Diseases of the Skin

Basic Anatomy of the Skin

The skin is the largest organ of the human body and serves as a primary barrier against microbial invasion. It consists of two main layers: the epidermis (outer layer) and the dermis (inner layer). The skin also contains sweat glands, sebaceous glands, hair follicles, and associated structures.

• Epidermis: Outermost layer, composed of keratinized cells providing a tough, protective barrier.

• Dermis: Thicker, inner layer containing connective tissue, blood vessels, nerves, and glands.

• Functions: Protection, temperature regulation, sensation, and synthesis of vitamin D.

Common Terms Used to Describe Skin Lesions

• Vesicle: Small, fluid-filled lesion (≤1 cm in diameter).

• Bulla: Larger, fluid-filled lesion (>1 cm in diameter).

• Macule: Flat, discolored spot on the skin.

• Papule: Raised, solid lesion (≤1 cm in diameter).

• Pustule: Raised lesion containing pus.

• Sty: Infection of an eyelash follicle (usually caused by Staphylococcus species).

• Furuncle (boil): Localized region of pus surrounded by inflamed tissue, often involving a hair follicle.

• Carbuncle: Deep, interconnected group of furuncles, often more severe and involving deeper tissues.

Bacterial Skin Infections

Staphylococcal Skin Infections

Staphylococcus species are common skin flora but can cause a variety of skin infections, especially when the skin barrier is breached.

• Coagulase-positive organisms: Produce the enzyme coagulase, which clots fibrin in blood. Example: Staphylococcus aureus.

• Coagulase-negative organisms: Do not produce coagulase. Example: Staphylococcus epidermidis.

Features Contributing to Pathogenicity of Staphylococcus aureus

• Production of toxins (e.g., exfoliative toxin, toxic shock syndrome toxin).

• Enzymes such as coagulase, hyaluronidase, and lipases.

• Ability to evade host immune responses (e.g., Protein A binds IgG).

• Formation of biofilms on medical devices.

Staphylococcal Diseases

• Impetigo: Superficial, highly contagious skin infection, often in children; presents as vesicles that rupture and form honey-colored crusts.

• Scalded Skin Syndrome: Caused by exfoliative toxin; widespread blistering and peeling of the skin, primarily in infants and young children.

• Folliculitis: Infection of hair follicles, presenting as small, red, pus-filled bumps.

• Toxic Shock Syndrome: Acute, life-threatening illness caused by toxic shock syndrome toxin-1 (TSST-1); symptoms include fever, rash, hypotension, and multi-organ involvement.

Streptococcal Skin Infections

Streptococcus species, especially Streptococcus pyogenes (Group A Streptococcus), are important causes of skin and soft tissue infections.

• Hemolysis Patterns:

  • Alpha-hemolytic: Partial hemolysis, greenish discoloration on blood agar.

  • Beta-hemolytic: Complete hemolysis, clear zone on blood agar (e.g., S. pyogenes).

  • Gamma-hemolytic: No hemolysis.

Features Contributing to Pathogenicity of Streptococcus pyogenes

• Production of streptolysins (lyse red and white blood cells).

• Enzymes such as hyaluronidase and streptokinase.

• M protein (antiphagocytic factor).

• Capsule formation.

Streptococcal Diseases

• Erysipelas: Acute infection of the upper dermis; presents as raised, red, well-demarcated areas, often with fever and chills.

• Necrotizing Fasciitis: Rapidly spreading infection of the fascia and subcutaneous tissue; often called "flesh-eating disease." Requires prompt surgical intervention.

Pseudomonas aeruginosa Infections

Pseudomonas aeruginosa is a Gram-negative, opportunistic pathogen that thrives in moist environments and is notorious for infecting burns and wounds.

• Produces pigments (e.g., pyocyanin, which gives a blue-green color).

• Can cause pseudomonal dermatitis and hot tub folliculitis.

• Highly resistant to many antibiotics.

Clostridium perfringens and Gas Gangrene

• Gangrene: Death of body tissue due to loss of blood supply, often complicated by infection.

• Gas gangrene: Caused by Clostridium perfringens; bacteria produce toxins and gas in tissues, leading to rapid tissue destruction.

• Pathogenesis: Spores enter wounds, germinate in anaerobic conditions, and produce alpha toxin (lecithinase) that destroys cell membranes.

• Treatment: Surgical removal of dead tissue (debridement), high-dose intravenous antibiotics, and sometimes hyperbaric oxygen therapy.

Acne Formation and Types

• Comedonal acne: Blockage of hair follicles by sebum and dead cells; forms whiteheads and blackheads.

• Inflammatory acne: Involves infection by Cutibacterium acnes (formerly Propionibacterium acnes), leading to inflamed papules and pustules.

• Nodular cystic acne: Severe form with large, painful nodules or cysts deep within the skin.

Viral Skin Infections

Chickenpox (Varicella)

• Organism: Varicella-zoster virus (VZV), a herpesvirus.

• Disease presentation: Fever, malaise, and a characteristic vesicular rash that appears in crops.

• Distinguishing features: Lesions at different stages (macules, papules, vesicles, crusts) present simultaneously.

Shingles (Herpes Zoster)

• Organism: Reactivation of latent VZV.

• Disease presentation: Painful, localized vesicular rash, usually in a single dermatome.

• Distinguishing features: Often preceded by pain or tingling; risk increases with age or immunosuppression.

Human Herpesvirus I (HSV-1) Infections

• Route of Transmission (ROT): Direct contact with infected secretions or lesions.

• Herpes labialis: "Cold sores" or fever blisters around the mouth.

• Herpes gladiatorum: Skin infection common among wrestlers.

• Herpes whitlow: Painful infection of the fingers, often in healthcare workers.

Measles (Rubeola)

• Organism: Measles virus (a paramyxovirus).

• Disease presentation: Fever, cough, coryza, conjunctivitis, followed by a maculopapular rash.

• Unique signs/symptoms: Koplik's spots (white lesions on buccal mucosa), high risk of complications (e.g., pneumonia, encephalitis).

Rubella (German Measles)

• Organism: Rubella virus (a togavirus).

• Disease presentation: Mild fever, rash, lymphadenopathy.

• Risks: Congenital rubella syndrome (CRS) if infection occurs during pregnancy, leading to severe birth defects.

Smallpox

• Organism: Variola virus (a poxvirus).

• Disease presentation: High fever, malaise, and a characteristic pustular rash with lesions at the same stage of development.

• Unique features: Pathogenesis involves viral replication in lymphoid tissue, followed by viremia and skin lesions; eradicated worldwide by vaccination.

Warts

• Organism: Human papillomaviruses (HPV).

• Disease presentation: Benign, hyperkeratotic growths on the skin or mucous membranes.

• Distinguishing features: Common, plantar, and flat warts; transmission by direct contact.

Fungal Skin Infections

Dermatophytes and Tinea Infections

• Dermatophytes: Fungi that infect keratinized tissues (skin, hair, nails).

• Tinea infections: Also known as "ringworm," classified by location:

• Route of Transmission (ROT): Direct contact with infected individuals, animals, or contaminated surfaces.

Additional info: This guide expands on the learning objectives by providing definitions, clinical features, and examples for each major disease and organism. For exam preparation, focus on distinguishing features, pathogenesis, and clinical presentations of each infection.