some produce toxins (e.g., aflatoxin, ergot) and proteases that damage host tissues.

• Capsules prevent phagocytosis.

Protozoa

• Cause symptoms by their presence and waste products.

• Evade defenses by digesting cells, growing in phagocytes, and antigenic variation.

Helminths

• Use host tissues for growth, produce large masses, and release waste products that cause symptoms.

Algae

• Some produce neurotoxins (e.g., saxitoxin) causing paralytic shellfish poisoning.

Portals of Exit

Definition and Examples

Portals of exit are routes by which pathogens leave the host to infect new hosts. Common portals include:

• Respiratory tract (coughing, sneezing)

• Gastrointestinal tract (feces, saliva)

• Genitourinary tract (urine, secretions)

• Skin

• Blood (via arthropod bites, needles)

Summary Diagram

The process of microbial pathogenicity involves entry, evasion of defenses, damage to host cells, and exit from the host. Each step is crucial for the establishment and spread of infectious disease.