Microbial Diseases of the Skin and Eyes: Structured Study Notes

Microbial Diseases of the Skin and Eyes

Overview

This chapter explores the major types of microbial diseases affecting the skin and eyes, including bacterial, viral, fungal, and parasitic infections. It covers lesion classification, key pathogens, disease mechanisms, clinical features, and treatment strategies.

Types of Skin Lesions

Classification and Clinical Relevance

• Rashes and Lesions: Not all indicate skin infection; some may signal systemic disease.

• Lesion Variation: Useful for describing disease conditions.

Lesion Type	Description
Vesicle	Small, fluid-filled elevation
Bulla	Large vesicle (>1 cm)
Macule	Flat, red spot
Papule	Raised, red bump
Pustule	Contains pus

Bacterial Diseases of the Skin

Staphylococcal Infections

• Staphylococcus aureus: Most pathogenic strains are coagulase-positive.

• Coagulase-negative Staph (e.g., S. epidermidis): Pathogenic only when skin barrier is breached.

• Toxin Production:

  • Enterotoxins: Affect GI tract

  • Leukocidins: Destroy phagocytic leukocytes

  • Exfoliative toxin: Causes scalded skin syndrome (outer skin peels off)

  • Toxic Shock Syndrome Toxin-1 (TSST-1): Produces fever, vomiting, rash, and shock

Common S. aureus Skin Infections

• Folliculitis: Infection of hair follicles (pimples)

• Sty: Infection of eyelash follicle

• Furuncle: Localized pus, inflamed tissue (abscess/boil); may become systemic

• Carbuncle: Extensive tissue invasion, often with fever; failed containment of furuncle

Streptococcal Infections

• Group A, β-hemolytic Strep (Streptococcus pyogenes): Possesses multiple virulence factors:

  • Hemolysins (α and β)

  • M protein

  • Erythrogenic toxins

  • Streptokinases

  • Hyaluronidases

  • Leukocidins

Key Streptococcal Skin Infections

• Impetigo: Pustules that rupture and crust; highly contagious, direct contact spread

• Erysipelas: Reddish lesions and fever; may progress to tissue damage and septicemia

• Necrotizing Fasciitis (IGAS): "Flesh-eating" bacteria; rapid tissue destruction, high mortality (20-50%)

Pseudomonas Infections

• Pseudomonas aeruginosa: Notable for antimicrobial resistance and biofilm formation

• Dermatitis: Often contracted from hot tubs; self-limiting, itchy rash

• Otitis externa (Swimmer's ear): Infection of the outer ear canal

• Opportunistic Infections: Major concern in burn units and chronic pulmonary disease

Biofilm Structure

Component	Description
Cell Cluster	Aggregated bacterial cells
Streamer	Filamentous extensions
Channel	Allows fluid movement
Void	Empty space within biofilm

Acne

• Etiologic Agent: Propionibacterium acnes; utilizes sebum as nutrient

• Types: Comedonal, inflammatory, nodular cystic

• Treatments: Benzoyl peroxide, tretinoin, tazorac, antimicrobials (clindamycin, erythromycin), isotretinoin (teratogenic)

Viral Diseases of the Skin

Warts (Papillomas)

• Causative Agent: Papillomaviruses (over 50 types)

• Transmission: Direct contact

• Treatment: Cryotherapy, electrodessication, acid, topical gels (podofilox, Aldara), laser removal

Smallpox

• Causative Agent: Variola virus

• Transmission: Respiratory route

• Incubation: ~2 weeks; followed by fever, influenza-like symptoms, then skin lesions

• Eradication: No animal reservoir; aggressive vaccination

Chickenpox and Shingles

• Causative Agent: Human herpesvirus 3 (varicella-zoster)

• Transmission: Respiratory route

• Clinical Course: Vesicles form, fill with pus, rupture, and scab; virus remains latent in nerve cells

• Shingles: Reactivation; affects cutaneous sensory nerves (unilateral), severe pain, postherpetic neuralgia

• Complication: Reye's syndrome (vomiting, brain dysfunction, coma, death); risk increased by aspirin use

• Vaccine: Live, attenuated (introduced 1995)

Herpes Simplex

• HSV-1: Oral/respiratory transmission; cold sores; latent infection

• HSV-2: Sexual transmission; genital herpes; latent infection

• Complications: Both may cause encephalitis

• Treatment: Acyclovir

Measles (Rubeola)

• Causative Agent: Rubeola virus

• Transmission: Respiratory route

• Incubation: 10-12 days

• Symptoms: Cold-like, then macular rash (face → trunk → extremities)

• Diagnostic: Koplik's spots (red lesions with white specks in oral cavity)

• Complications: Ear infections, pneumonia, encephalitis, death

• Vaccine: MMR (measles, mumps, rubella)

Rubella (German Measles)

• Causative Agent: Rubella virus

• Transmission: Respiratory route

• Incubation: 2-3 weeks

• Symptoms: Mild fever, small rash

• Congenital Rubella Syndrome: Fetal damage (deafness, heart defects, cataracts, mental delays, death) if infection occurs in first trimester

• Vaccine: MMR (introduced 1969)

Fungal Diseases of the Skin

Dermatophytoses (Ringworm, Tinea)

• Tinea capitis: Scalp

• Tinea cruris: Jock itch

• Tinea pedis: Athlete's foot

Pityriasis and Malassezia furfur

• Depigmented or hyperpigmented "scaly" skin patches

• Treated with topical ointments

• May cause fungemia

Candidiasis

• Manifestations: Vaginitis, thrush

• Treatment: Topical miconazole, clotrimazole, nystatin

• Systemic Disease: In immunosuppressed individuals, may become fulminating and fatal

Parasitic Diseases of the Skin

Scabies

• Causative Agent: Sarcoptes scabiei mite

• Pathogenesis: Mite burrows under skin, lays eggs; intense itching, risk of secondary infection

• Transmission: Direct contact; mite lifespan ~25 days

• Treatment: Topical Kwell®, oral ivermectin for resistant cases

Microbial Diseases of the Eye

Conjunctivitis ("Pinkeye")

• Infection Site: Conjunctiva (mucous membrane lining eyelid and eyeball)

• Causative Agents: Bacteria, viruses, protozoans

• Risk: Soft contact lens wearers

• Examples:

  • Acanthamoeba keratitis: Protozoan from water, soil

  • Pseudomonas spp. (esp. P. aeruginosa)

  • Staphylococcus aureus

Neonatal Gonorrheal Ophthalmia

• Causative Agent: Neisseria gonorrhoeae

• Transmission: Passage through birth canal

• Prevention: Eyes washed with 1% silver nitrate or antibiotics

• Complication: Blindness

Trachoma

• Causative Agent: Chlamydia trachomatis (intracellular parasite)

• Transmission: Direct contact, flies

• Clinical Course: Corneal abrasion, scarring, blindness

• Treatment: Antibiotics (tetracycline)

Summary Table: Major Microbial Diseases of Skin and Eyes

Disease	Causative Agent	Key Features	Treatment
Impetigo	Streptococcus pyogenes	Pustules, crusting, contagious	Topical antibiotics
Scalded Skin Syndrome	Staphylococcus aureus	Peeling skin, neonates	IV antibiotics
Chickenpox/Shingles	Varicella-zoster virus	Vesicular rash, latency, pain	Vaccine, antivirals
Measles	Rubeola virus	Macular rash, Koplik's spots	MMR vaccine
Ringworm	Dermatophytes	Scaly patches	Topical antifungals
Scabies	Sarcoptes scabiei	Itching, burrows	Kwell®, ivermectin
Trachoma	Chlamydia trachomatis	Corneal scarring, blindness	Tetracycline

Key Terms and Concepts

• Coagulase: Enzyme produced by pathogenic staphylococci; distinguishes S. aureus from other species.

• Biofilm: Structured community of bacteria encased in a self-produced polymeric matrix, conferring resistance to antimicrobials.

• Latent Infection: Virus remains dormant in host cells, can reactivate (e.g., herpesviruses).

• Attenuated Vaccine: Live virus weakened for immunization purposes.

Additional info:

• Some details on pathogenesis, treatment, and epidemiology have been expanded for clarity and completeness.