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 enter, survive, and damage the host. Understanding these processes is essential for microbiology students to grasp the basis of infectious diseases and host-pathogen interactions.

How Microorganisms Enter a Host

Portals of Entry

Pathogens must enter the host through specific portals to initiate infection. The main portals of entry include:

• Mucous membranes: Entry via respiratory, gastrointestinal, and genitourinary tracts. The respiratory tract is the most common portal (e.g., influenza, pneumonia).

• Skin: Although unbroken skin is a strong barrier, some microbes enter through hair follicles or sweat gland ducts (e.g., trachoma, conjunctivitis).

• Parenteral route: Microbes are deposited directly into tissues beneath the skin or mucous membranes, often via bites, cuts, or injections (e.g., tetanus, hepatitis viruses).

Preferred Portal of Entry

Most pathogens have a preferred portal of entry. If they enter by another route, disease may not occur.

Numbers of Invading Microbes

ID50 and LD50

• ID50 (Infectious Dose 50): The number of pathogen cells or virions required to cause infection in 50% of a test population. It measures the virulence of a microbe.

• LD50 (Lethal Dose 50): The amount of toxin required to kill 50% of a test population. It measures the potency of a toxin.

Example: The LD50 for botulinum toxin in mice is 0.03 ng/kg, indicating extreme potency.

Adherence to Host Tissues

Mechanisms of Adherence

Adherence is a critical step in infection. Pathogens use surface molecules called adhesins (ligands) to bind specifically to complementary receptors on host cells. These adhesins may be located on the glycocalyx or fimbriae.

• If adhesins or receptors are altered, infection can often be prevented.

• Microbes can form biofilms, communities that adhere to surfaces and are protected by a glycocalyx. Biofilms enhance microbial survival and resistance (e.g., dental plaque).

How Pathogens Penetrate Host Defenses

Capsules and Cell Wall Components

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

• M protein: Resists phagocytosis (e.g., Streptococcus pyogenes).

• Opa protein: Facilitates attachment to host cells (e.g., Neisseria gonorrhoeae).

• Mycolic acid: Waxy lipid in cell walls resists digestion (e.g., Mycobacterium tuberculosis).

Enzymes as Virulence Factors

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

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

• Hyaluronidase: Digests polysaccharides holding cells together, facilitating tissue invasion.

• Collagenase: Breaks down collagen in connective tissue.

• IgA proteases: Destroy IgA antibodies, aiding in immune evasion.

Antigenic Variation

Some pathogens alter their surface antigens, rendering antibodies ineffective. This allows them to evade the immune response.

Penetration into Host Cells

• Invasins: Surface proteins that rearrange actin filaments, causing membrane ruffling and facilitating entry (e.g., Salmonella).

• Some bacteria use actin to move between cells (e.g., Shigella, Listeria).

Biofilms and Immune Evasion

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

How Bacterial Pathogens Damage Host Cells

Mechanisms of Host Cell Damage

1. Using host nutrients (e.g., iron via siderophores)

2. Direct damage to host cells

3. Production of toxins

4. Inducing hypersensitive reactions

Siderophores

Siderophores are proteins secreted by pathogens to scavenge iron from the host, which is essential for bacterial growth.

Toxins

• Toxins: Poisonous substances produced by microorganisms that can cause fever, shock, and other symptoms.

• Toxigenicity: The 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 Gram-positive, that are highly potent and specific in action.

• Genes for exotoxins are often carried on plasmids.

• Types of exotoxins:

  • A-B toxins: Consist of an active (A) and binding (B) component (e.g., diphtheria toxin).

  • Membrane-disrupting toxins: Cause cell lysis (e.g., hemolysins, leukocidins).

  • Superantigens: Trigger excessive immune responses, leading to shock.

  • Genotoxins: Damage DNA, potentially leading to cancer.

Table: Diseases Caused by Exotoxins

Disease	Bacterium	Type of Exotoxin	Mechanism
Botulism	Clostridium botulinum	A-B	Neurotoxin prevents nerve impulse transmission; flaccid paralysis.
Tetanus	Clostridium tetani	A-B	Blocks muscle relaxation pathway; uncontrollable contractions.
Diphtheria	Corynebacterium diphtheriae	A-B	Inhibits protein synthesis in nerve, heart, kidney cells.
Toxic shock syndrome	Staphylococcus aureus	Superantigen	Causes fluid loss, low blood pressure.
Traveler’s diarrhea	E. coli, Shigella spp.	A-B	Enterotoxin causes diarrhea.
Anthrax	Bacillus anthracis	A-B	Shock, reduced immune response.

Endotoxins

• Lipid A component of lipopolysaccharide (LPS) in Gram-negative bacteria.

• Released during bacterial death or multiplication.

• Cause general symptoms: fever, shock, weakness, and can trigger miscarriages.

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

Table: Comparison of Exotoxins and Endotoxins

Property	Exotoxins	Endotoxins
Chemistry	Proteins (A-B structure)	Lipid A (LPS)
Source	Gram-positive & Gram-negative	Gram-negative
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	Larger

Pathogenic Properties of Viruses, Fungi, Protozoa, Helminths, and Algae

Viruses

• Cause cytopathic effects (CPE) such as cell death, inclusion bodies, syncytia formation, and chromosomal changes.

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

Fungi

• Produce toxic metabolic products (e.g., aflatoxin, ergot alkaloids).

• Can provoke allergic responses and inhibit protein synthesis (e.g., trichothecene toxins).

• Capsules prevent phagocytosis.

Protozoa

• Cause disease by growing in host tissues, producing waste products, and evading immune responses via 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) that cause paralytic shellfish poisoning.

Portals of Exit

Major Portals of Exit

Pathogens leave the host via specific portals, often the same as the portals of entry:

• Respiratory tract: Coughing and sneezing

• Gastrointestinal tract: Feces and saliva

• Genitourinary tract: Urine and genital secretions

• Skin

• Blood: Via arthropod bites or contaminated needles

Additional info: This guide expands on the original notes with definitions, examples, and tables for clarity and exam preparation.