Lecture 19

Question 1

Question: What are the layers of the skin, and their primary function?

Answer 1

Answer:
Stratum corneum (outermost): Dead cells providing a barrier.
Epidermis: Protects from pathogens and water loss.
Dermis: Contains nerves, vessels, and appendages.
Subcutis/Subcutaneous (hypodermis): Insulation and energy storage.
Fascia: Connective tissue separating skin from muscles.

Question 2

Question: What appendages are found in the skin, and what role do they play?

Answer 2

Answer:
Hair follicles: Produce hair, involved in thermoregulation.
Sweat glands: Secrete sweat to regulate body temperature and excrete waste.
Sebaceous glands: Produce sebum for skin lubrication.
Immune Role: Appendages can be sites for microbial infections.

Question 3

Question: How does the skin prevent infections, and what host factors promote infections?

Answer 3

Answer:
Prevention:
- Physical barrier: Stratum corneum and tight junctions in epidermis.
- Chemical barrier: Sebum, sweat (antimicrobial peptides).
- Immune cells: Langerhans cells in the epidermis, mast cells in the dermis.
Promotion:
- Disrupted barriers: Wounds, cuts.
- Excessive moisture: Promotes microbial growth.
- Poor circulation or immune suppression.

Question 4

Question: What are the main virulence factors of Staphylococcus aureus?

Answer 4

Answer:
Exfoliative toxins: Cause skin peeling (e.g., in Scalded Skin Syndrome).
Protein A: Inhibits phagocytosis by binding antibodies.
Coagulase: Converts fibrinogen to fibrin, allowing bacteria to clot plasma and evade immune detection.

Question 5

Question: How is Staphylococcus aureus transmitted?

Answer 5

Answer:
Spread person-to-person.
Colonization in nares (20-40%), then spreads to skin.
Can also infect via direct contact with infected wounds or contaminated surfaces.

Question 6

Question: What are the common diseases caused by Staphylococcus aureus?

Answer 6

1. Impetigo:
   
   • A superficial skin infection that is often seen in children.
   • It presents as red sores or blisters that can ooze and form a yellowish crust.
   • This infection does not scar and typically heals with antibiotic treatment.
2. Staphylococcal Scalded Skin Syndrome (SSSS):
   
   • Caused by S. aureus strains that produce exfoliative toxins, which lead to the peeling of the outer skin layer.
   • Primarily affects young children and infants.
   • No scarring occurs, as the deeper layers of skin are not damaged.
3. Furuncles (boils):
   
   • A deep infection of hair follicles, resulting in a painful, pus-filled lump.
   • Boils are generally localized infections but can be painful and inflamed.
4. Carbuncles:
   
   • Larger abscesses that form when multiple furuncles merge together.
   • They indicate a more severe infection and may suggest a systemic infection (meaning the bacteria might spread beyond the local site).
   • Carbuncles often require more aggressive treatment and may need to be drained.

Question 7

Question: What are the characteristics of Streptococcus pyogenes infections?

Answer 7

Answer:
Present on mucous membranes (mouth, throat) in <5% of people.
Causes rapidly spreading infections like erysipelas.
Can also cause impetigo, often co-infected with S. aureus.

Question 8

Question: What skin diseases are caused by Streptococcus pyogenes?

Answer 8

Answer:
Impetigo: Co-infection with S. aureus, affects epidermis.
Erysipelas: Affects the dermis, bright red lesions due to an immune reaction.
Cellulitis: Infection spreads to the deeper dermis and subcutaneous layers, causing hot, red skin with fever.

Question 9

Question: What virulence factors contribute to the pathogenicity of S. pyogenes?

Answer 9

Answer:
M-protein: Prevents phagocytosis.
Streptolysins: Break down red and white blood cells.
Hyaluronidase: Spreads infection by breaking down connective tissue.

Question 10

Question: What is the standard treatment for skin infections caused by Staphylococcus aureus and Streptococcus pyogenes?

Answer 10

Answer:
Antibiotics such as penicillin or cephalosporins are commonly used.
In severe cases, drainage of abscesses (e.g., furuncles or carbuncles) may be necessary.
Topical antibiotics for superficial infections like impetigo.

Question 11

Question: What causes acne vulgaris, and how is it treated?

Answer 11

Answer:
Cause: Overproduction of sebum by sebaceous glands, leading to blocked pores and infection by Propionibacterium acnes.
Treatment: Antibiotics, antibacterial agents, or retinoids like isotretinoin to reduce oil production.

Question 12

Question: What is folliculitis, and what causes it?

Answer 12

Answer:
Folliculitis: Infection and inflammation of hair follicles.
Cause: Most commonly caused by Staphylococcus aureus.
Special form: Stye on the base of the eyelid.

Question 13

Question: How can skin infections be prevented?

Answer 13

Answer:
Hygiene: Regular washing to remove pathogens.
Wound care: Proper care of cuts and abrasions to prevent microbial entry.
Barrier protection: Using moisturizers to maintain skin integrity, especially in dry or cracked skin.

Question 14

Question: What is necrotizing fasciitis and what layers of the skin are involved?

Answer 14

Answer:
Necrotizing fasciitis is a severe infection that affects the superficial fascia and subcutaneous fat.
It leads to rapid tissue destruction and can result in gangrene or patchy anesthesia due to tissue undermining.

Question 15

Question: What are the two main causes of necrotizing fasciitis?

Answer 15

Answer:
Streptococcus pyogenes: Known as “flesh-eating bacteria”, produces toxins that damage tissues, leading to rapid spread.
Mixed infection of facultative and obligate anaerobes (often from bowel origin, e.g., Clostridium perfringens): Produces bubbles of gas and alpha toxins, causing tissue destruction.

Question 16

Question: How does Streptococcus pyogenes contribute to necrotizing fasciitis?

Answer 16

Answer:
Produces toxins that rapidly destroy host tissues.
Can lead to systemic infection and complications like streptococcal toxic shock syndrome (TSLS).

Question 17

Question: How does Clostridium perfringens contribute to necrotizing fasciitis?

Answer 17

Answer:
Clostridium perfringens invades tissues, producing gas bubbles.
The bacteria release alpha toxin, which can enter the bloodstream, causing hemolysis (destruction of red blood cells) and death.
Antibiotics often cannot cure it, and surgical intervention such as amputation may be necessary.

Question 18

Question: Why is Staphylococcus aureus such a significant skin pathogen?

Answer 18

Answer:
It has tough Gram-positive cell walls, providing protection against environmental stress.
It can tolerate high salt concentrations (7.5% NaCl), wide pH ranges (4-10), and varying temperatures, making it highly adapted to the skin.

Question 19

Question: What virulence factors make Staphylococcus aureus a successful pathogen?

Answer 19

Answer:
Cell capsule: A small, thin layer that prevents phagocytosis by immune cells.
Toxins: Including TSST-1, exfoliatin, and enterotoxins, which damage host tissue and cause inflammation.
Invasins: Enzymes like coagulase and staphylokinase help break down host tissues and facilitate bacterial spread.
Immunoglobulin-binding proteins (Protein A): Bind to and neutralize antibodies.
Adhesins: Promote adherence to host tissues and help evade immune response.

Question 20

Question: How is Staphylococcus aureus transmitted and where is it most commonly found?

Answer 20

Answer:
The anterior nares (nostrils) are the main reservoir of S. aureus in humans.
About 20% of people are persistent carriers, while others are intermittent carriers or non-carriers.
Persistent carriers have a higher density of S. aureus and are at increased risk of endogenous infections.

Question 21

Question: How is Staphylococcus aureus skin infection treated?

Answer 21

Answer:
No vaccines are available.
Treatment depends on severity: Topical or oral antibiotics for simple skin infections, and broad-spectrum intravenous antibiotics for more severe cases.
Prolonged treatment is often required, and treatment failure is common, especially in resistant strains.

Question 22

Question: What is MRSA, and why is it a problem in treating Staphylococcus aureus infections?

Answer 22

Answer:
MRSA stands for methicillin-resistant Staphylococcus aureus, a strain resistant to commonly used antibiotics.
These strains are often multi-drug resistant, making infections harder to treat.

Question 23

Question: How can MRSA be controlled and prevented?

Answer 23

Answer:
Good hygiene and proper wound care can reduce the spread.
In healthcare settings, screening carriers and using antibiotic stewardship (limiting unnecessary antibiotic use) are key preventive measures.

Question 24

Suggest three ways in which pathogens can penetrate the barrier of the stratum corneum.

Answer 24

Answer:
Trauma: Physical damage like cuts or abrasions can disrupt the skin barrier.
Inflammation: Conditions like eczema or dermatitis can compromise the integrity of the stratum corneum.
Excess moisture (maceration): Prolonged exposure to moisture softens the skin, making it more susceptible to infection.

Question 25

Explain to a non-scientist/lay person how low pH and high salt conditions naturally found on the skin help protect us from pathogens.

Answer 25

Answer:
The skin has low pH because it produces acids, making it harder for harmful bacteria to survive. Most bacteria prefer neutral pH environments.
The skin also has high salt levels, especially from sweat. This salt creates a harsh environment that many bacteria cannot tolerate, helping prevent infections.

Question 26

Explain the term “endogenous infection” in the context of persistent carriers of Staphylococcus aureus.

Answer 26

Answer:
An endogenous infection occurs when bacteria that are naturally present ( opportunisitc) on or inside the body, like Staphylococcus aureus, cause an infection.

It’s termed endogenous because the source of the infection is within the body itself — specifically, from bacteria that were peacefully colonizing but have now turned harmful due to a change in conditions (like a weakened immune barrier or open wound).

In persistent carriers, S. aureus lives in areas like the nose, and if it spreads to a break in the skin or an area of compromised immunity, it can cause an infection from within the person’s own body rather than from an external source.

Question 27

Barriers to infection

Answer 27

Physical Barrier: Stratum corneum (SC)
Intact stratum corneum and epidermis
Shedding of stratum corneum
Host immune system
Environment:
Low skin pH - acidic due to high sebum
High salt from sweat gland
Resident microbes:
Compete for nutrients and space
Stimulate the immune system

Question 28

Skin infection

Answer 28

Skin infection occurs when the skin barrier is disrupted by:
Trauma
Inflammation
Maceration due to excess moisture
or in skin pores and follicles