Microbial Diseases of the Skin and Eyes: Structure, Function, and Pathogenesis

Skin Structure and Function

Normal Anatomy and Microbial Flora

The skin is the body's largest organ, serving as a physical barrier against pathogens and environmental hazards. Its structure includes the epidermis, dermis, and subcutaneous layers, each with distinct functions.

• Epidermis: Outermost layer, provides waterproofing and protection.

• Dermis: Contains connective tissue, blood vessels, and nerves.

• Subcutaneous Layer: Fat and connective tissue for insulation and cushioning.

• Normal Microbiota: Includes Staphylococcus epidermidis, Propionibacterium acnes, and others. These organisms help prevent colonization by pathogens.

Key Point: The skin's acidic pH, salty environment, and presence of lysozyme contribute to its antimicrobial properties.

Bacterial Diseases of Skin

Staphylococcal Infections

Staphylococcus aureus and Staphylococcus epidermidis are common skin pathogens. They differ in pathogenicity and clinical presentation.

• Gram Reaction: Gram-positive cocci.

• Shape/Arrangement: Clusters (grape-like).

• Coagulase Reaction: S. aureus is coagulase-positive; S. epidermidis is coagulase-negative.

• Diseases: Folliculitis, boils (furuncles), carbuncles, impetigo, scalded skin syndrome.

Example: S. aureus can cause impetigo, a contagious skin infection characterized by pustules and honey-colored crusts.

Streptococcal Infections

Streptococcus pyogenes (Group A strep) is a major cause of skin infections.

• Gram Reaction: Gram-positive cocci.

• Arrangement: Chains.

• Diseases: Erysipelas, cellulitis, necrotizing fasciitis.

• Virulence Factors: M protein, streptolysins, hyaluronidase.

Example: Necrotizing fasciitis is a rapidly progressing infection that destroys fascia and muscle tissue.

Other Bacterial Skin Diseases

• Pseudomonas aeruginosa: Causes infections in burn patients; produces blue-green pigment.

• Acne: Primarily caused by Propionibacterium acnes; involves inflammation of hair follicles.

Viral Diseases of Skin

Warts

Warts are benign skin growths caused by human papillomavirus (HPV).

• Transmission: Direct contact.

• Treatment: Removal by cryotherapy, acids, or surgery.

Chickenpox and Shingles

Both are caused by Varicella-Zoster Virus (VZV).

• Chickenpox: Primary infection; causes vesicular rash.

• Shingles: Reactivation of latent VZV; causes painful dermatomal rash.

Difference: Chickenpox is common in children; shingles occurs in older adults or immunocompromised individuals.

Herpes Simplex Virus (HSV)

• HSV-1: Causes oral herpes (cold sores).

• HSV-2: Causes genital herpes.

• Difference: Location and mode of transmission.

Measles (Rubeola)

• Symptoms: Fever, cough, Koplik spots, maculopapular rash.

• Complications: Pneumonia, encephalitis.

• Prevention: MMR vaccine.

Rubella

• Symptoms: Mild rash, fever.

• Congenital Rubella Syndrome: Major threat to developing fetus; causes birth defects.

• Prevention: MMR vaccine.

Fungal Diseases of Skin

Cutaneous Mycoses

Fungal infections of the skin are commonly caused by dermatophytes.

• Examples: Ringworm (tinea), athlete's foot.

• Symptoms: Itching, scaling, redness.

• Treatment: Topical or oral antifungals.

Candidiasis

• Agent: Candida albicans.

• Diseases: Diaper rash in infants, oral thrush.

Parasitic Diseases of Skin

Scabies

• Cause: Infestation by Sarcoptes scabiei mite.

• Symptoms: Intense itching, rash.

• Treatment: Topical scabicides.

Eye Diseases

Bacterial and Viral Conjunctivitis

• Conjunctivitis: Inflammation of the conjunctiva; "pink eye".

• Causes: Bacterial (Staphylococcus, Streptococcus), viral (adenovirus).

• Symptoms: Redness, discharge, irritation.

Other Eye Diseases

• Ophthalmia neonatorum: Severe conjunctivitis in newborns, often caused by Neisseria gonorrhoeae.

• Trachoma: Chronic infection by Chlamydia trachomatis; leading cause of blindness worldwide.

Table: Comparison of Major Skin Pathogens

Pathogen	Type	Common Diseases	Key Features
Staphylococcus aureus	Bacteria	Impetigo, boils, scalded skin syndrome	Coagulase-positive, clusters
Streptococcus pyogenes	Bacteria	Erysipelas, cellulitis, necrotizing fasciitis	Chains, M protein, hemolysis
Varicella-Zoster Virus	Virus	Chickenpox, shingles	Latent infection, vesicular rash
Candida albicans	Fungus	Diaper rash, oral thrush	Yeast, opportunistic
Sarcoptes scabiei	Parasite	Scabies	Mite, burrows in skin

Additional info:

• Some questions reference the role of normal microbiota and immune responses, which relate to chapters on innate and adaptive immunity.

• Questions about transmission, treatment, and prevention connect to epidemiology and antimicrobial drugs.