Chapter 15: MICROBIAL MECHANISMS OF PATHOGENICITY

What is pathogenicity?

The ability of a microbe to cause disease.

What is virulence?

The degree or intensity of pathogenicity; how severe the disease-causing ability is.

What are virulence factors?

Microbial traits that help pathogens enter, survive, spread, and damage the host.

What are the 4 major stages in the big picture of pathogenicity?

Portal of entry, Penetration/evasion of host defenses, Damage to host, Portal of exit.

How do pathogens overcome host defenses?

By entering the host, attaching to tissues, hiding from immunity, spreading through tissues, and exiting to infect new hosts.

What is the 1st line of defense?

Prevents entry (skin, mucous membranes, tears, saliva, stomach acid, microbiota).

What is the 2nd line of defense?

Limits spread through inflammation, fever, phagocytes, and antimicrobial proteins.

What is the 3rd line of defense?

Specific immunity involving B cells, T cells, antibodies, and immune memory.

What is a portal of entry?

The route a pathogen uses to enter the body.

What are the main portals of entry?

respiratory tract, GI tract, genitourinary tract, conjunctiva, skin breaks, parenteral route (needles, bites, wounds).

What does parenteral route mean?

Direct entry beneath skin or mucous membranes through punctures, injections, bites, or surgery.

What is ID50?

Infectious dose required to infect 50% of a population.

What is LD50?

Lethal dose required to kill 50% of a population.

What does a lower ID50 or LD50 mean?

The pathogen or toxin is more virulent / potent.

Why is adherence important in infection?

Without attachment, microbes are removed by mucus, urine, tears, saliva, and peristalsis.

What are adhesins?

Molecules on pathogen surfaces that bind host cell receptors.

What are host receptors usually made of?

Sugars such as mannose.

What bacterial structures help with adherence?

fimbriae, pili, capsules, flagella.

What is a biofilm?

A community of microbes attached to a surface and protected by an extracellular matrix.

Why are biofilms dangerous?

They resist antibiotics, disinfectants, and immune cells.

Example of microbial cooperation in adherence?

Streptococcus mutans + Actinomyces in dental plaque/cavities.

What is a portal of exit?

The route pathogens leave the host.

Common portals of exit?

respiratory droplets, feces, urine, blood, skin lesions, genital secretions.

Why is a capsule a virulence factor?

It prevents phagocytosis and helps bacteria evade the immune system.

What line of defense does the capsule mainly avoid?

2nd line defense (phagocytes).

Examples of encapsulated pathogens?

Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Bacillus anthracis, Yersinia pestis.

What does mycolic acid do in Mycobacterium tuberculosis?

Makes it waxy, resistant to drying, disinfectants, and immune destruction.

What does Opa protein do in Neisseria gonorrhoeae?

Helps adherence and antigenic variation.

What does M protein do in Streptococcus pyogenes?

Prevents phagocytosis and complement activation.

Difference between endoenzymes and exoenzymes?

Endoenzymes = remain inside cell; Exoenzymes = secreted outside cell.

What does coagulase do?

Causes clot formation around bacteria to hide from immune cells.

What do kinases do?

Break down clots to allow bacterial spread.

What does hyaluronidase do?

Breaks down connective tissue to help bacteria spread.

What does collagenase do?

Breaks down collagen in tissues.

What do IgA proteases do?

Destroy IgA antibodies on mucous membranes.

What is antigenic variation?

Pathogens change their surface antigens to avoid immune recognition.

What line of defense does antigenic variation avoid?

3rd line defense (adaptive immunity).

Important example of antigenic variation?

Influenza virus / flu vaccine changes yearly.

What are invasins?

Proteins that allow bacteria to enter host cells.

What is membrane ruffling?

Rearrangement of host cell membrane/cytoskeleton that pulls bacteria into the cell.

Why does being inside host cells help bacteria?

Antibodies and phagocytes cannot easily reach them.

What are siderophores?

Iron-binding molecules secreted by bacteria to steal iron from the host.

Why is iron important?

Bacteria need it for growth and metabolism.

What is direct damage?

Tissue damage caused by bacterial invasion, growth, and nutrient theft.

What are exotoxins?

Protein toxins released outside the bacterium.

Are exotoxins potent in small amounts?

Yes, extremely potent.

Are exotoxins target specific?

Yes — specific tissues/organs.

What is an A-B toxin?

B = binds host cell; A = active toxic enzyme portion.

What do membrane-disrupting toxins do?

Create holes in cell membranes causing lysis.

What do superantigens do?

Cause massive cytokine release and can lead to shock.

What does botulinum toxin cause?

Flaccid paralysis.

What does tetanus toxin cause?

Spastic paralysis.

What does Vibrio enterotoxin cause?

Severe watery diarrhea.

What does diphtheria toxin do?

Inhibits protein synthesis.

What does staphylococcal enterotoxin cause?

Food poisoning, nausea, vomiting.

Where are endotoxins found?

Gram-negative outer membrane.

What part of endotoxin is toxic?

Lipid A of LPS.

Are endotoxins proteins?

No, they are lipid-based.

When are endotoxins released?

When bacterial cells die or cell wall is damaged.

Common endotoxin symptoms?

Fever, chills, aches, shock, low blood pressure.

Why can endotoxins cause septic shock?

Lipid A stimulates cytokine release → increased vascular permeability + hypotension.

How do plasmids increase pathogenicity?

They carry virulence genes and can be transferred to other bacteria.

What is lysogenic conversion?

A bacteriophage adds virulence genes to bacteria.

Important examples?

Diphtheria toxin and cholera toxin genes can come from phages.

How do viruses cause pathogenicity?

Enter host cells, reproduce, evade immunity, and cause cytopathic effects.

Examples of cytopathic effects?

cell death, lysis, syncytia, inclusion bodies, abnormal cell shape.

How do fungi cause disease?

Tissue invasion, inflammation, and toxin production.

How do protozoa cause disease?

Cell invasion and nutrient theft.

How do helminths cause disease?

Mechanical tissue damage and obstruction.