Module Seven: Skin and Eye Infections

Anatomy and Microbiota of the Skin

• Skin Layers:

  • Epidermis: The outermost, superficial layer. It is composed of stratified squamous epithelium.
    • The epidermis is avascular (lacks a blood supply).
    • As cells push upward through the layers, they die.
    • Stratum Corneum: The outermost layer of the epidermis, consisting of dead, keratinized cells that are constantly shed. Skin microbiota feed on this material.
  • Dermis: Located beneath the epidermis, composed of connective tissue.
    • The dermis is vascular (has a blood supply) and contains nerves and glands.
    • Sudoriferous Glands: Sweat glands.
    • Sebaceous Glands: Glands associated with hair follicles that produce sebum (oil).
  • Hypodermis: The layer beneath the dermis containing connective tissue and fatty (adipose) tissue. It is also vascular and overlies the fascia and muscle.

• Skin Microbiota (Human Genome Project):

  • DNA analysis was used to identify the inhabitants of various body parts.
  • Protective Functions:
    • Produce antimicrobial substances to prevent invasion by other organisms.
    • Act as strong competitors against non-normal inhabitants.
  • Environmental Factors: The skin is a hostile environment with a low pH (around $5$) and high salt content due to sweat.
  • Distribution: Populations vary by moisture levels. For example, Staphylococcus is more prevalent in moist areas like the crease of the knee, the heel of the foot, the genital area, and the crook of the elbow.

Bacterial Skin Infections: Staphylococcus aureus

• Characteristics: Staphylococcus aureus is a gram-positive cocci.

• Virulence Factors:

  • Staphylolysin: A form of hemolysis that kills skin cells and white blood cells (WBCs), contributing to pus formation.
  • Coagulase: A clotting protein that helps in abscess formation. S. aureus is coagulase-positive.
  • Leukocidins: Chemicals that kill white blood cells, also contributing to the formation of pus (debris and dead WBCs).
  • Toxic Shock Syndrome Toxin (TSST): An exotoxin and superantigen. Symptoms include fever, diarrhea, vomiting, desquamation (exfoliation/shedding of outer skin layers), systemic problems, drops in blood pressure, and heart failure.

• Superficial Infections:

  • Folliculitis: Localized infection of the hair follicles resulting in red bumps or pus-filled papilla.
    • Scenario: Can occur in improperly treated hot tubs; warm water opens follicles to bacteria, which are then trapped when the skin cools.
    • Symptoms: Local discomfort, itching, fever, and chills.
  • Furuncle (Boil): A deeper, painful, warm, red lesion in the region of a hair follicle, appearing as a pus-filled nodule.
  • Carbuncle: A massive sore or abscess formed when multiple furuncles combine. This deeper infection can lead to systemic issues.
  • Impetigo: Can be caused by Staph or Strep. Introduced through skin breaks (scratching, chapped lips). Highly contagious via direct contact or fomites.
    • Forms vesicles/pustules that rupture and leak yellowish fluid, which then crusts over.
    • Nonbullous: Vesicles and pustules that rupture and crust.
    • Bullous: Large, fluid-filled blisters and draining lesions.
  • Treatment: Topical or oral antibiotics depending on severity.

Bacterial Skin Infections: Streptococcus

• Characteristics: Gram-positive cocci found in pairs or chains.

• Virulence Factors:

  • M Protein: Found on the surface; aids in tissue adherence and inhibits phagocytosis.
  • Capsule: Helps avoid phagocytosis.
  • Coagulase Negative: Unlike Staph, Strep is coagulase-negative.
  • Lancefield Groups: Classification based on cell wall carbohydrates (Groups A through M).
  • Hemolysis Types:
    • Alpha hemolysis: Incomplete breakdown of red blood cells, resulting in an olive-green color on blood agar.
    • Beta hemolysis: Complete destruction of red blood cells, appearing clear on blood agar (e.g., Group A beta-hemolytic strep).

• Streptococcal Skin Infections:

  • Erysipelas: Infection of the dermis entering through skin breaks (bites, incisions, shaving). Causes painful, fiery red, raised areas with very sharp borders (edema). Can cause fever and chills.
  • Cellulitis: A deeper infection of the dermis and hypodermis.
    • Symptoms: Redness, fever, swollen lymph nodes, pain, and edema.
    • Lymphangitis: Visible red lines indicating the disease is spreading along lymphatic vessels toward the heart. This is a very serious sign.
  • Acute Necrotizing Fasciitis: The infection spreads through the dermis and hypodermis into the underlying fascia (connective tissue over muscle) and the muscle itself.
    • Toxins can induce fever, tachycardia (fast heart rate), and catastrophic hypotension.
    • Treatment: Debridement (surgical removal of dead tissue), healing from the inside out (leaving the wound open), IV antibiotics, or amputation.
    • Mortality Rate: Between $40\%$ and $60\%$.

Other Bacterial Skin Infections

• Gas Gangrene (Myonecrosis):

  • Causative Agent: Clostridium perfringens, an anaerobic, spore-forming organism.
  • Pathophysiology: Thrives in tissues with reduced circulation (e.g., diabetics with peripheral circulation issues). Produces proteases that break down proteins, generating gas that "blows apart" tissues.
  • Symptoms: Foul odor (worse than rotten chicken), local destruction, and edema.
  • Treatment: Debridement, high-level oxygen in a hyperbaric chamber (oxygen is lethal to this anaerobe), and amputation.

• Acne:

  • Causative Agent: Propionibacterium acnes, a normal skin inhabitant that thrives on sebum (oil).
  • Pathophysiology: Population explosion occurs during teen years or pregnancy due to increased oil production.
  • Comedones:
    • Whiteheads: Opening is closed over with pus forming.
    • Blackheads: Open to the air; lipids oxidize and turn black.
  • Symptoms: Papules (raised bumps) and pustules.
  • Treatment: Salicylic acid (to dry skin), retinoids, or topical/oral antibiotics (e.g., erythromycin).

• Cutaneous Anthrax (Woolsorter's Disease):

  • Causative Agent: Bacillus anthracis (soil-borne, contains a capsule and produces toxins causing edema).
  • Transmission: Occupational hazard for those handling animal products (like wool) where spores enter skin breaks.
  • Symptoms: A nodule forms at the entry site, eventually developing into an Eschar (a black necrotic sore).
  • Treatment: Antibiotics. Vaccines are available for military or high-risk individuals.

Viral Skin Infections

• Human Papillomavirus (HPV):

  • Causes Papillomas (Warts), which are benign epithelial growths.
  • Transmission: Direct contact.
  • Treatment: Often self-limiting, but can be treated with salicylic acid, liquid nitrogen (freezing), or surgical removal.

• Oral Herpes:

  • Causative Agent: Herpes Simplex Virus (HSV-1 or HSV-2).
  • Symptoms: Fever blisters or cold sores in the mouth/lip region.
  • Pathophysiology: Virus migrates up the trigeminal nerve (cranial nerve) to lay dormant in sensory neurons. It reactivates due to stress, illness, or trauma, causing a tingling or burning sensation as the virus migrates back down the neuron.
  • Treatment: Topical or oral antivirals (can be used prophylactically).

• Fifth Disease (Erythema Infectiosum):

  • Causative Agent: Human Parvovirus B19.
  • Transmission: Respiratory droplets or direct contact.
  • Symptoms: "Slapped face" (very red cheeks), low-grade fever, malaise, and a maculopapular rash (raised red bumps) that develops a lacy appearance as it resolves.
  • Risks: Can cross the placenta in pregnant women, potentially leading to stillbirth.

• Roseola (Sixth Disease):

  • Causative Agent: Human Herpesvirus 6 or 7.
  • Symptoms: Cold-like symptoms followed by a high fever ($> 100^{\circ}F$, often $102^{\circ}F$ to $103^{\circ}F$). After the fever, a maculopapular rash appears on the torso.

• Molluscum Contagiosum:

  • Causative Agent: Molluscum contagiosum virus (MCV) (a poxvirus).
  • Symptoms: Waxy, pearl-like nodules with a central depression.
  • Treatment: Lesion removal.

Fungal Skin Infections (Mycoses)

• Tinea (Ringworm):

  • Causative Agents (Dermatophytes): One of three genera: Trichophyton, Epidermophyton, or Microsporum.
  • Named by Location:
    • Tinea corporis: Body.
    • Tinea capitis: Scalp.
    • Tinea pedis: Feet (Athlete's foot).
    • Tinea barbae: Beard region (Barber's itch).
    • Tinea cruris: Groin (Jock itch).
    • Tinea unguium: Nails.
  • Treatment: Topical antifungal creams; oral antifungals for severe cases or nail fungus (though harsh).

• Cutaneous Candidiasis:

  • Causative Agent: Candida albicans (yeast).
  • Pathophysiology: Opportunistic infection that thrives in warm, moist environments (diaper rash, armpits, skin folds).
  • Symptoms: Red, itchy, weepy rash with exudate.

• Sporotrichosis:

  • Causative Agent: Sporothrix shenckii.
  • Transmission: Introduced via puncture wounds (e.g., rose thorns). Occupational hazard for gardeners.
  • Symptoms: A purplish nodule that may ulcerate. This is a deeper infection of the dermis/hypodermis.
  • Treatment: Oral antifungals for $3$ to $6$ months.

Eye Infections

• Eye Anatomy Review:

  • Lacrimal Apparatus: Includes the lacrimal gland (produces tears), lacrimal ducts, and nasolacrimal duct (drains into the nasal cavity).
  • Conjunctiva: Mucous membrane covering the eyeball and inner eyelids.
  • Cornea: Transparent portion over the lens and iris.

• Conjunctivitis (Pink Eye):

  • Bacterial Conjunctivitis: Caused by Haemophilus influenzae (HiB), Staph aureus, or Strep pneumoniae. Characterized by purulent discharge (yellow, green, or white) that can glue eyes shut. Treated with antibiotic drops like erythromycin.
  • Neonatal Conjunctivitis: Newborns are treated with antibiotics immediately upon birth to prevent:
    • Gonococcal ophthalmia neonatum: Caused by Neisseria gonorrhoeae.
    • Inclusion conjunctivitis: Caused by Chlamydia trachomatis.
  • Viral Conjunctivitis: Caused by adenovirus (localized) or systemic viruses (rhinovirus/cold, varicella-zoster/chickenpox). Characterized by watery discharge. Usually self-limiting.

• Trachoma (Granular Conjunctivitis):

  • Causative Agent: Chlamydia trachomatis (not acting as an STD in this context).
  • Transmission: Personal contact, shared fomites (towels), or flies. Prevalent in Africa.
  • Impact: Leading cause of preventable blindness. Chronic inflammation causes eyelids to swell and lashes to turn inward, rubbing and damaging the cornea over $10$ to $15$ years.

• Onchocerciasis (River Blindness):

  • Causative Agent: Onchocerca volvulus (a filarial roundworm).
  • Transmission: Bites from black flies; endemic in Central Africa and parts of Central/South America.
  • Pathophysiology: Larvae migrate to the cornea and conjunctiva, causing inflammation and immune responses that lead to blindness.
  • Treatment: Antihelminthic drugs such as ivermectin.

Questions & Discussion

• Dialogue Fragment: "Did you pick it up for me? I'm assuming you like the smell… What the fuck? You're going out and how do I smell? Do I smell good? I need to make sure I scrub myself really good. Because why? Would be uncomfortable. It's fucking a tattoo shop. I don't wanna stink. He's gonna be sitting there tattooing me for six months."
• Dialogue Fragment: "I'll kiss you. I'll kiss you."
• Dialogue Fragment: "Watch all the bedding today. Mhmm."
• Dialogue Fragment: "Do me one favor. Once the washer slips off…"