Microbial Diseases of the Skin and Eyes

Structure and Function of the Skin

The skin acts as a primary physical barrier against microbial invasion. Its structure and secretions play crucial roles in both supporting and inhibiting microbial growth.

• Epidermis: The outermost layer, rich in keratin, provides a waterproof barrier.

• Dermis: The inner layer contains hair follicles, sweat ducts, and oil glands, which can serve as entry points for microorganisms.

• Sebum and Perspiration: These secretions can inhibit microbial growth due to their chemical properties, but also provide nutrients for certain microbes.

• Mucous Membranes: Epithelial cells lining body cavities secrete mucus, forming another protective barrier.

Example: Moist areas of the skin, such as underarms, support larger populations of bacteria compared to dry areas.

Normal Microbiota of the Skin

The skin hosts a diverse microbiota, primarily composed of bacteria adapted to harsh conditions such as dryness and high salt concentrations.

• Gram-positive cocci (e.g., Staphylococcus and Micrococcus) predominate.

• Cutibacterium species metabolize oil and colonize hair follicles.

• Malassezia furfur (a yeast) grows on oily secretions and may cause dandruff.

• Normal microbiota are resistant to desiccation and are not completely removed by washing.

Ecological Role: Normal microbiota can inhibit the colonization of pathogenic organisms by competing for nutrients and space.

Microbial Diseases of the Skin

Skin diseases can be caused by bacteria, viruses, fungi, or parasites. Lesions are classified by their appearance:

• Vesicles: Small, fluid-filled lesions.

• Bullae: Vesicles larger than 1 cm.

• Macules: Flat, reddened lesions.

• Papules: Raised lesions.

• Pustules: Raised lesions containing pus.

Bacterial Diseases of the Skin

• Staphylococcus epidermidis: Coagulase-negative, common on skin, generally non-pathogenic.

• Staphylococcus aureus: Pathogenic strains produce coagulase, enterotoxins, leukocidins, and exfoliative toxin. Causes localized infections (sties, pimples, carbuncles) and toxemias (scalded skin syndrome, toxic shock syndrome).

• Impetigo: Highly contagious superficial infection, often caused by S. aureus.

• Streptococci: Classified by hemolytic enzymes and cell wall antigens. Group A beta-hemolytic streptococci (e.g., Streptococcus pyogenes) produce M protein, deoxyribonuclease, streptokinases, and hyaluronidase, causing severe tissue destruction.

• Pseudomonas aeruginosa: Produces endotoxin and exotoxins; causes otitis externa ("swimmer's ear"), dermatitis, and infections in burns. Characterized by blue-green pus (pyocyanin pigment).

• Cutibacterium acnes: Metabolizes fatty acids, causing inflammatory acne.

• Mycobacterium ulcerans: Causes deep-tissue ulceration (Buruli ulcer).

Viral Diseases of the Skin

• Papillomaviruses: Cause warts (benign growths); spread by direct contact; may regress or be removed.

• Variola virus (Smallpox): Transmitted by respiratory route; causes variola major and minor; eradicated by vaccination.

• MPOX (Monkeypox): Similar to smallpox; transmitted by direct contact or respiratory secretions; increased incidence in 2022.

• Varicella-zoster virus (HHV-3): Causes chickenpox (vesicular rash); can remain latent and reactivate as shingles (painful vesicular rash along sensory nerves). Treated with acyclovir; vaccine available.

• Herpes simplex virus (HSV-1): Causes cold sores; remains latent in nerve cells; can cause encephalitis. Treated with acyclovir.

• Measles virus: Transmitted by respiratory route; causes macular rash and Koplik's spots; complications include pneumonia and encephalitis. Vaccine provides long-term immunity.

• Rubella virus: Causes red rash and light fever; can cause congenital rubella syndrome if contracted during pregnancy. Vaccine available.

Fungal Diseases of the Skin and Nails

• Dermatomycoses: Fungi colonizing the epidermis, causing ringworm (tinea) of hair, skin, and nails. Common genera: Microsporum, Trichophyton, Epidermophyton.

• Diagnosis by microscopic examination or culture of skin scrapings.

• Candida albicans: Causes thrush (oral mucosa) and vaginitis; treated with topical antifungals.

Parasitic Infestations of the Skin

• Scabies: Caused by mites burrowing and laying eggs in the skin.

• Pediculosis: Infestation by lice (Pediculus humanus).

Microbial Diseases of the Eye

The eye is protected by the conjunctiva, a mucous membrane lining the eyelid and covering the eyeball. Several microbial diseases can affect the eye, often originating from skin or respiratory tract microbiota.

Inflammation of the Eye Membranes: Conjunctivitis

• Conjunctivitis: Inflammation of the conjunctiva; caused by various bacteria; can be transmitted by contaminated contact lenses.

Bacterial Diseases of the Eye

• Ophthalmia neonatorum: Caused by Neisseria gonorrhoeae; transmitted from mother to infant during birth.

• Inclusion conjunctivitis: Caused by Chlamydia trachomatis; transmitted during birth or in unchlorinated swimming water.

• Trachoma: Caused by C. trachomatis; transmitted by hands, fomites, or possibly flies; leading cause of infectious blindness worldwide.

Table: Selected Microbial Diseases of the Skin and Eyes

Key Definitions

• Conjunctivitis: Inflammation of the conjunctiva ("pinkeye").

• Ophthalmia neonatorum: Severe conjunctivitis in newborns, usually caused by N. gonorrhoeae.

• Inclusion conjunctivitis: Conjunctival infection by C. trachomatis.

• Trachoma: Chronic conjunctivitis leading to scarring and blindness.

Additional Info:

• Antibiotics have largely replaced silver nitrate for preventing ophthalmia neonatorum due to broader efficacy and fewer side effects.

• Head lice (Pediculus humanus capitis) are not known to transmit diseases.

• "Fifth disease" is so named because it was the fifth identified childhood rash-causing disease.

• Penicillin use can disrupt normal microbiota, predisposing to Candida overgrowth (candidiasis).