Microbial Diseases of the Skin and Eyes

Structure and Normal Microbiota of the Skin and Eyes

The skin and eyes possess unique anatomical features and resident microbiota that influence susceptibility to infection.

• Skin Structure: The skin consists of three main layers:

  • Epidermis: The outermost protective layer composed of tightly packed epithelial cells.

  • Dermis: Contains blood vessels, nerves, hair follicles, sweat, and oil glands.

  • Hypodermis (Subcutaneous tissue): The deepest layer, primarily composed of fat and connective tissue.

• Normal Skin Microbiota: Common organisms include Staphylococcus epidermidis, other staphylococci, and Corynebacterium species. Moist areas support more microbial growth.

• Eye Structure: The conjunctiva lines the eyelids and covers the anterior eye; the cornea is the transparent front surface that focuses light.

• Normal Eye Microbiota: The eye harbors few microbes due to antimicrobial tears (containing lysozyme) and mechanical flushing. Coagulase-negative staphylococci and Corynebacterium may be present.

Skin as a Physical and Chemical Barrier

The skin provides both physical and chemical defenses against microbial invasion.

• Physical Barrier: The epidermis forms a continuous, tightly packed layer that blocks pathogen entry. Regular shedding of skin cells removes attached microbes.

• Chemical Barrier: Sweat and sebum create an acidic environment; skin secretions contain antimicrobial substances (e.g., lysozyme, fatty acids).

• Microbial Competition: Normal microbiota compete with pathogens for space and nutrients, inhibiting colonization by harmful microbes.

Common Bacterial Skin Infections

Several bacteria can cause characteristic skin infections, often distinguished by their clinical presentation.

• Staphylococcus aureus:

  • Impetigo: Superficial sores with honey-colored crusts.

  • Scalded Skin Syndrome: Exfoliative toxins cause widespread skin peeling.

  • Toxic Shock Syndrome: TSST-1 toxin leads to fever, hypotension, and shock.

• Streptococcus pyogenes:

  • Erysipelas: Raised, red, sharply demarcated skin infection.

  • Necrotizing Fasciitis: Rapidly progressing infection destroying deeper tissues; a medical emergency.

  • Impetigo: Can also be caused by this organism.

Pseudomonas aeruginosa in Burn Infections and Nosocomial Disease

Pseudomonas aeruginosa is a significant opportunistic pathogen, especially in healthcare settings.

• Characteristics: Gram-negative, aerobic rod; produces blue-green pigment (pyocyanin).

• Clinical Importance: Infects burns and damaged tissue; forms biofilms and is resistant to many antibiotics.

• Nosocomial Infections: Common cause of healthcare-associated infections, especially in immunocompromised patients.

Major Viral Diseases Affecting the Skin

Viruses can cause a variety of skin diseases, some with systemic implications.

• Measles (Rubeola): Spread by respiratory droplets; causes fever, rash, and Koplik spots in the mouth.

• Rubella (German Measles): Usually milder; infection during pregnancy can cause congenital defects (e.g., hearing, heart problems).

• Chickenpox (Varicella-Zoster Virus): Itchy vesicular rash; virus can reactivate later as shingles (painful, one-sided rash).

• Warts (Human Papillomaviruses, HPV): Benign skin growths; some types linked to cancers.

• Smallpox (Variola Virus): Eradicated worldwide through vaccination.

Fungal Infections of the Skin and Their Causative Agents

Fungi can infect superficial layers of the skin, hair, and nails, as well as mucous membranes.

• Dermatomycoses (Ringworm): Caused by dermatophytes such as Trichophyton, Microsporum, and Epidermophyton.

  • Tinea pedis: Athlete's foot.

  • Tinea cruris: Jock itch.

• Candidiasis: Overgrowth of Candida albicans; causes oral thrush and moist skin-fold infections.

• Sporotrichosis: Caused by Sporothrix schenckii species complex; often enters through skin after contact with contaminated plant material.

Normal Eye Microbiota and Eye Infections

The eye is protected by physical and chemical defenses, but several infections can occur.

• Normal Microbiota: Limited due to tears, blinking, and lysozyme.

• Conjunctivitis: Inflammation of the conjunctiva; may be viral, bacterial, allergic, or irritant-induced.

• Keratitis: Inflammation or infection of the cornea; can be caused by bacteria, viruses, fungi, or parasites.

• Trachoma: Chronic infection with Chlamydia trachomatis; repeated infections cause scarring, in-turned eyelashes, and potential blindness.

Transmission and Prevention of Skin and Eye Infections

Modes of transmission and prevention strategies vary by pathogen type.

Eye Infection Prevention: Avoid sharing eye makeup or towels; handle contact lenses properly.

Clinical Symptoms and Pathogen Mechanisms

Recognizing clinical features helps identify causative agents and underlying mechanisms.

• Honey-colored crusts: Impetigo (often Staph. aureus or Strep. pyogenes).

• Peeling skin: Staphylococcal exfoliative toxin (scalded skin syndrome).

• Fever and shock: Toxin-mediated toxic shock syndrome.

• Rapid, severe pain/tissue damage: Necrotizing fasciitis.

• Blue-green wound drainage: Pseudomonas aeruginosa.

• Itchy vesicles: Chickenpox.

• Painful one-sided rash: Shingles.

• Ring-shaped scaly rash: Dermatophyte infection (ringworm).

• Intense itching with mites: Scabies (Sarcoptes scabiei).

• Red conjunctiva: Conjunctivitis.

• Corneal pain/inflammation: Keratitis.

• Repeated eye infection with scarring: Trachoma (Chlamydia trachomatis).

Summary Table: Key Pathogens and Associated Diseases

Microbial Diseases of the Nervous System

Major Bacterial Diseases of the Nervous System

The nervous system is vulnerable to several severe bacterial infections, often with rapid progression and high morbidity.

• Bacterial Meningitis: Inflammation of the meninges (protective membranes covering the brain and spinal cord).

  • Key Pathogens: Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae.

  • Symptoms: Fever, headache, stiff neck, photophobia, altered mental status.

• Tetanus: Caused by Clostridium tetani; its neurotoxin (tetanospasmin) blocks inhibitory neurotransmitter release, resulting in painful muscle stiffness and spasms.

• Botulism: Caused by Clostridium botulinum; its neurotoxin blocks acetylcholine release at neuromuscular junctions, causing flaccid paralysis.

Additional info: Bacterial meningitis is a medical emergency requiring prompt diagnosis and treatment. Vaccines are available for some causative agents (e.g., Neisseria meningitidis, Streptococcus pneumoniae).