Microbial Diseases of the Skin and Eyes

Chapter 21: Microbial Diseases of the Skin and Eyes

Introduction

  • Focus: Understanding microbial diseases affecting the skin and eyes.
  • Objective: Identify specific bacteria, viruses, fungi, and parasites causing diseases in these areas, differentiate causes of conjunctivitis, discuss normal skin microbiota, and review skin and eye structure and function.

Structure and Function of the Skin

Overview
  • Function of the Skin: Acts as a physical barrier against microbial invasion.
    • Considered the first line of defense against pathogens.
    • Skin secretions maintain acidity which hampers bacterial growth.
    • Intact skin is generally impermeable to most microbes.
    • Breaks in the skin provide entry points for pathogens.
    • **Environmental Influence: **
    • Dry regions (e.g., scalp) support minimal microbial growth.
    • Moist areas (e.g., armpit, groin) facilitate larger microbial populations.
Skin Layers
  1. Epidermis:
    • Thin outer layer composed of epithelial cells.
    • Keratin: Waterproofing protein that covers the epidermal surface.
  2. Dermis:
    • Thick inner layer made primarily of connective tissue.
    • Contains hair follicles, sweat ducts, and sebaceous glands which can be pathways for microbes.
Skin Secretions
  • Perspiration:
    • Provides moisture and nutrients for microbial growth.
    • Contains salt which inhibits most microorganisms.
    • Lysozyme present which breaks down bacterial cell walls.
  • Sebum:
    • Excreted by oil glands, contains fatty acids that inhibit pathogens.
    • Can create a favorable environment for certain microbes.
  • Mucous Membranes:
    • Line body cavities open to the exterior, comprised of tightly packed epithelial cells that secrete mucus.
    • Contain ciliated cells to trap and move debris out.

Normal Microbiota of the Skin

Characteristics
  • Skin microbiota are capable of resisting drying (desiccation) and high salt concentrations.
  • Predominant bacteria are gram-positive cocci:
    • Examples include Staphylococcus epidermidis and other Staphylococcus species.
  • Washing the skin does not completely remove microbiota.
Other Bacterial Species and Yeasts
  • Cutibacterium (formerly Propionibacterium) acnes:
    • Inhabits hair follicles, anaerobic cell metabolism; breaks down oils and produces acids to maintain low skin pH.
  • Corynebacterium xerosis:
    • Aerobic organism occupying the skin surface.
  • Malassezia furfur:
    • Causes dandruff; a type of yeast associated with skin flora.

Diseases of the Skin

Diagnosis
  • Rashes and skin lesions may not indicate infection; can stem from internal conditions.
  • Preliminary diagnosis relies on visible appearances of lesions and rashes.
Types of Skin Lesions
  1. Vesicles: Fluid-filled lesions.
  2. Bullae: Larger vesicles over 1 cm in diameter.
  3. Macules: Flat, reddened lesions.
  4. Papules: Raised lesions.
  5. Pustules: Raised lesions containing pus.

Bacterial Diseases of the Skin

Major Genera
  • Staphylococcus: Spherical clusters, responsible for common skin infections.
  • Streptococcus: Spherical chains, also linked with superficial infections.
Staphylococcus
  • Coagulase: Enzyme that clots fibrin in blood; useful in differentiating types of Staphylococcus.
  • Staphylococcus epidermidis:
    • Comprises 90% of the normal skin microbiota.
    • Produces biofilm on medical devices (e.g., catheters).
    • Coagulase-negative.
  • Staphylococcus aureus:
    • Coagulase-positive, produces damaging toxins, can lead to conditions like sepsis or toxic shock syndrome, especially noted in tampon-associated cases.
    • MRSA: Methicillin-resistant strains pose significant public health issues.
Specific Conditions Caused by Staphylococcus aureus
  • Impetigo: Nonbullous crusting sores.
  • Scalded Skin Syndrome: Bullous impetigo caused by toxins responsible for exfoliation.
Streptococcus
  • Hemolysins: Cause lysis of red blood cells.
  • Beta-hemolytic streptococci: Group A through T classified based on cell wall carbohydrates.
  • Streptococcus pyogenes:
    • Known as Group A streptococci (GAS); has 80 immunological types.
    • Produces several virulence factors:
    • Streptolysins: Lyse red blood cells.
    • M proteins: Aid in adherence and immune evasion.
    • Streptokinases: Dissolve blood clots.
Specific Conditions Caused by Streptococcus Pyogenes
  • Necrotizing Fasciitis: Labeled as “flesh-eating disease.” Exotoxin A acts as a superantigen leading to an aggressive immune response.
  • Streptococcal Toxic Shock Syndrome: Shares similarities with staphylococcal toxic shock syndrome (TSS).
Acne
  • Prevalence: Most common skin condition.
  • Pathophysiology: Skin cells plus sebum block hair follicles; sebum production is hormonally regulated, not dietary.
  • Types of Acne:
    1. Comedonal (Mild): Simple blockage; responsive to topical treatments.
    2. Inflammatory (Moderate): Caused by Propionibacterium acnes; manifests as inflammation due to fatty acids. Treatable with antibiotics and benzoyl peroxide.
    3. Nodular Cystic (Severe): Deep inflamed lesions with pus, requiring more extensive treatment.

Viral Diseases of the Skin

General Insights
  • Transmission commonly occurs via respiratory routes, affecting children and fetuses more than adults.
Smallpox
  • Causative Agent: Orthopoxvirus.
  • Forms: Variola major (20-60% mortality) and Variola minor (<1% mortality).
  • Transmitted via respiratory route, progresses into bloodstream, infecting skin.
  • Eradication: Vaccination led to complete eradication from human population; noted as a potential bioterrorism agent.
Chickenpox
  • Causative Agent: Herpesvirus varicella-zoster (human herpesvirus 3).
  • Transmitted via respiratory route; can lead to severe complications like Reye’s syndrome.
  • Latency: Virus can become dormant in central nerve ganglia, reactivating as shingles.
  • Prevention: Live attenuated vaccine is effective; breakthrough infections can occur post-vaccination.
Shingles
  • Caused by reactivation of varicella-zoster virus; lesions appear on the chest and back.
  • Risk Factors: Aging, stress, or immunocompromised state.
  • Prevention/Treatment: Zoster vaccine (Shingrix) and antivirals like acyclovir.
Herpes Simplex Virus
  • Types: HSV-1 primarily oral/respiratory transmission; HSV-2 primarily sexual.
  • Prevalence: 90% of U.S. population infected with HSV-1; manifests as cold sores or fever blisters.
  • Complications: Can result in herpes encephalitis; treated with acyclovir.
Rubella
  • Commonly known as German measles; caused by Rubivirus rubella (RuV).
  • Congenital Rubella Syndrome: Risk to fetus, leading to various defects and a mortality rate of 15% within the first year.
  • Prevention: MMR vaccination is effective, contraindicated in pregnancy.

Fungal Diseases of the Skin

Mycosis
  • General term for any fungal infection, specifically targeting cutaneous regions including the outer epidermis, hair, and nails.
  • Dermatomycoses: Informally termed tineas or ringworm, include specific types such as:
    • Tinea capitis: Scalp ringworm.
    • Tinea cruris: Jock itch.
    • Tinea pedis: Athlete’s foot.
    • Tinea unguium: Ringworm of nails.
  • Treatment: Generally involves topical antifungals like miconazole and clotrimazole.
Candida albicans
  • Associated with yeast infections.
  • Can lead to vaginal candidiasis and oral thrush, often following antibiotic treatments that disrupt normal flora.
  • Risk Factors: Occurs in immunocompromised individuals.

Parasitic Diseases of the Skin

Scabies
  • Caused by Sarcoptes scabiei mites.
  • Mites burrow into skin leading to inflammation and lesions; transmitted via direct contact.
  • Treatment: Permethrin, an insecticide effective against mites and their eggs.
Pediculosis (Lice)
  • Defined as an infestation by lice; specifically, Pediculus humanus capitis (head louse) and P. h. corporis (body louse).
  • Feed on blood and lay eggs (nits) attached to hair shafts.
  • Treatment: Utilizes topical insecticides like permethrin or pyrethrin.

Diseases of the Eyes

Conjunctivitis (Pinkeye)
  • Inflammation of conjunctiva; commonly found due to Haemophilus influenzae (not related to influenza).
  • Can stem from bacterial infections tied to unsanitary contact lens use or adenoviruses (highly contagious).
Bacterial Diseases of the Eye

  1. Ophthalmia Neonatorum:

    • Caused by Neisseria gonorrhoeae and Chlamydia trachomatis.
    • Characterized by pus formation, may lead to corneal ulceration and blindness if untreated.
    • Transmission occurs during passage through the birth canal; treat newborn eyes preventively.

  2. Chlamydia trachomatis:

    • Causes Inclusion conjunctivitis; an obligate intracellular parasite.
    • Transmission during birth or through contaminated swimming water.
    • Also causes Trachoma: leading global cause of blindness due to corneal scarring from infection; spread via hand contact or flies.