15.3 Virulence Factors of Bacterial and Viral Pathogens

Virulence factors

Pathogen product that assists in ability to cause infection and disease

• dictate how severe & extensive a disease is

• Some have more than one = more virulent

Adhesion factors

Type of virulence factor

Exoenzymes

Type of virulence factor

Toxins

Type of virulence factor

Immune evasion

Type of virulence factor

Adhesins

proteins that aid in attachment to host cell receptors

Where are adhesins found?

Found in all microbial types

(viral, fungal, bacterial, etc.)

• Commonly found on fimbriae or pili

• Can initiate biofilm formation in some species

many pathogens achieve invasion via

bloodstream

-bacteremia

Bacteria in blood

-viremia

Viruses in blood

-toxemia

Toxins in blood

-septicemia

Bacteria present and multiplying in blood

Patients w/ septicemia

Can lead to shock (life-threatening decrease in BP)

What happens when bacteria are engulfed by immune system phagocytes?

They release tumor necrosis factor causing severe inflammatory reaction and loss of fluid from the circulatory system, leading to shock.

Exoenzymes

Extracellular enzymes used to invade host tissues.

Types of Exoenzymes

Glycohydrolases, nucleases, phospholipases, proteases.

Collagenase

Produced by C. perfringens causing necrosis and gas gangrene.

Toxins

Biological poisons that assist in ability to invade and cause tissue damage (toxigenicity).

Endotoxins

Lipopolysaccharides that trigger host inflammatory responses; can cause severe fever and shock.

Exotoxins

Proteins mostly produced by Gram (+) bacteria; targets receptors on specific cells.

detecting endotoxins

1. Limulus amebocyte lysate (LAL) Test

2. ELISA

Limulus amebocyte lysate (LAL) Test

Blood cells of the horseshoe crab mixed with patient's serum; observed chromogenically or by coagulation.

ELISA

Enzyme-linked immunosorbent assay; uses antibodies to detect endotoxins.

Exotoxins

Can be further divided into intracellular targeting, membrane-disrupting, and superantigen.

Intracellular targeting

Exotoxins with A & B regions for activity and binding; examples include diphtheria & botulinum toxin.

Membrane-disrupting

Also known as phospholipases that degrade bilayer membrane; examples include Bacillus anthracis & Rickettsia spp.

Hemolysins and Leukocidins

Can target RBC, WBC, and other cells.

Superantigen

Triggers excessive production of cytokines by immune cells; examples include Staphylococcus aureus and Toxic Shock Syndrome.

Host evasion

Mechanisms to evade phagocytosis

Capsules

Enlarge bacterial cell so phagocytes cannot engulf pathogens

Proteases

Digest host antibody molecules

Mycolic acid

Helps evade phagolysosomes in acid fast bacteria (M. tuberculosis)

Coagulase positive microbes

Can coagulate blood cells to keep immune cells out of reach

Antigenic variation

Alteration of cell surface proteins to hide from immune cell recognition

HIV glycoprotein 120

For binding to CD4 T-cells

Influenza virus mutation

High mutation of envelope spikes allows for antigenic variation

Antigenic drift

Result of point mutations causing slight changes in spike proteins (H & N)

Antigenic shift

Major change in spike proteins due to gene reassortment