Overview of Skin Structure and Defenses

Skin Anatomy and Protective Mechanisms

The skin is the largest organ of the human body and serves as a primary barrier against microbial invasion. Its structure and secretions provide both physical and chemical defenses.

• Epidermis: Outermost layer, composed of tightly packed epithelial cells, provides a physical barrier.

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

• Subcutaneous Layer: Fatty tissue that insulates and cushions the body.

• Defensive Secretions:

  • Melanin: Pigment that offers UV protection.

  • Perspiration (Sweat): Acidic pH and high salt content inhibit microbial growth; contains lysozyme which breaks down bacterial cell walls.

  • Sebum: Oily, acidic substance from sebaceous glands that inhibits some pathogens.

Normal Skin Microbiota

The skin hosts a diverse microbiome, with different microbes predominating in oily, moist, or dry regions. These normal residents help prevent colonization by pathogens.

• Common Bacterial Phyla: Actinobacteria, Bacteroidetes, Proteobacteria.

• Common Genera: Staphylococcus, Pseudomonas, Janthinobacterium, Malassezia (fungus).

• Locations: Epidermis, sweat glands, sebaceous glands, hair follicles.

• Factors Influencing Microbiota: Skin type (oily, moist, dry), hygiene, environment, and host genetics.

Clinical Terminology of Skin Lesions and Rashes

Understanding lesion terminology is essential for diagnosis and communication in clinical microbiology.

• Lesion: Any change or abnormality in the skin, localized or widespread.

• Primary Lesion: Directly associated with a disease process (e.g., vesicle, macule, papule, pustule, cyst).

• Secondary Lesion: Develops from a primary lesion or as a result of external factors (e.g., crust, scale, ulcer).

• Rash: Widespread eruption of lesions, may be symptomatic or asymptomatic.

Viral Skin Infections

Chickenpox and Shingles

Chickenpox and shingles are caused by the varicella-zoster virus (VZV), a member of the Herpesviridae family. The virus is highly contagious and can establish latency in nerve cells.

• Transmission: Respiratory droplets and direct contact with lesions.

• Pathogenesis: Virus enters via the respiratory tract, spreads through the blood, and causes skin lesions. It can become latent in peripheral nerves and reactivate as shingles.

• Prevention: Chickenpox vaccine (since 1995) and shingles vaccine (Zostavax®) for older adults.

Smallpox

Smallpox, caused by the variola major virus (Poxviridae), was a highly contagious and deadly disease eradicated by vaccination.

• Transmission: Respiratory droplets or contact with contaminated fomites.

• Symptoms: High fever, fatigue, followed by pustular rash.

• Mortality: Up to 30%.

• Eradication: WHO vaccination program ended natural cases by 1980.

Herpes Simplex Virus 1 (HSV-1)

HSV-1 causes oral herpes, characterized by cold sores. The virus can establish latency in nerves and reactivate, causing recurrent lesions.

• Transmission: Saliva, contaminated fomites.

• Symptoms: Painful, itchy vesicular lesions on lips, sore throat, flu-like symptoms.

• Latency: Virus remains dormant in nerves and can reactivate.

Measles (Rubeola)

Measles is a highly contagious viral disease with significant complications, preventable by the MMR vaccine.

• Transmission: Respiratory droplets.

• Symptoms: Fever, sore throat, dry cough, Koplik's spots in the mouth, maculopapular rash starting on the face and spreading.

• Complications: Hospitalization, brain swelling, death.

• Prevention: MMR vaccine (live attenuated).

Warts (Papillomavirus)

Warts are benign skin growths caused by human papillomaviruses (HPV). They are generally harmless and self-limiting.

• Transmission: Direct or indirect contact through skin abrasions.

• Treatment: Cryotherapy, cauterization, laser removal, chemicals.

Bacterial Skin Infections

Acne (Propionibacterium acnes)

Acne is a common skin condition caused by the proliferation of Propionibacterium acnes in clogged pores, leading to inflammation.

• Mechanism: Sebum and dead skin cells block pores, allowing bacterial growth and inflammation.

• Treatment: Retinoids, antibiotics, benzoyl peroxide, photodynamic and laser therapies.

Staphylococcus aureus Infections

Staphylococcus aureus is a Gram-positive coccus that forms clusters and produces numerous virulence factors, causing a range of skin diseases.

• Virulence Factors: Coagulase, catalase, hemolysins, M-protein.

• Diseases: Impetigo, cellulitis, abscesses.

• Laboratory Characteristics: Catalase positive, mannitol fermenter, beta-hemolytic, coagulase positive.

Staphylococcal Impetigo

• Highly contagious, especially in children.

• Symptoms: Superficial pus-filled vesicles, honey-colored crusted lesions.

• Treatment: Topical antibiotics.

Cellulitis

• Infection of lower dermis and subcutaneous fat.

• Symptoms: Red, swollen, painful skin, fever, may spread to lymph nodes and bloodstream.

• Treatment: Antibiotics; MRSA complicates treatment.

Pseudomonas Infections

Pseudomonas aeruginosa is a Gram-negative rod, opportunistic pathogen, and common cause of healthcare-associated infections (HAIs).

• Virulence: Forms biofilms, produces pigments (e.g., pyocyanin), highly antibiotic-resistant.

• Diseases: Wound infections, otitis externa (swimmer's ear).

• Treatment: Meticulous wound care, antibiotics, silver creams, debridement.

Otitis Externa (Swimmer's Ear)

• Infection of the outer ear canal, often due to P. aeruginosa.

• Risk Factors: Poorly maintained pools, hot tubs.

• Treatment: Antibiotic eardrops.

Gas Gangrene (Clostridium perfringens)

Gas gangrene is a severe infection of deep wounds by Clostridium perfringens, an anaerobic, endospore-forming bacterium.

• Symptoms: Foul-smelling gas, tissue destruction, shock, kidney failure.

• Treatment: Antibiotics, debridement, amputation, hyperbaric oxygen.

Cutaneous Anthrax (Bacillus anthracis)

Cutaneous anthrax is caused by Bacillus anthracis, a Gram-positive, endospore-forming bacterium.

• Transmission: Entry through skin wounds, contact with infected animals or products.

• Symptoms: Solid nodule, blackened ulcer (eschar), painless but may itch.

• Treatment: Antibiotics.

Fungal Skin Infections

Cutaneous Mycoses

Fungal infections of the skin are usually superficial and caused by Candida species or dermatophytes.

• Dermatophytes: Infect skin, hair, and nails, causing brittleness and flaking.

• Treatment: Removal of affected hair, topical or oral antifungals.

Cutaneous Candidiasis

• Agent: Candida albicans, a unicellular yeast.

• Symptoms: Bright red macular rash, white pustules, itching/burning, scaling.

• Treatment: Keep area dry, topical creams.

Tinea Infections (Ringworm)

• Transmission: Skin abrasions, fomites, contact with infected people or animals.

• Symptoms: Scaly, blistered, discolored, or inflamed skin; hair discoloration or bald patches.

• Treatment: OTC topical antifungals, prescription oral drugs.

Parasitic Skin Infections

Leishmaniasis

Leishmaniasis is caused by protozoan Leishmania species, transmitted by sand fly bites. It is prevalent in tropical and subtropical regions.

• Cutaneous Leishmaniasis: Painless ulcers at bite sites, can persist for months or years, may scar.

• Mucocutaneous Leishmaniasis: Lesions in mucous membranes, severe disfigurement.

• Visceral Leishmaniasis: Systemic spread, fatal if untreated.

• Prevention: Avoid sand fly bites (protective clothing, repellents).

• Treatment: Chemotherapy (species-dependent).

Structure, Defenses, and Infections of the Eyes

Eye Structure and Defense Mechanisms

The eye is protected by multiple anatomical and chemical barriers that limit microbial colonization.

• Cornea: Multiple layers of epithelial cells at the front of the eye.

• Conjunctiva: Mucous membrane covering the eyeball and lining the eyelids.

• Lacrimal Gland: Produces tears containing lysozyme (breaks down bacterial cell walls) and lactoferrin (binds iron).

Normal Eye Microbiota

The ocular surface is colonized by a limited but distinct microbiome, with different species dominating the conjunctiva and cornea.

Viral Conjunctivitis

Viral conjunctivitis is most commonly caused by adenoviruses and is highly contagious.

• Symptoms: Red or pink, swollen conjunctiva; itchy, painful, watery eyes; usually starts in one eye.

• Transmission: Direct contact, fomites, vertical transmission (HSV in neonates).

• Prevention: Hand hygiene, avoid sharing personal items.

• Treatment: Usually self-limiting; avoid contact lenses during infection.

Bacterial Conjunctivitis

Bacterial conjunctivitis is caused by various bacteria, including Haemophilus influenzae, Staphylococcus, and Streptococcus species.

• Symptoms: Similar to viral conjunctivitis, but with pus-like discharge (often green or yellow), eyelids may be crusted.

• Treatment: Often mild and self-resolving; antibiotic eye drops or ointment may be used.

Neonatal Bacterial Conjunctivitis

• Causes: Neisseria gonorrhoeae or Chlamydia trachomatis, acquired during birth.

• Prevention: Antibiotic drops for all newborns in the U.S.

Trachoma

Trachoma is a chronic conjunctivitis caused by certain types of Chlamydia trachomatis. It is the leading infectious cause of blindness worldwide.

• Symptoms: Rough conjunctiva, scarring, inward-turning eyelids, corneal damage.

• Transmission: Contaminated fomites, hands, or flies.

• Prevention: Facial hygiene, environmental improvements.

• Treatment: Oral azithromycin, ophthalmic ointments, surgery for eyelid correction.

Keratitis

Keratitis is inflammation of the cornea, caused by viruses, bacteria, protozoa, or helminths.

• Viral Keratitis: Most commonly due to HSV-1; symptoms include conjunctivitis, eye pain, blurred vision, sensitivity to light.

• Bacterial Keratitis: Often caused by Pseudomonas aeruginosa or Staphylococcus aureus; associated with improper contact lens use.

• Protozoan Keratitis: Acanthamoeba species; linked to poor contact lens hygiene.

• Helminthic Keratitis (River Blindness): Onchocerca volvulus larvae migrate to the eye, causing inflammation and blindness.

• Treatment: Antivirals (trifluridine, acyclovir), antibiotics, antiparasitics (ivermectin, doxycycline).