Lecture 6 Strep Throat

Strep Throat

Strep throat = common, transmissible disease.

• Caused by the bacterium Streptococcus pyogenes.

• S. pyogenes can normally be part of the human microbiome.

  • A microorganism can be part of the microbiomeAND still cause disease under certain conditions.

Streptococcus pyogenes

• Gram positive

• Firmicute (Bacillota)

• Aerotolerant anaerobe → Can survive with oxygen → But does fermentation, not respiration

• Cocci = round shape

• Strepto = forms chains

What makes S. pyogenes infective?

S. pyogenes is infectious because it has a variety of virulence factors.

These virulence factors:

• Permit adhesion to the host cell wall → Allows the bacteria to stick to the throat cells.

• Evade the host immune system → Helps it avoid being recognized and destroyed.

• Spread into tissues → Allows it to move deeper beyond the surface.

• Cause host cell damage → Through exotoxins and secreted enzymes.

Most of the damage to host cells is caused by:

• Exotoxins
  Secreted enzymes

Streptolysin

Streptolysin

Streptolysin is an exotoxin produced by Streptococcus pyogenes.

• It binds to host cell membranes and forms pores.

• These pores damage and kill cells (cytotoxicity).

• It is responsible for hemolysis (red blood cell destruction).

So hemolysis is just one type of cytotoxicity.

S. pyogenes as an invasive infection

• Strep throat (localized infection):

  • The bacteria attach to throat cells and multiply on the surface.

  • Streptolysin damages nearby cells and causes inflammation → sore throat.

• Invasive infection:

  • If the bacteria evade the immune system and break through the epithelial barrier, they can enter the bloodstream.

  • Once in the blood, they spread to tissues and can cause severe disease (sepsis, toxic shock, necrotizing fasciitis).

Big idea:
Localized infection = surface damage.
Invasive infection = immune evasion + bloodstream spread.

What is the immune system?

The body's defense against disease-causing organisms.

What is innate immunity?

Innate immunity is the immune system's generic response to any invasion (does not target specific pathogens)

1. Anatomical Barriers (First Line)
Prevent microbes from entering the body.

Examples:
Skin, mucus, tears, saliva, stomach acid.

Function:
Block, trap, wash away, or chemically kill microbes.

---

2. Inflammation (Second Line)
Occurs when microbes enter tissue.

• Immune cells recognize pathogens using PRRs.

• They release signaling molecules (histamine, cytokines).

• Blood vessels dilate and become leaky.

Results:
Redness, swelling, heat, pain.
Immune cells move in to fight infection.

---

3. Complement System
Proteins in the blood that activate in a chain reaction.

• Functions:

  • Punch holes in bacteria.

  • Tag bacteria for destruction (opsonization).

  • Recruit immune cells.

Pattern Recognition Receptors (PRRs)

Immune cells have Pattern Recognition Receptors (PRRs).

• PRRs detect PAMPs (Pathogen-Associated Molecular Patterns).

PAMPs are:

• Molecules found on microbes
• Not found on human cells
• Common across many pathogens

Examples:
LPS (Gram-negative bacteria)
Flagellin
CpG DNA
Lipoproteins

---

What happens when PRRs bind PAMPs?

When a PRR detects a PAMP:

• Immune cells release signaling molecules (histamine, cytokines)
• Blood vessels dilate
• Immune cells are recruited
• Inflammation occurs

What are immunoglobulins?

antibodies

• bind to molecules (antigens) associated with pathogens

• This binding or opsonization allows the immune system to recognize the pathogen

• The recognized pathogen is then inactivated/destroyed

How do horseshoe crab blood cells work?

• Limulus Amoebocyte Lysate (LAL)
  Extract from horseshoe crab blood cells.

Horseshoe crabs rely heavily on innate immunity (they do not have adaptive immunity like humans).

---

What does it recognize?

• Lipopolysaccharide (LPS) from Gram-negative bacteria
• Some bacterial toxins

LPS is a PAMP.

So this is similar to PRRs detecting PAMPs in humans.

---

What happens when LPS is detected?

It triggers immediate coagulation (clotting).

---

Why clot?

Clotting:

• Traps bacteria
• Prevents spread
• Localizes infection

This is an innate immune defense.

humoral immunity

Humoral immunity is the antibody-mediated immune response that occurs in body fluids like blood and serum.

cell-mediated immunity

Cell-mediated immunity is the immune response carried out by immune cells (especially T cells) that directly target and destroy infected cells.

Adaptive Immune System

(Acquired immune system)

• Acquired through prior exposure to pathogens or proteins (immunization)

• Responses to recognized pathogens are usually specialized to those pathogens

B and T lymphocytes (B and T Cells)

(Adaptive Immunity)

B and T cells are immune cells that recognize specific pathogens.

When they detect a pathogen:

• They activate
• They multiply (clonal expansion)
• They create many identical cells that target that same pathogen

---

What they do:

B cells
• Produce antibodies
• Antibodies bind to pathogens and help the immune system destroy them

T cells
• Help coordinate the immune response
• Some T cells directly kill infected cells

What are M protein

• M protein is a surface virulence factor of S. pyogenes.

• It helps the bacteria adhere to host cells.

• It helps the bacteria evade the immune system by preventing opsonization.

Opsonization = tagging a pathogen so immune cells can recognize and eat it.

How do pathogens breach the layers of defense

1⃣ “Go Dark”

Avoid being recognized.

How?

• Change surface molecules
• Hide or reduce exposed antigens

Result:

• Less antibody binding
• Harder for immune cells to detect

Big idea: They avoid detection.

---

2⃣ “Fight On”

Resist immune attack after being detected.

Examples:

• Prevent phagocytosis (like M protein) — when immune cells engulf and destroy bacteria
• Survive inside immune cells

Big idea: Even if the immune system sees them, they don’t die easily.

---

3⃣ “Blind Them”

Interfere with immune communication.

Examples:

• Block immune signaling
• Interfere with complement

Big idea: Weaken or delay the immune response.

Characteristics of Staphylococcus aureus

Not Always a Pathogen

What it is:

• Gram positive

• Firmicute

• Facultative anaerobe (can live with or without oxygen)

30% of people are carriers

About 1 in 3 people normally carry S. aureus:

• On skin

• In the nasal passages

• In the lower reproductive tract of women

And they are completely healthy.

So this bacterium is part of the normal microbiome for many people.

Why does Staphylococcus aureus (s.aureus) become a pathogen?

Any breach of anatomical immunity (epidermis) can permit entry of S. aureus

• It is normally part of the microbiome (on skin, nose).

• The skin is an innate immune barrier.

• When the barrier is broken (cut, burned, surgically removed, etc.), the bacteria can enter deeper tissue.

• Once inside, it can use virulence factors to evade the immune system and cause infection.

Staphylococcus aureus in conjunctivitis

The eyes are a moist environment, warm and protected

• It's anaerobic (no oxygen)

• Tis created a good environment for bacteria

Conjunctivitis is transmitted through touching eyes and transmitting pus to others, and contact lens misuse

Is conjunctivitis an STD?

It can be, but most cases are not.

Most conjunctivitis:

• Viral (adenovirus)

• Not sexually transmitted

👉 Normal microbiome members