Group A Strep and External Bacterial Structures

Exam Information

Exam 2 Updates
- No matching section for diseases, unlike Exam 1.
- Always refer to study guides for disease-specific topics, as they are tailored to pertinent information.

Group A Strep (Streptococcus pyogenes)

Bacterial Characteristics

Causative Agent: Streptococcus pyogenes (identifying as Group A Strep, or GAS).
Gram Stain: Gram-positive bacterium.
Motility: Non-motile.
Morphology: "Strepto-" indicates chains of spheres (cocci), or occasionally diplococcus (pairs of circles).
Capsule: Usually present.
Oxygen Requirement: Facultative anaerobe – meaning it grows with or without oxygen, but prefers oxygen.
Hemolysis: Beta-hemolysis positive – this means it breaks down red blood cells (erythrocytes).
- Visual Identification: When plated on blood agar (containing erythrocytes), S. pyogenes will create a clear halo around its colonies, indicating the breakdown of blood cells. This is a distinctive diagnostic feature, as few bacteria possess this capability.

Virulence Factors (Enzymes)

These enzymes, identified by their "-ase" suffix, are crucial virulence factors that aid the bacteria in causing disease and spreading throughout the body.
- Streptokinase: Breaks down blood clots. The body's immune system often walls off infections with blood clots; streptokinase allows S. pyogenes to penetrate these barriers and spread.
- Hyaluronidase: Breaks down hyaluronic acid, a component of connective tissue found throughout the body, allowing the bacteria to move through tissues.
- Deoxyribonucleases (DNases): Dissolves DNA. This is highly damaging to host cells, as DNA is essential for cell survival, leading to cell death.

Spectrum of Infections

S. pyogenes can cause a range of infections:
- Asymptomatic Carriers: Approximately 5-15\% of infected individuals show no symptoms or disease state.
- Mild Illnesses:
  - Strep throat.
  - Impetigo.
- Severe/Invasive Cases:
  - Scarlet fever (systemic, non-invasive).
  - Necrotizing fasciitis (flesh-eating disease).
  - Toxic Shock Syndrome (TSS).

Epidemiology and Risk Factors

Incidence:
- Millions of mild S. pyogenes infections occur annually in the U.S.
- Approximately 9500 severe or invasive cases are reported each year.
Factors for Severe Forms:
- Compromised Barriers: Breaks in the skin barrier (e.g., cuts, wounds) provide entry points for the bacteria, potentially leading to necrotizing fasciitis.
- Immunocompromise: Healthcare-associated infections or chronic illnesses can make individuals more susceptible to severe S. pyogenes infections.

Specific Diseases Caused by S. pyogenes

1. Strep Throat

Symptoms: Scratchy throat, pustules (white spots on tonsils/back of throat), swollen lymph nodes (especially in the neck region).
Complication: If untreated, S. pyogenes can migrate from the throat to the heart, potentially causing heart valve damage, a severe complication known as rheumatic fever. This underscores the importance of prompt diagnosis and antibiotic treatment.
Anecdote: Instructor had tonsils removed at 18 due to recurrent strep throat, recommending against adult tonsillectomy due to difficulty.

2. Impetigo

Characteristics: A common bacterial skin infection, particularly prevalent in children ("kiddos") who are more susceptible.
Susceptibility: Athletes (e.g., high school football players) can also be prone to impetigo due to close contact and potential skin abrasions.
Symptoms: Characterized by a distinctive, often unsightly, rash usually appearing on the face.

3. Scarlet Fever

Association: Often a complication developing after a child has had strep throat.
Symptoms:
- Systemic Rash: The body is covered head-to-toe in a red rash, caused by a toxin produced by S. pyogenes that breaks down red blood cells. This indicates a systemic spread from a localized throat infection.
- Strawberry Tongue: Similar to the pustules of strep throat, but present on the tongue, often towards the back.
Progression: Untreated scarlet fever can further progress to rheumatic fever, leading to long-term complications, particularly affecting joints.

4. Necrotizing Fasciitis (Flesh-Eating Disease)

Pathology: An aggressive infection where the bacteria rapidly consumes layers of tissue, including skin, muscle, and fat, often affecting limbs.
Causative Factor: Caused by certain strains of S. pyogenes that produce exotoxin A.
Mortality: Extremely high; almost 50\% of individuals die from necrotizing fasciitis, even with medical treatment.
Treatment: Due to rapid spread, amputations are often necessary to halt the progression of the disease. Time is of the essence in treatment.
Diagnosis: Blood cultures are drawn to identify the bacteria in the bloodstream before drastic measures like amputation are taken.
Precaution: Class included cautionary warning for graphic images of necrotizing fasciitis.

5. Toxic Shock Syndrome (TSS)

Distinction: This form of TSS is distinct from the menstrual product-associated TSS.
Pathology: A severe, systemic condition characterized by a cascade of events leading to organ failure (e.g., kidneys) and a drop in blood pressure, as the body reacts severely to the infection.
Mortality: Very high, with a 1 in 2 mortality rate even after treatment has commenced.

Treatment for S. pyogenes Infections

Antibiotics: S. pyogenes is highly susceptible to antibiotics, particularly those in the penicillin family (e.g., amoxicillin). As of current knowledge, it has not developed resistance.
Severe Cases: For severe infections like necrotizing fasciitis, treatment extends beyond antibiotics to include interventions like surgical debridement and amputation to prevent further tissue destruction and systemic spread.
Impetigo Treatment: Often treated with topical antibiotic creams (e.g., clindamycin) rather than oral antibiotics, when possible.

External Bacterial Structures (Continuation)

Flagella Review (Eukaryotic vs. Prokaryotic)

Prokaryotic Flagella:
- Motion: Rotates like a helicopter propeller.
- Composition: A hollow tube made of flagellin protein.
Eukaryotic Flagella:
- Motion: Wave-like manner.
- Composition: Made of microtubulin protein, with an internal 9+2 arrangement (nine pairs forming a circle with two in the middle), and is not hollow.

Axial Filament

Nature: Technically a type of flagella, but it is an internal flagella, meaning the bacterial outer membrane surrounds it.
Specificity: Exclusively found in spirochetes (bacteria with a spring-like shape).
Function: The rotation of this internal flagella causes the entire spirochete cell to rotate, allowing it to "drill" through host tissues.
Example: Treponema pallidum, the bacterium causing syphilis, uses its axial filament to penetrate tissue during infection.
Distinction: Named differently due to its unique internal location and mechanism of action/composition compared to typical flagella.

Fimbriae and Pili

General Characteristics: These are external appendages, shorter and more hair-like than flagella. Both are made of pillin protein.
Specificity: Primarily found in Gram-negative bacteria.

Fimbriae

Abundance: Bacteria can produce many fimbriae, often 100 or more all over the cell, or concentrated at the ends (polar position).
Primary Function: Crucial for attachment to host tissues and surfaces.
- Clinical Significance: For example, Neisseria gonorrhoeae (which causes gonorrhea) relies heavily on fimbriae for attachment; without them, it cannot cause disease. Studies have shown transferring fimbriae from N. gonorrhoeae to E. coli enables E. coli to infect gonorrhea's target tissues.
Biofilm Formation: Also play a role in the initial attachment and subsequent formation of biofilms.

Pili (Sex Pilus)

Structure: Longer than fimbriae but shorter than flagella. These are less numerous, typically one or two per cell.
Primary Function: Conjugation (often referred to as "bacteria sex") – the direct transfer of DNA between two adjacent bacterial cells. A pilus forms a bridge between cells, allowing genetic material to pass from a donor to a recipient cell.
- (Details of the conjugation process will be covered in a later lecture on bacterial genetics).