Microbial Diseases of the Skin and Eyes

Overview

This section covers the structure and defenses of the skin, the normal microbiota, and the major bacterial, viral, fungal, and parasitic diseases affecting the skin and eyes. It also includes laboratory methods for identification and clinical considerations for diagnosis and treatment.

The Skin: Structure and Defenses

Skin Structure

• Epidermis: Thin outer layer of epithelial cells, contains keratin (protective protein).

• Dermis: Thick inner layer, contains hair follicles, sweat, and oil ducts.

Skin Secretions

• Perspiration: Provides moisture, contains lysozyme and antimicrobial peptides.

• Sebum: Contains lipids, proteins, and salt; provides nutrients but fatty acids can inhibit microbes.

Normal Microbiota of the Skin

• Skin is resistant to drying and high salt concentrations.

• Common Gram-positive cocci: Staphylococcus, Micrococcus

• Gram-positive rods (diphtheroids): Propionibacterium acnes, Corynebacterium xerosis

• Note: Some normal microbiota can cause disease under certain conditions (opportunistic pathogens).

Skin Lesions

• Vesicle: Small fluid-filled lesion

• Bulla: Large (>1 cm) vesicle

• Macule: Flat, reddened lesion

• Papule: Raised lesion

• Pustule: Raised lesion containing pus

Bacterial Diseases of the Skin

Staphylococcal Skin Infections

• Staphylococci: Gram-positive cocci, irregular clusters.

• Divided by coagulase activity:

  • Coagulase-negative: Staphylococcus epidermidis (common on skin, forms biofilms on catheters, pathogenic when skin is broken).

  • Coagulase-positive: Staphylococcus aureus (most pathogenic, produces toxins, evades host defenses, causes various skin infections).

• Antibiotic resistance:

  • MRSA (Methicillin-resistant S. aureus): produces beta-lactamase.

  • VRSA (Vancomycin-resistant S. aureus): modification of target molecule.

Diseases Caused by Staphylococcus aureus

• Folliculitis: Infection of hair follicle (sty if eyelash).

• Furuncle (boil): Localized pus surrounded by inflamed tissue.

• Carbuncle: Progression of furuncle to deeper tissue inflammation.

• Impetigo: Highly contagious, spread by direct contact, common in children, isolated pustules that become crusted.

• Toxin-mediated diseases:

  • Exfoliative toxin A: Causes bullous impetigo at infection site.

  • Exfoliative toxin B: Causes scalded skin syndrome at other sites.

Streptococcal Skin Infections

• Streptococci: Gram-positive cocci, form chains, secrete enzymes and toxins (hemolysins).

• Beta-hemolytic streptococci: Streptococcus pyogenes (Group A streptococci)

  • Produce hemolysin, streptolysin, streptokinase, DNase.

Diseases Caused by Streptococcus pyogenes

• Erysipelas: Infection of dermal layer, often preceded by strep throat, treated with beta-lactam antibiotics.

• Necrotizing fasciitis: "Flesh-eating bacteria," rapid tissue destruction, caused by exotoxin A (superantigen), may require amputation.

• Streptococcal toxic shock syndrome: M protein activates neutrophils, leading to shock and organ damage.

Infections by Pseudomonads

• Pseudomonas spp.: Gram-negative rods, resistant to many antibiotics and disinfectants.

• Produce exotoxins, endotoxin, and pyocyanin (blue-green pigment, forms reactive oxygen species).

• Grow in biofilms.

• Associated with:

  • Dermatitis (self-limiting rash, ~2 weeks)

  • Otitis externa (swimmer's ear)

  • Opportunistic infections in burn victims, cystic fibrosis patients, and catheterized patients

Acne

• Caused by clogged pores/hair follicles due to shed skin cells and sebum.

• Propionibacterium acnes: Anaerobic, Gram-positive rod, produces fatty acids that stimulate immune response.

• Treated with antibiotics or ointments to reduce sebum formation.

Viral Diseases of the Skin

Chickenpox and Shingles

• Chickenpox: Caused by Herpesvirus varicella-zoster (polyhedral, enveloped virus).

• Virus enters respiratory system, spreads to skin, forms vesicles that fill with pus and scab over.

• Shingles: Reactivation of latent varicella-zoster virus, usually on torso, causes painful vesicles.

• Shingles is a latent infection.

Herpes Simplex Virus

• HSV-1: Polyhedral, enveloped virus, causes cold sores and herpetic keratitis.

• Latent in trigeminal nerve ganglion; reactivated by fever, stress, or UV exposure.

• ~90% of population infected, most subclinical.

• Treatments: Acyclovir (nucleoside analog), docosanol (prevents viral fusion), hydrocolloid patch (promotes healing). No cure.

Measles

• Caused by Morbilivirus (helical, enveloped virus).

• Enters respiratory system, spreads to skin, causes macular rash.

• Symptoms: Fever, rash, cough, runny nose, sore throat.

• Long-term effects: Disables immune system, secondary infections, subacute sclerosing panencephalitis (fatal), blindness, deafness.

• Prevention: MMR vaccine (measles, mumps, rubella), herd immunity at 95% vaccination.

Fungal Diseases of the Skin

Cutaneous Mycoses

• Fungi colonize hair, nails, or outer epidermis.

• Caused by dermatophytes (grow on keratin).

• Tineas (ringworm):

  • Tinea cruris: Groin area ("jock itch")

  • Tinea pedis: Feet ("athlete's foot")

  • Tinea unguium: Fingernails or toenails

Parasitic Diseases of the Skin

Scabies

• Caused by Sarcoptes scabiei (mite).

• Mites burrow under skin to lay eggs, causing various lesions (may lead to secondary infections).

• Diagnosed by microscopic observation; treated with topical agents.

Lice

• Pediculus humanus capitis (head lice): Adapted to grasp hairs, bite scalp to consume blood.

• Itching caused by sensitization to louse saliva.

• Treatment: Topical agents and physical removal.

Diseases of the Eye

Conjunctivitis (Pink Eye)

• Inflammation of the conjunctiva.

• Caused by bacterial (Haemophilus influenzae, others), viral (adenoviruses), or allergies.

Bacterial Diseases of the Eye

• Ophthalmia neonatorum: Caused by Neisseria gonorrhoeae, infection of infant's eyes during childbirth, leads to pus and corneal ulceration. Treated with antibiotics after birth.

• Trachoma: Caused by Chlamydia trachomatis (not STI strains), leads to conjunctivitis and trichiasis (inward turning of eyelashes). Treated with antibiotics or surgery.

Laboratory Identification and Clinical Applications

• Gram stain: Differentiates bacteria by cell wall structure.

• Cultural isolation: Growth on selective media to identify pathogens.

• Biochemical testing: Determines metabolic properties of bacteria.

• Antibiotic resistance testing: Disk diffusion test to determine susceptibility.

• Lab report structure: Introduction, Methods, Results, Discussion (including dichotomous key, interpretation, and suggested treatments).

Table: Key Microbial Diseases of the Skin and Eyes

Key Terms and Concepts

• Biofilm: Community of microorganisms attached to a surface, encased in a self-produced matrix.

• Opportunistic pathogen: Normally harmless microbe that causes disease under certain conditions.

• Latent infection: Pathogen remains inactive in the body and can reactivate later (e.g., shingles).

• Superantigen: Toxin that causes excessive immune activation (e.g., exotoxin A in necrotizing fasciitis).

• Herd immunity: Protection of a population from infection when a sufficient proportion is immune.

Example: Disk Diffusion Antibiotic Susceptibility Test

• Bacteria are spread on an agar plate; antibiotic disks are placed on the surface.

• Zones of inhibition indicate susceptibility; no zone indicates resistance.

• Used to determine appropriate antibiotic therapy (e.g., MRSA identification).

Formulas and Equations

• Zone of Inhibition (mm): Measured diameter around antibiotic disk where bacteria do not grow.

• Interpretation: Compare measured zone to standard tables to determine susceptibility or resistance.

Summary Table: Common Pathogens and Their Features

Additional info: Laboratory identification of unknown bacteria involves Gram staining, culturing, biochemical tests, and antibiotic susceptibility testing. Interpretation of results is essential for diagnosis and treatment selection.